Extracellular fluid volume and glomerular filtration rate in 1878 healthy potential renal transplant donors: effects of age, gender, obesity and scaling

The physiological basis of renal nuclear medicine

Renal physiology underpins renal nuclear medicine, both academic and clinical. Clearance, an important concept in renal physiology, comprises tissue uptake rate of tracer (tissue clearance), disappearance rate from plasma (plasma clearance), appearance rate in urine (urinary clearance) and disappearance rate from tissue. In clinical research, steady-state plasma clearances of para-amino-hippurate and inulin have been widely used to measure renal blood flow (RBF) and glomerular filtration rate (GFR), respectively. Routinely, GFR is measured at non-steady state as plasma clearance of a filtration agent, such as technetium-99m diethylenetriaminepentaacetic acid. Scaled to three-dimensional whole body metrics rather than body surface area, GFR in women is higher than in men but declines faster with age. Age-related decline is predominantly from nephron loss. Tubular function determines parenchymal transit time, which is important in renography, and the route of uptake of technetium-99m dimercaptosuccinic acid, which is via filtration. Resistance to flow is defined according to the pressure-flow relationship but in renography, only transit time can be measured, which, being equal to urine flow divided by collecting system volume, introduces further uncertainty because the volume is also unmeasurable. Tubuloglomerular feedback governs RBF and GFR, is regulated by the macula densa, mediated by adenosine and renin, and can be manipulated with proximal tubular sodium-glucose cotransporter-2 inhibitors. Other determinants of renal haemodynamics include prostaglandins, nitric oxide and dopamine, while protein meal and amino acid infusion are used to measure renal functional reserve. In conclusion, for measuring renal responses to exogenous agents, steady-state para-amino-hippurate and inulin clearances should be replaced with rubidium-82 and gallium-68 EDTA for measuring RBF and GFR.

Has the time come for age-adapted glomerular filtration rate criteria to define chronic kidney disease: how soon is now?

PURPOSE OF REVIEW

The conventional definition of chronic kidney disease (CKD) primarily relies on the identification of albuminuria or a decline in estimated glomerular filtration rate (eGFR). For many years, a straightforward eGFR threshold of <60 ml/min/1.73 m 2 has been widely adopted as the standard for defining CKD. Nonetheless, this criterion fails to consider the natural aging process of the kidney, and this oversight may affect the accurate diagnosis of kidney disease particularly at the extremes of age.

RECENT FINDINGS

The fixed eGFR threshold of <60 ml/min/1.73 m 2 for defining CKD misses crucial opportunities for risk prevention. Studies have revealed that the eGFR threshold at which the risks for adverse long-term health outcomes such as mortality, cardiovascular events, and kidney failure begin to rise varies substantially by age. Specifically, this threshold is lower for the elderly and higher for young adults. Consequently, this results in the over-diagnosis of kidney disease in the elderly and the under-diagnosis of kidney disease in young adults.

SUMMARY

To address these limitations of the current CKD definition, we discuss a number of proposed age-adapted eGFR criteria and weigh their pros and cons against the current, simple, and universally accepted approach.

Errors in measurement of glomerular filtration rate using the slope-intercept technique and their identification

BACKGROUND

GFR measured from plasma sampling may be expressed as slope-intercept GFR (SI-GFR) and scaled to body surface area (mGFR/BSA) or as GFR per unit extracellular fluid volume (mGFR/ECV), which is based only on half-time. Measurement errors comprise 3 categories. Pre-injection error arises from error in administered marker and is suspected when mGFR/BSA and mGFR/ECV disagree. Injection errors include 'tissued' injections. Post-injection errors include inaccurate sample timing, inaccurate pipetting, sample haemolysis and sampling through long IV lines through which marker was administered. The aim of the study was to evaluate the impact of errors on mGFR.

METHODS

We compared mGFR/BSA with mGFR/ECV in 898 patients undergoing routine investigation. To investigate post-injection error, we took two further patient datasets with r values (correlation coefficient of the 3-sample fit) of 1.0 and introduced errors, in isolation, into each of the 3 recorded sample values, as follows: pipetting (volume) errors of -20%, -10%, -5%, 5%, 10% and 20%, and timing errors of -15 min, -10 min, -5 min, 5 min, 10 min and 15 min.

RESULTS

The correlation between mGFR/BSA and mGFR/ECV was close and independent of r. Post-injection error depended on the time of the sample in which it occurred. r correlated poorly with error magnitude for both volume and timing errors. When a 'rogue' sample is suspected its error needed to be substantial for it to be identified by single sample estimates applied to the other samples.

CONCLUSION

SI-GFR is resistant to post-injection timing and volume errors but not to pre-injection error.

Recommendations for living donor kidney transplantation

This Guide for Living Donor Kidney Transplantation (LDKT) has been prepared with the sponsorship of the Spanish Society of Nephrology (SEN), the Spanish Transplant Society (SET), and the Spanish National Transplant Organization (ONT). It updates evidence to offer the best chronic renal failure treatment when a potential living donor is available. The core aim of this Guide is to supply clinicians who evaluate living donors and transplant recipients with the best decision-making tools, to optimise their outcomes. Moreover, the role of living donors in the current KT context should recover the level of importance it had until recently. To this end the new forms of incompatible HLA and/or ABO donation, as well as the paired donation which is possible in several hospitals with experience in LDKT, offer additional ways to treat renal patients with an incompatible donor. Good results in terms of patient and graft survival have expanded the range of circumstances under which living renal donors are accepted. Older donors are now accepted, as are others with factors that affect the decision, such as a borderline clinical history or alterations, which when evaluated may lead to an additional number of transplantations. This Guide does not forget that LDKT may lead to risk for the donor. Pre-donation evaluation has to centre on the problems which may arise over the short or long-term, and these have to be described to the potential donor so that they are able take them into account. Experience over recent years has led to progress in risk analysis, to protect donors' health. This aspect always has to be taken into account by LDKT programmes when evaluating potential donors. Finally, this Guide has been designed to aid decision-making, with recommendations and suggestions when uncertainties arise in pre-donation studies. Its overarching aim is to ensure that informed consent is based on high quality studies and information supplied to donors and recipients, offering the strongest possible guarantees.

Factors Associated with Antiretroviral Therapy Toxicity Out-Comes in Patients with and without Hypertension

BACKGROUND

Negative effects of antiretroviral therapy (ART) drugs on HIV/AIDS patients are one of the major health issues in the therapeutic treatment of this communicable disease. This holds particularly for people living with HIV (PLHIV) who might have a non-communicable disease (like hypertension), which also requires a lifetime treatment. In this study, we investigated the association between hypertension and other possible factors on ART toxicity markers in patients with hypertension, compared to those without hypertension.

METHODS

This retrospective longitudinal study reviewed chronic patient files of 525 patients (of which 222 were hypertensive) who satisfied the inclusion criteria and were on ART at a hospital in central Eswatini. Specific levels of estimated glomerular filtration (eGFR), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were used as drug toxicity markers. To analyze the longitudinal data between the exposure of interest and outcome variables, a Generalized Estimated Equation method was employed.

RESULTS

Participants with hypertension had decreased eGFR compared to those without hypertension (β = -2.22; p-value = 0.03). There was no significant association between ALT, AST and hypertension (p-value = 0.34 and 0.20, respectively). Factors that were found to have a significant association with ART toxicity markers included age, sex, ART duration, hypertension treatment and time of study. The eGFR was found to be significantly increasing over the study period (p-value < 0.001) for all participants. The significance was consistent in both hypertensive and non-hypertensive participants independently (p-value = 0.002 and <0.001, respectively). The overall trends of ALT and AST over time were also significant (p-value = 0.003 and <0.001, respectively).

CONCLUSIONS

Patients with hypertension had decreased eGFR, and there was a significant association of eGFR with time of the study. Special attention, therefore, to monitor calamities which are indicated by a decrease of eGFR (like renal impairment) should be given in PLHIV on ART with hypertension, especially more so if they were on ART for longer time.

Assessing Renal Function for Kidney Donation. How Low Is Too Low?

Kidney transplantation (KT) is the treatment of choice for patients with end-stage kidney disease (ESKD) with decreased morbi-mortality, improved life quality, and reduced cost. However, the shortage of organs from deceased donors led to an increase in KT from living donors. Some stipulate that living donors have a higher risk of ESKD after donation compared with healthy non-donors. The reason for this is not clear. It is possible that ESKD is due to the nephrectomy-related reduction in glomerular filtration rate (GFR), followed by an age-related decline that may be more rapid in related donors. It is essential to assess donors properly to avoid rejecting suitable ones and not accepting those with a higher risk of ESKD. GFR is a central aspect of the evaluation of potential donors since there is an association between low GFR and ESKD. The methods for assessing GFR are in continuous debate, and the kidney function thresholds for accepting a donor may vary according to the guidelines. While direct measurements of GFR (mGFR) provide the most accurate evaluation of kidney function, guidelines do not systematically use this measurement as a reference. Also, some studies have shown that the GFR decreases with age and may vary with gender and race, therefore, the lower limit of GFR in patients eligible to donate may vary based on these demographic factors. Finally, it is known that CrCl overestimates mGFR while eGFR underestimates it, therefore, another way to have a reliable GFR could be the combination of two measurement methods.

Estimating extracellular fluid volume in healthy individuals: evaluation of existing formulae and development of a new equation

Introduction

Several clinical settings require an accurate estimation of the physiologically expected extracellular fluid volume (ECFV). We aimed to analyze the performances of existing ECFV-estimating equations and to develop a new equation.

Methods

The performances of 11 ECFV-estimating equations were analyzed in 228 healthy kidney donor candidates (Bichat Hospital, Paris, France) who underwent ECFV measurement using the distribution volume of ⁵¹Cr-labeled EDTA (⁵¹Cr-EDTA). An equation was developed using a penalized linear modeling approach (elastic net regression) and externally (Tenon Hospital, Paris, France, N = 142) validated.

Results

Participants from Bichat (mean age 45.2 ± 12.0 years, 43.0% men) and Tenon (47.8 ± 10.3 years, 29.6% men) hospitals had a mean measured ECFV of 15.4 ± 2.8 l and 15.1 ± 2.1 l, respectively. Available ECFV-estimating formulae have highly variable precision and accuracy. The new equation incorporating body weight, height, sex, and age had better precision and accuracy than all other equations in the external validation cohort, with a median bias of −0.20 (95% CI: −0.35 to −0.05) l versus −2.63 (−2.87 to −2.42) l to −0.57 (− 0.83 to −0.40) l and 0.21 (0.12 to 0.43) l to 2.89 (2.65 to 3.11) l, for underestimating and overestimating equations, respectively, an interquartile range for the bias of 0.88 (0.70 to 1.08) l versus 0.91 (0.71 to 1.20) l to 1.93 (1.67 to 2.25) l, and an accuracy within 10% of 90.9% (83.8 to 94.4) versus 88.0% (81.0 to 92.3) to 8.5% (4.2 to 13.4). These results were consistent across subgroups defined by sex, body mass index (BMI), body surface area (BSA), age, and ethnicity.

Conclusion

We developed and validated a new equation to estimate the individual reference value of ECFV, which is easily usable in clinical practice. Further validation in cohorts including individuals of extreme age and corpulence remains needed.

[Living donor nephrectomy: The French guidelines from CTAFU]

OBJECTIVE

To propose surgical recommendations for living donor nephrectomy.

METHOD

Following a systematic approach, a review of the literature (Medline) was conducted by the CTAFU regarding functional and anatomical assessment of kidney donors, including which side the kidney should be harvested from. Distinct surgical techniques and approaches were evaluated. References were considered with a predefined process to propose recommendations with the corresponding levels of evidence.

RESULTS

The recommendations clarify the legal and regulatory framework for kidney donation in France. A rigorous assessment of the donor is one of the essential prerequisites for donor safety. The impact of nephrectomy on kidney function needs to be anticipated. In case of modal vascularization of both kidneys without a relative difference in function or urologic abnormality, removal of the left kidney is the preferred choice to favor a longer vein. Mini-invasive approaches for nephrectomy provide faster donor recovery, less donor pain and shorter hospital stay than open surgery.

CONCLUSION

These French recommendations must contribute to improving surgical management of candidates for kidney donation.

New gender-specific formulae for estimating extracellular fluid volume from height and weight in adults

AIMS

First, to derive gender-specific formulae for estimation of extracellular fluid volume (eECV) and second, compare eECV as a scaling metric for slope-intercept glomerular filtration rate (GFR) with estimated body surface area (eBSA), lean body mass (eLBM) and total body water (eTBW).

METHODS

GFR and 'slope-only' GFR (GFR/ECV), both single compartment-corrected, were measured in a previously published multicentre database of healthy potential kidney transplant donors. Measured ECV (mECV) was obtained as ratio GFR-to-GFR/ECV. Formulae for eECV in men and women were derived from the relationship of mECV with height and weight and expressed as eECV = a.weight.height. In a population of prospective kidney transplant donors from a single centre, eECV was compared with mECV. GFR was scaled to eECV, eBSA, eLBM and eTBW, estimated from previously published formulae.

RESULTS

In men and women, respectively, a was 0.0755 and 0.0399, x was 0.6185 and 0.6065 and y was 0.4982 and 0.6217. In the single centre, biases (±precisions) of eECV against mECV in men and women were 0.26 (±1.68) and 0.31 (±1.67) l. Mean GFR/eBSA was higher in men but mean GFR/eLBM and GFR/eTBW were higher in women. Mean GFR/ECV and mean GFR/eECV were very similar between the two genders. GFR/ECV and GFR/eECV showed correlations with each other that were almost identical between men and women.

CONCLUSIONS

New formulae are described for estimating eECV. Scaling GFR to eECV is more physiological than scaling to eBSA and accounts for gender. eECV used for measuring GFR from a single blood sample should be gender-specific.

Extracellular fluid volume: A suitable indexation variable to assess impact of bariatric surgery on glomerular filtration rate in patients with chronic kidney disease

BACKGROUND

Bariatric surgery (BS) might be a nephroprotective treatment in obese patients with chronic kidney disease (CKD), and the non-linear relation between body surface area (BSA) and extracellular fluid volume (ECFV) in obese people raises the question of the most relevant way to scale glomerular filtration rate (GFR) for assessing renal function changes after BS.

METHODS

We screened 1774 BS candidates and analysed 10 consecutive participants with CKD stage 3. True GFR (mGFR), measured by the renal clearance of 51Cr-ethylenediaminetetraacetic acid (EDTA), was scaled either to BSA (mGFRBSA) or to ECFV measured by 51Cr-EDTA distribution volume (mGFRECFV) before and one year after BS.

RESULTS

The 10 candidates for BS had a mean body mass index of 43.3 ± 3.6 kg/m2 and a mean GFR of 48 ± 8 mL/min/1.73 m2. Six participants had a sleeve gastrectomy and four had a Roux-en-Y gastric bypass. One year after BS, ECFV decreased (23.2 ± 6.2 to 17.9 ± 4.3 L, p = 0.001), absolute mGFR was not significantly modified (74 ± 23 versus 68 ±19 mL/min), mGFRBSA did not change significantly (53 ± 18 versus 56 ± 17 mL/min/1.73 m2) whereas mGFRECFV significantly increased (42 ± 13 versus 50 ± 14 mL/min/12.9 L, p = 0.037). The relation between mGFRECFV and mGFRBSA was different from the identity line before (p = 0.014) but not after BS (p = 0.09).

CONCLUSION

There is a difference between mGFRBSA and mGFRECFV following BS and the latter might better reflect the adequacy between renal function and corpulence.

Reference values of glomerular filtration rate for healthy adults in southern China: a cross-sectional survey

Background:

Currently the global data on the glomerular filtration rate of healthy adults are insufficient, with relatively little data for other races and countries. Especially in China, there are no such figures.

Methods:

In this cross-sectional study, we included healthy Han adults in southern China. Participants completed a lifestyle and medical history questionnaire and had their blood pressure measured, and blood and urine samples collected. Serum creatinine was measured and used to estimated glomerular filtration rate (eGFR) by the Modification of Diet in Renal Disease (MDRD) and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formulae. The normal range of eGFR is described, and the influence of gender and age on eGFR is analyzed by the statistical method.

Results:

We provided the largest sample size of eGFR research in China at present. The mean age of the 20,930 healthy individuals was 40.9 ± 12.3 years, 58.8% were women. The eGFR_MDRD for women and men were 111.3 ± 17.4 mL/min per 1.73 m² and 103.3 ± 15.9 mL/min per 1.73 m², respectively. The eGFR_CKD-EPI for women and men were 110.3 ± 12.1 mL/min per 1.73 m² and 103.8 ± 13.3 mL/min per 1.73 m², respectively. The eGFR_MDRD of women and men in all age groups decreased continuously by 7.3 ml/min/1.73 m²/decade and 4.4 ml/min/1.73 m²/decade, respectively. The eGFR_CKD-EPI of women and men in all age groups decreased continuously by 8.4 ml/min/1.73 m²/decade and 6.9 ml/min/1.73 m²/decade.

Conclusions:

The eGFR of women is higher than men and with the increasing age, the eGFR of women declines faster than men.

Renal length z-score for the detection of dysfunction in children with solitary functioning kidney

AIM

To evaluate whether renal length z-scores predict renal dysfunction in children with a solitary functioning kidney (SFK).

METHODS

In a single-centre retrospective cohort of children with SFK, we correlated body mass index z-scores, extracellular volume and lean body mass to renal length z-scores. We grouped these z-scores to other markers of renal dysfunction (proteinuria, hypertension, extracellular volume and abnormal estimated glomerular function rate [eGFR]) and analysed renal length z-score with multivariate analysis, receiver-operated characteristics (ROC) plots and Youden's index to determine an appropriate cut-off.

RESULTS

111 children had a median follow-up 5.08 years, eGFR 80.8 mL/min/1.73 m² , and age at last follow-up 7.4 (3.8-13.4 years). The median renal length z-scores of those without any renal dysfunction (n = 37, 25.1%) were greater (+3.66, interquartile range 3.02-4.47) than those with renal dysfunction (median 3.11, interquartile range 1.76-4.11, P = .0107, Mann-Whitney test). Using a cut-off of z-score of >+1.911, the odds ratio for having no renal dysfunction was 0.07 (95% CI 0.002-0.459, P = .0010). However, accuracy of the renal length z-score was poor (ROC curve 0.6488).

CONCLUSION

In this cohort of children with SKF, using the renal length z-score as a biomarker of renal dysfunction at 7 years of age is not recommended.

GFR in Healthy Aging: an Individual Participant Data Meta-Analysis of Iohexol Clearance in European Population-Based Cohorts

BACKGROUND

Population mean GFR is lower in older age, but it is unknown whether healthy aging is associated with preserved rather than lower GFR in some individuals.

METHODS

We investigated the cross-sectional association between measured GFR, age, and health in persons aged 50-97 years in the general population through a meta-analysis of iohexol clearance measurements in three large European population-based cohorts. We defined a healthy person as having no major chronic disease or risk factors for CKD and all others as unhealthy. We used a generalized additive model to study GFR distribution by age according to health status.

RESULTS

There were 935 (22%) GFR measurements in persons who were healthy and 3274 (78%) in persons who were unhealthy. The mean GFR was lower in older age by -0.72 ml/min per 1.73 m² per year (95% confidence interval [95% CI], -0.96 to -0.48) for men who were healthy versus -1.03 ml/min per 1.73 m² per year (95% CI, -1.25 to -0.80) for men who were unhealthy, and by -0.92 ml/min per 1.73 m² per year (95% CI, -1.14 to -0.70) for women who were healthy versus -1.22 ml/min per 1.73 m² per year (95% CI, -1.43 to -1.02) for women who were unhealthy. For healthy and unhealthy people of both sexes, both the 97.5th and 2.5th GFR percentiles exhibited a negative linear association with age.

CONCLUSIONS

Healthy aging is associated with a higher mean GFR compared with unhealthy aging. However, both the mean and 97.5 percentiles of the GFR distribution are lower in older persons who are healthy than in middle-aged persons who are healthy. This suggests that healthy aging is not associated with preserved GFR in old age.

CKD: A Call for an Age-Adapted Definition

Current criteria for the diagnosis of CKD in adults include persistent signs of kidney damage, such as increased urine albumin-to-creatinine ratio or a GFR below the threshold of 60 ml/min per 1.73 m2 This threshold has important caveats because it does not separate kidney disease from kidney aging, and therefore does not hold for all ages. In an extensive review of the literature, we found that GFR declines with healthy aging without any overt signs of compensation (such as elevated single-nephron GFR) or kidney damage. Older living kidney donors, who are carefully selected based on good health, have a lower predonation GFR compared with younger donors. Furthermore, the results from the large meta-analyses conducted by the CKD Prognosis Consortium and from numerous other studies indicate that the GFR threshold above which the risk of mortality is increased is not consistent across all ages. Among younger persons, mortality is increased at GFR <75 ml/min per 1.73 m2, whereas in elderly people it is increased at levels <45 ml/min per 1.73 m2 Therefore, we suggest that amending the CKD definition to include age-specific thresholds for GFR. The implications of an updated definition are far reaching. Having fewer healthy elderly individuals diagnosed with CKD could help reduce inappropriate care and its associated adverse effects. Global prevalence estimates for CKD would be substantially reduced. Also, using an age-specific threshold for younger persons might lead to earlier identification of CKD onset for such individuals, at a point when progressive kidney damage may still be preventable.

Evaluation of the sensitivity of R1ρ MRI to pH and macromolecular density

The tumor microenvironment is characteristically acidic and this extracellular acidosis is known to play a role in carcinogenesis and metastasis and can affect tumor chemosensitivity and radiosensitivity. Intracellular pH has been used as a possible biomarker of salvageable tissue in ischemic stroke. A non-invasive MRI-based approach for the determination and imaging of cerebral pH would be a powerful tool in cancer diagnosis and monitoring, as well as stroke treatment planning. Several pH-based MRI imaging approaches have been proposed but for these to be useful, disentangling the effects of pH from other parameters which may affect the measured MRI signal is crucial to ensure accuracy and specificity. R1 relaxation in the rotating frame (R1ρ) is an example of a method that has been proposed to probe pH in vivo using MRI. In this study, we have investigated the relationship between R1ρ, pH, and macromolecular density in vitro using phantoms and in human volunteers. Here we show that the rate of R1ρ relaxation (=1/T1ρ) varies with pH but only in the presence of macromolecules. At constant pH, phantom macromolecular density inversely correlated with R1ρ. R1ρ imaging of the normal human brain demonstrated regional heterogeneity with significant differences between structurally distinct regions, which are likely to be independent of pH. For example, R1ρ was higher in the basal ganglia compared to grey matter and higher in grey matter compared to white matter. We conclude that R1ρ cannot be reliably used to image tissue pH without deconvolution from the effects of local tissue macromolecular composition.

Glomerular filtration rate: new age- and gender- specific reference ranges and thresholds for living kidney donation

BACKGROUND

There is a need for a large, contemporary, multi-centre series of measured glomerular filtration rates (mGFR) from healthy individuals to determine age- and gender-specific reference ranges for GFR. We aimed to address this and to use the ranges to provide age- and gender-specific advisory GFR thresholds considered acceptable for living kidney donation.

METHODS

Individual-level data including pre-donation mGFR from 2974 prospective living kidney donors from 18 UK renal centres performed between 2003 and 2015 were amalgamated. Age- and gender-specific GFR reference ranges were determined by segmented multiple linear regression and presented as means ± two standard deviations.

RESULTS

Males had a higher GFR than females (92.0 vs 88.1 mL/min/1.73m2, P < 0.0001). Mean mGFR was 100 mL/min/1.73m2 until 35 years of age, following which there was a linear decline that was faster in females compared to males (7.7 vs 6.6 mL/min/1.73m2/decade, P = 0.013); 10.5% of individuals aged > 60 years had a GFR < 60 mL/min/1.73m2. The GFR ranges were used along with other published evidence to provide advisory age- and gender-specific GFR thresholds for living kidney donation.

CONCLUSIONS

These data suggest that GFR declines after 35 years of age, and the decline is faster in females. A significant proportion of the healthy population over 60 years of age have a GFR < 60 mL/min/1.73m2 which may have implications for the definition of chronic kidney disease. Age and gender differences in normal GFR can be used to determine advisory GFR thresholds for living kidney donation.

Educational review: measurement of GFR in special populations

IMPORTANCE

Changes in kidney function are typically followed by the sequential estimation of glomerular filtration rate (eGFR). Formulae for eGFR work well on a population basis, but there are well-known conditions where they do not work.

OBJECTIVE

The purpose of this review is to summarize the existing literature on special populations in the pediatric age range and provide recommendations on how to estimate GFR in these populations.

FINDINGS

The reliability of creatinine depends on muscle mass, while cystatin C (not widely available) is limited by inflammation and changes in protein catabolism. Various dietary factors can alter eGFR. Renal function in neonates changes drastically every day, and there are currently no satisfactory reference intervals for routine pediatric use. Gender effects and conditions such as wasting disease and obesity require alternative ways to obtain eGFR. In oncology patients, chemotherapy may negatively affect renal function, and nuclear GFR measurements may be necessary. For body builders, high muscle mass may lead to underestimation of eGFR using creatinine.

CONCLUSIONS AND RELEVANCE

Clinicians should be aware of special populations that may yield misleading eGFRs with conventional creatinine-based formulae, and that the alternative methods may be more appropriate for some populations.

A Physiologically-Based Pharmacokinetic Model to Predict Human Fetal Exposure for a Drug Metabolized by Several CYP450 Pathways

BACKGROUND

Pregnant women and their fetuses are exposed to numerous drugs; however, they are orphan populations with respect to the safety and efficacy of drugs. Therefore, the prediction of maternal and fetal drug exposure prior to administration would be highly useful.

METHODS

A physiologically-based pharmacokinetic (PBPK) model for nevirapine, which is metabolized by the cytochrome P450 (CYP) 3A4, 2B6 and 2D6 pathways, was developed to predict maternal and fetal pharmacokinetics (PK). The model was developed in both non-pregnant and pregnant women, and all physiological and enzymatic changes that could impact nevirapine PK were taken into account. Transplacental parameters estimated from ex vivo human placenta perfusion experiments were included in this PBPK model. To validate the model, observed maternal and cord blood concentrations were compared with predicted concentrations, and the impact of fetal clearance on fetal PK was investigated.

RESULTS

By implementing physiological changes, including CYP3A4, 2D6 and 2B6 inductions, we predicted a clearance increase of 21 % in late pregnancy. The PBPK model successfully predicted the disposition for both non-pregnant and pregnant populations. Parameters obtained from the ex vivo experiments allowed the prediction of nevirapine concentrations that matched observed cord blood concentrations. The fetal-to-maternal area under the curve ratio (0-24 h interval) was 0.77, and fetal metabolism had no significant effect on fetal PK.

CONCLUSIONS

The PBPK approach is a useful tool for quantifying a priori the drug exposure of metabolized drugs during pregnancy, and can be applied to evaluate alternative dosing regimens to optimize drug therapy. This approach, including ex vivo human placental perfusion parameters, is a promising approach for predicting human fetal exposure.

Development of a Whole-Body Physiologically Based Pharmacokinetic Approach to Assess the Pharmacokinetics of Drugs in Elderly Individuals

Background

Because of the vulnerability and frailty of elderly adults, clinical drug development has traditionally been biased towards young and middle-aged adults. Recent efforts have begun to incorporate data from paediatric investigations. Nevertheless, the elderly often remain underrepresented in clinical trials, even though persons aged 65 years and older receive the majority of drug prescriptions. Consequently, a knowledge gap exists with regard to pharmacokinetic (PK) and pharmacodynamic (PD) responses in elderly subjects, leaving the safety and efficacy of medicines for this population unclear.

Objectives

The goal of this study was to extend a physiologically based pharmacokinetic (PBPK) model for adults to encompass the full course of healthy aging through to the age of 100 years, to support dose selection and improve pharmacotherapy for the elderly age group.

Methods

For parameterization of the PBPK model for healthy aging individuals, the literature was scanned for anthropometric and physiological data, which were consolidated and incorporated into the PBPK software PK-Sim^®. Age-related changes that occur from 65 to 100 years of age were the main focus of this work. For a sound and continuous description of an aging human, data on anatomical and physiological changes ranging from early adulthood to old age were included. The capability of the PBPK approach to predict distribution and elimination of drugs was verified using the test compounds morphine and furosemide, administered intravenously. Both are cleared by a single elimination pathway. PK parameters for the two compounds in younger adults and elderly individuals were obtained from the literature. Matching virtual populations—with regard to age, sex, anthropometric measures and dosage—were generated. Profiles of plasma drug concentrations over time, volume of distribution at steady state (V_ss) values and elimination half-life (t_½) values from the literature were compared with those predicted by PBPK simulations for both younger adults and the elderly.

Results

For most organs, the age-dependent information gathered in the extensive literature analysis was dense. In contrast, with respect to blood flow, the literature study produced only sparse data for several tissues, and in these cases, linear regression was required to capture the entire elderly age range. On the basis of age-informed physiology, the predicted PK profiles described age-associated trends well. The root mean squared prediction error for the prediction of plasma concentrations of furosemide and morphine in the elderly were improved by 32 and 49 %, respectively, by use of age-informed physiology. The majority of the individual V_ss and t_½ values for the two model compounds, furosemide and morphine, were well predicted in the elderly population, except for long furosemide half-lifes.

Conclusion

The results of this study support the feasibility of using a knowledge-driven PBPK aging model that includes the elderly to predict PK alterations throughout the entire course of aging, and thus to optimize drug therapy in elderly individuals. These results indicate that pharmacotherapy and safety-related control of geriatric drug therapy regimens may be greatly facilitated by the information gained from PBPK predictions.

Electronic supplementary material

The online version of this article (doi:10.1007/s40262-016-0422-3) contains supplementary material, which is available to authorized users.

The diagnostic value of rescaled renal biomarkers serum creatinine and serum cystatin C and their relation with measured glomerular filtration rate

BACKGROUND

Serum creatinine (Scr) is the major contributing variable in glomerular filtration rate (GFR) estimating equations. Serum cystatin C (ScysC) based GFR estimating (eGFR)-equations have also been developed. The present study investigates the relation between 'rescaled' levels of these renal biomarkers (with reference interval of [0.67-1.33]) and measured GFR (mGFR).

METHODS

We evaluated the diagnostic ability to detect impaired kidney function of the rescaled renal biomarkers in 8584 subjects from 12 cohorts with measured GFR, standardized Scr and ScysC. We calculated sensitivity and specificity of the rescaled biomarkers to identify kidney disease, with reference to a fixed (60mL/min/1.73m²) as well as an age-dependent threshold for mGFR.

RESULTS

The upper reference limit of 1.33 for rescaled renal biomarkers is closely related to the age-dependent threshold for defining kidney status by mGFR with sensitivity and specificity for the rescaled biomarkers close to 90% for all ages. If the fixed threshold of 60mL/min/1.73m² for mGFR is used, then lower specificity in children and sensitivity in older adults are observed.

CONCLUSIONS

Impaired kidney function can be diagnosed by rescaled renal biomarkers instead of eGFR-equations using the fixed threshold of 1.33 for all ages, consistent with an age-dependent threshold of mGFR.

Sex differences in renin-angiotensin-aldosterone system affect extracellular volume in healthy subjects

Several studies reported sex differences in aldosterone. It is unknown whether these differences are associated with differences in volume regulation. Therefore we studied both aldosterone and extracellular volume in men and women on different sodium intakes. In healthy normotensive men ( n = 18) and premenopausal women ( n = 18) we investigated plasma aldosterone, blood pressure, and extracellular volume (¹²⁵I-iothalamate), during both low (target intake 50 mmol Na⁺/day) and high sodium intake (target intake 200 mmol Na⁺/day) in a crossover setup. Furthermore, we studied the adrenal response to angiotensin II infusion (0.3, 1.0, and 3.0 ng·kg^-1·min^-1 for 1 h) on both sodium intakes. Men had a significantly higher plasma aldosterone, extracellular volume, and systolic blood pressure than women during high sodium intake ( P < 0.05). During low sodium intake, extracellular volume and blood pressure were higher in men as well ( P < 0.05), whereas the difference in plasma aldosterone was no longer significant ( P = 0.252). The adrenal response to exogenous angiotensin II was significantly lower in men than in women on both sodium intakes. Constitutive sex differences in the regulation of aldosterone, characterized by a higher aldosterone and a lower adrenal response to exogenous angiotensin II infusion in men, are associated with a higher extracellular volume and blood pressure in men. These findings suggest that sex differences in the regulation of aldosterone contribute to differences in volume regulation between men and women.

Central obesity associates with renal hyperfiltration in the non-diabetic general population: a cross-sectional study

Background

Obesity is a risk factor for end-stage renal disease. Renal hyperfiltration, defined as an abnormally high glomerular filtration rate (GFR), is a link in the causal chain between diabetes and chronic kidney disease. Whether obesity is associated with hyperfiltration in the non-diabetic general population, remains unresolved due to a lack of consensus regarding the definition of hyperfiltration and the limited precision of high-range GFR estimations with creatinine and/or cystatin C.

Methods

1555 middle-aged participants without diabetes, renal or cardiovascular disease were enrolled from the general population in the Renal Iohexol Clearance Survey from the 6th Tromsø Study (RENIS-T6) between 2007 and 2009. Obesity was assessed using the body mass index (BMI), waist circumference (WC) and the waist-hip ratio (WHR). GFR was measured by iohexol clearance. Dichotomous variables for hyperfiltration were based on two alternative definitions using unadjusted GFR (mL/min) above the 90th percentile. The 90th percentile was age-, sex- and height-specific in one definition and age-, sex-, height- and weight-specific in the other.

Results

In multivariable adjusted logistic regression models, only WHR was consistently associated with hyperfiltration based on both definitions. For the definition based on the age-, sex-, height- and weight-specific 90th percentile, the association with the WHR (odds ratios (95 % confidence intervals)) for hyperfiltration was 1.48 (1.08–2.02) per 0.10 WHR increase.

Conclusions

Central obesity is associated with hyperfiltration in the general population. The WHR may serve as a better indicator of the renal effects of obesity than BMI or WC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12882-016-0386-4) contains supplementary material, which is available to authorized users.

Incorporating bolus and infusion pharmacokinetics into the ICING insulin model

The ICING model has been successfully used to guide clinical decisions on insulin administration in critical illness. However, insulin pharmacokinetics in the ICING model can be improved to better describe both intravenous (IV) bolus and infusion insulin administration. Patient data from 217 Dynamic Insulin Sensitivity and Secretion Tests (DISST) and 36 Intravenous Glucose Tolerance Tests (IVGTT) from independent dietary intervention studies was used to fit model parameters to a model structure that conforms to known behaviour. The DISST tests measured both endogenous and exogenous IV insulin bolus responses, while the IVGTT measured exogenous IV insulin infusion dynamics. Unidentifiable parameters were given physiologically justified values, with knowledge on relative insulin clearance rates used to constrain parameter values. The resulting whole-cohort description was able to simultaneously describe both IV bolus and infusion dynamics, and improves ICING model descriptive capability. Improved infusion dynamics will allow better description of subcutaneous insulin, the insulin administration route favoured in outpatient care of diabetes.

Normalization of glomerular filtration rate in obese children

BACKGROUND

Glomerular filtration rate (GFR) is conventionally indexed to body surface area (BSA), but this may lead to biased results when applied to subjects of abnormal body size. The aim of our study was to examine the impact of normalization to the BSA and alternative body size descriptors on measured and estimated GFR in overweight and obese children.

METHODS

This was a cross-sectional study of 313 children aged 8-9 years old. GFR was measured by 24-h creatinine clearance (CrCl) and additionally estimated from serum creatinine and cystatin C (CysC) using the combined Zappitelli formula, both as absolute values and adjusted to various body size descriptors. The results were compared between 163 normal-weight, 89 overweight and 61 obese children.

RESULTS

Compared to the normal-weight children, mean absolute GFR (both measured and estimated) was higher in the overweight and obese children, whereas BSA-adjusted GFR was lower. Linear regression models fitted in normal-weight children revealed equally close associations between absolute GFR and squared height, ideal body weight (IBW) and BSA derived from IBW. Normalization of GFR to the IBW-derived BSA completely eliminated the discrepancy between absolute and BSA-indexed GFR in overweight and obese children.

CONCLUSIONS

Indexing of GFR to BSA calculated from the ideal-rather than actual-body weight is a promising approach to avoid overcorrection when studying obese children. Further studies should assess the accuracy of this approach across the full range of age and BMI distribution.

Pitfalls and Limitations of Radionuclide Renal Imaging in Adults

To understand pitfalls and limitations in adult renography, it is necessary to understand firstly the physiology of the kidney, especially the magnitude and control of renal blood flow, glomerular filtration rate and tubular fluid flow rate, and secondly the pharmacokinetics and renal handling of the three most often used tracers, Tc-99m-mercaptoacetyltriglycine (MAG3), Tc-99m-diethylene triamine pentaacetic acid (DTPA) and Tc-99m-dimercaptosuccinic acid (DMSA). The kidneys may be imaged dynamically with Tc-99m-MAG3 or Tc-99m-DTPA, with or without diuretic challenge, or by static imaging with Tc-99m-DMSA. Protocols are different according to whether the kidney is native or transplanted. Quantitative analysis of dynamic data includes measurement of renal vascularity (important for the transplanted kidney), absolute tracer clearance rates, differential renal function (DRF) and response to diuretic challenge. Static image reveals functional renal parenchymal damage, both focal and global, is useful in the clinical management of obstructive uropathy, renal stone disease and hypertension (under angiotensin converting enzyme inhibition), and is the preferred technique for determining DRF. Diagnosis based on morphological appearances is important in transplant management. Even though nuclear medicine is now in the era of hybrid imaging, renal imaging remains an important subspecialty in nuclear medicine and requires a sound basing in applied physiology, the classical supporting discipline of nuclear medicine.

Removal of body surface area normalisation improves raw-measured glomerular filtration rate estimation by the Chronic Kidney Disease Epidemiology Collaboration equation and drug dosing in the obese

BACKGROUND/AIM

We aimed to compared estimated glomerular filtration rate (eGFR) according to the Chronic Kidney Disease Epidemiology Collaboration equation (CKD-EPI), with (mL/min/1.73 m(2) ) and without body surface area (BSA) normalisation (CKD-EPI_noBSA, mL/min) against measured (99m) Technetium - diethylenepentaacetic acid (Tc-DTPA GFR) (mL/min) in 222 individuals, including 80 with malignancy.

METHODS

BSA was calculated for each individual using the Du Bois equation. The CKD-EPI and CKD-EPI_noBSA equations were compared with measured Tc-DTPA GFR with respect to bias, proportion within 30% of GFR (P30) and root mean square error for predicting levels of GFR, and concordance in relation to carboplatin dosing.

RESULTS

The mean (SD) for BSA and measured GFR for the entire group was 1.99 (0.25) m(2) and 127 (41) mL/min respectively. The P30 for Tc-DTPA GFR was significantly higher with the CKD-EPI_noBSA (80%) than with the CKD-EPI equation (63%, P = 0.0001). In those with body mass index (BMI) > 30 kg/m(2) , the P30 values for the CKD-EPI_noBSA and CKD-EPI were 74% and 42% respectively (P < 0.0001). Carboplatin dosing concordance for the cancer patients using the CKD-EPI and CKD-EPI_noBSA equation was 71% and 56% respectively (P = 0.07). In 78 individuals with BMI > 30 kg/m(2) , concordance in relation to carboplatin dosing using CKD-EPI_noBSA was 65% compared with 26% with the CKD-EPI (P < 0.0001).

CONCLUSION

The CKD-EPI without normalisation (CKD-EPI_noBSA) equation was superior to the CKD-EPI equation in estimating raw-measured Tc-DTPA GFR (mL/min).

Prediction of human fetal pharmacokinetics using ex vivo human placenta perfusion studies and physiologically based models

AIMS

Pregnant women can be exposed to numerous drugs during the gestational period. For obvious ethical reasons, in vivo studies of fetal exposure to drugs are limited. Information about the transplacental transfer of drugs prior to their administration to pregnant women would be highly useful. In the present study, a novel approach was developed quantitatively predict or to predict the fetal exposure to drugs administered to the mother quantitatively.

METHODS

Transplacental parameters estimated from ex vivo human placenta perfusion experiments were implemented in pregnancy-physiologically based pharmacokinetic (p-PBPK) models in order to predict fetal PK. Thereafter, fetal PK profiles for two antiretroviral drugs, tenofovir (TFV) and emtricitabine (FTC) were simulated. These predictions were then compared to observed cord blood concentrations, to validate these models.

RESULTS

Parameters obtained from the ex vivo experiments enabled a good prediction of observed cord blood concentrations without additional a scaling factor. Moreover, a sensitivity analysis showed that fetal predictions were sensitive to changes in transplacental parameters values obtained ex vivo.

CONCLUSION

The integration of ex vivo human placental perfusion parameters in a p-PBPK model should be a promising new approach for predicting human fetal exposure to xenobiotics.

Association between ratio of measured extracellular volume to expected body fluid volume and renal outcomes in patients with chronic kidney disease: a retrospective single-center cohort study

Background

Excess extracellular volume is a major clinical problem in patients with chronic kidney disease (CKD). However, whether the extracellular volume status is associated with disease progression is unclear. We investigated the association between the extracellular volume status and renal outcomes.

Methods

We performed a retrospective cohort study of 149 patients with CKD who underwent bioelectrical impedance analysis (BIA) from 2005 to 2009. Patients were categorized according to tertiles of extracellular volume status. The extracellular volume status was assessed by examining the ratio of extracellular water measured by BIA (ECW_BIA) to the total body water calculated using the Watson formula (TBW_Watson). The main outcomes were adverse renal outcomes as defined by a decline of ≥50% from the baseline glomerular filtration rate or initiation of renal replacement therapy.

Results

A higher %ECW_BIA/TBW_Watson ratio tended to be associated with older age, male sex, diabetes mellitus, resistant hypertension, lower renal function, lower serum albumin levels, higher proteinuria levels, and a higher frequency of furosemide use. In the multivariate analysis, proteinuria remained independently associated with the %ECW_BIA/TBW_Watson ratio. Both the intracellular and extracellular water volumes decreased with age (correlation between ICW and age, r = -0.30, P < 0.001; correlation between ECW and age, r = -0.17, P = 0.03). Consequently, the %ECW_BIA in the body fluid composition increased with age. During a median follow-up of 4.9 years, patients in the highest tertile of the %ECW_BIA/TBW_Watson ratio were at greater risk of adverse renal outcomes (16.6 per 100.0 patient years) than were those in the lowest tertile (8.1 per 100.0 patient years) or second tertile (5.6 per 100.0 patient years) (log-rank P = 0.005). After adjustment for covariates, the %ECW_BIA/TBW_Watson ratio was significantly associated with adverse renal outcomes (hazard ratio, 1.21; 95 % confidence interval, 1.10–1.34; P < 0.001).

Conclusions

The ECW_BIA/TBW_Watson ratio was independently associated with adverse renal outcomes. Proteinuria was independently associated with the extracellular volume status. The balance between ICW and ECW changes with age in that the percentage of ECW content in the body fluid composition increases. Elderly patients with CKD may thus be susceptible to volume overload.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2369-15-189) contains supplementary material, which is available to authorized users.

Body mass index and body fat distribution as renal risk factors: a focus on the role of renal haemodynamics

Weight excess and/or central body fat distribution are associated with increased long-term renal risk, not only in subjects with renal disease or renal transplant recipients, but also in the general population. As the prevalence of weight excess is rising worldwide, this may become a main renal risk factor on a population basis, even more so because the risk extends to the overweight range. Understanding the mechanisms of this detrimental effect of weight excess on the kidneys is needed in order to design preventive treatment strategies. The increased risk associated with weight excess is partly attributed to associated comorbid conditions, such as hypertension, dyslipidaemia, insulin resistance and diabetes; however, current evidence supports a direct pathogenetic role for renal haemodynamics as well. Weight excess is associated with an altered renal haemodynamic profile, i.e. an increased glomerular filtration rate relative to effective renal plasma flow, resulting in an increased filtration fraction (FF). This renal haemodynamic profile is considered to reflect glomerular hyperfiltration and glomerular hypertension, resulting from a dysbalance between afferent and efferent arterial vasomotor balance. This unfavorable renal haemodynamic profile was found to be associated with renal outcome in experimental models and in human renal transplant recipients, and is associated with a blunted sodium excretion, and reversible by weight loss, renin-angiotensin-aldosterone system blockade or by dietary sodium restriction. More recent evidence showed that a central body fat distribution is also associated with an increased FF, even independent of overall weight excess. In this review, we provide an overview on current literature on the impact of weight excess and central body fat distribution on the renal haemodynamic profile in humans, and its possible role in progressive renal damage.

Evaluation of non-polynomial equations for one-compartment correction of slope-intercept GFR: theoretical prediction and experimental measurement

BACKGROUND

Polynomial equations for one-compartment correction of slope-intercept glomerular filtration rate (GFR) will underestimate values at high clearance rates. Non-polynomial correction equations that are independent of patient size and renal function would be advantageous and may have cross-species use.

MATERIALS AND METHODS

The study explored the theoretical basis of firstly the Jodal and Brochner-Mortensen one-compartment correction equation, replacing plasma volume with extracellular fluid volume, and secondly an equation described by Peters. One-compartment correction factors (a which is related to plasma volume and v which is related to extracellular fluid volume) which avoided the need for scaling to body size were developed. Both factors were determined from the biexponential clearance curve of the markers iohexol and (51)Cr-EDTA in humans and iohexol in cats and dogs. Relationships between a and v and filtration function and body size were then determined using data from humans, cats and dogs to assess their validity and compare this with theoretical predictions.

RESULTS

In all species, v was higher than a, as theoretically predicted. Both were significantly higher in humans than cats and dogs, ruling out cross-species use. Significant relationships were present between v and measures of filtration function in humans, but were weak with respect to a. Neither a nor v showed significant relationships with filtration function in animals or with body size in any species.

CONCLUSIONS

a and v (which are factors independent of body size) can be used interchangeably for correcting slope-intercept clearance. However values of both for humans are higher compared to cats and dogs. Therefore a single cross-species factor cannot be used.

Differential scaling of glomerular filtration rate and ingested metabolic burden: implications for gender differences in chronic kidney disease outcomes

BACKGROUND

Men commence dialysis with a higher estimated glomerular filtration rate (eGFR) than women and are more likely to transition from chronic kidney disease (CKD) to end-stage renal disease. We hypothesized that for a given estimated body surface area (BSA) men have a greater metabolic burden, and that consequently, the practice of indexing GFR to BSA results in gender differences in the degree of biochemical uraemia.

METHODS

Metabolic burden was assessed as estimated dietary protein, calorie, phosphorus, sodium and potassium intakes and urinary urea nitrogen excretion in the Chronic Renal Insufficiency Cohort, Modification of Diet in Renal Disease study, and National Health and Nutrition Examinations Surveys (NHANES) 1999-2010. Uraemia was characterized by serum biochemistry.

RESULTS

Per m(2) BSA, men had greater urea nitrogen excretion and intakes of all dietary parameters (P < 0.001 for all). For a given BSA-indexed iothalamate GFR or eGFR, male gender was associated with a 10-15% greater serum urea nitrogen (P < 0.001), giving men with a BSA-indexed GFR of 70-75 mL/min/1.73 m(2) the same serum urea nitrogen concentration as women with a GFR of 60 mL/min/1.73 m(2). However, indexing metabolic burden and GFR to alternative body size measures (estimated total body water, lean body mass or resting energy expenditure) abolished/reversed the gender associations. In NHANES, BSA-indexed eGFR distribution was very similar for men and women, so that adjusting for eGFR had little effect on the gender difference in serum urea.

CONCLUSIONS

Indexing GFR to BSA across genders may approximate nature's indexing approach, but gives men a greater ingested burden of protein, calories, sodium, phosphorus and potassium per mL/min GFR. This has implications for gender differences in CKD outcomes.

Effects of hydration on scintigraphic glomerular filtration rate measured using integral and plasma volume methods in dogs with suspected renal disease

The current standard scintigraphic method for estimating glomerular filtration rate (GFR) in dogs is the integral method, which normalizes renal GFR to body weight. The plasma volume method, that is normalizing GFR to plasma volume, has been reported to be more physiologically correct. The aim of this prospective study was to test the effect of hydration status on GFR measured by these two methods in a group of dogs with suspected renal disease. Eleven dogs were recruited. All dogs underwent standardized scintigraphic examinations before and after 15 ml/kg of fluid was administered intravenously at 5-7 ml/kg/min. Individual kidney GFR estimates (n = 22) were calculated using both methods and a consensus of two observers who were unaware of clinical findings. Individual kidney GFR increased significantly (P = 0.0008) after fluid administration using the integral method and individual kidney GFR using the plasma volume method remained constant. Percentage differences for individual kidney GFR before and after fluid administration were 31.4 ± 58.1% (change ± 95% CI) for the integral method and 0.1 ± 70% (change ± 95% CI) for the plasma volume method. Intravenously administered fluid increased individual kidney GFR from low to normal in 10 of 22 kidneys using the integral method and in 1 of 22 kidneys using the plasma volume method. Findings supported the use of the plasma volume method for scintigraphic calculation of GFR in dogs with suspected renal disease and indicated that errors of kidney status classification may more likely occur when the integral method is used.

The use of absolute values improves performance of estimation formulae: a retrospective cross sectional study

BACKGROUND

Estimation of Glomerular Filtration Rate (GFR) by equations such as Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) or Modification of Diet in Renal Disease (MDRD) is usually expressed as a Body Surface Area (BSA) indexed value (ml/min per 1.73 m²). This can have severe clinical consequences in patients with extreme body sizes, resulting in an underestimation in the case of obesity or an overestimation of GFR in the case of underweight patients. The aim of this study was to compare the performance of both estimation formula expressed in ml/min, instead of ml/min per 1.73 m², with a reference method.

METHODS

Retrospective single centre cross sectional study of 185 patients. GFR was measured with 51Cr-EDTA and estimated with CKD-EPI and MDRD. Bias, precision and accuracy of absolute estimated GFR was calculated.

RESULTS

Bias of CKD-EPI and MDRD formulae expressed as an absolute value was 0.49 and 0.27 ml/min respectively, which is lower than previously reported. Precision was 12.95 and 16.33 and accuracy expressed as P30 was over 92.43% for CKD-EPI. There were no significant differences in GFR between the reference method and the estimation formulae.

CONCLUSIONS

The performance of CKD-EPI and MDRD formulae can be significantly improved in the individual patient if the absolute values are used by removing the BSA normalization factor. Absolute estimated GFR by CKD-EPI is comparable to measured GFR, improving the performance of this formula in the assessment of individual kidney function, thus providing clinicians with an alternative to reference methods.

Higher extracellular fluid volume in women is concealed by scaling to body surface area

OBJECTIVE

The objective was to assess body surface area (BSA) for scaling extracellular fluid volume (ECV) in comparison with estimated lean body mass (LBM) and total body water (TBW) across a range of body mass indices (BMI).

METHODS

This was a multi-centre study from 15 centres that submitted raw data from routine measurement of GFR in potential kidney transplant donors. There were 819 men and 1059 women in total. ECV was calculated from slope-intercept and slope-only measurements of GFR. ECV was scaled using two methods: Firstly, division of ECV by the scaling variable (ratio method), and secondly the regression method of Turner and Reilly. Subjects were placed into five BMI groups: < 20, 20-24.9, 25-29.9, 30-34.9, and 35 + kg/m(2). LBM and TBW were estimated from previously published, gender-specific prediction equations.

RESULTS

Ratio and regression scaling gave almost identical results. ECV scaled to BSA by either method was higher in men in all BMI groups but ECV scaled to LBM and TBW was higher in women. There was, however, little difference between men and women in respect to ECV per unit weight in any BMI group, even though women have 10% more adipose tissue. The relations between TBW and BSA and between LBM and BSA, but not between LBM and TBW, were different between men and women.

CONCLUSION

Lean tissue in women contains more extracellular water than in men, a difference that is obscured by scaling to BSA. The likely problem with BSA is its insensitivity to body composition.

Simplified methods for assessment of renal function as the ratio of glomerular filtration rate to extracellular fluid volume

BACKGROUND

Instead of scaling glomerular filtration rate (GFR) to a body surface area of 1.73 m(2), it has been suggested to scale GFR to extracellular fluid volume (ECV). The ratio GFR/ECV has physiological meaning in that it indicates how often 'that which is to be regulated' (i.e. ECV) comes into contact with the 'regulator' (i.e. the kidneys).

AIM

The aim of the present study was as follows: to analyse two published calculation methods for determining ECV and GFR/ECV; to develop a new simple and accurate formula for determining ECV; and to compare and evaluate these methods.

MATERIALS AND METHODS

GFR was determined as (51)Cr-EDTA clearance. The study comprised 128 individuals (35 women, 66 men and 27 children) with a full (51)Cr-EDTA plasma concentration curve, determined from injection until 4-5 h p.i. Reference values for GFR and ECV were calculated from the full curve. One-pool approximations C/(1) and V(1) were calculated using only the final-slope curve. Four calculation methods were compared: simple one-pool values; GFR/ECV according to Peters and colleagues; ECV according to Brøchner-Mortensen (BM); and ECV according to a new method (JBM): y=2x-1, where x=Cl(1)/Cl and y=V(1)/ECV.

RESULTS

The new JBM method is accurate and can be explained theoretically. BM has a slight bias for high renal function. The Peters method had bias in our data. GFR/ECV had better precision than ECV alone, especially for BM and JBM, which were within -4% to +7% of the reference values (95% limits of agreement in adults).

CONCLUSION

GFR/ECV can be precisely determined, especially with the BM and JBM methods. Expressing GFR/ECV in unit %/h gives a simple interpretation. Normal ranges for GFR/ECV need to be established.

Early life influences kidney function at age 63-64 years, but so does adult body size: results from the newcastle thousand families birth cohort

BACKGROUND

It is suggested that impaired fetal growth can affect kidney development, resulting in fewer glomeruli being formed and reduced kidney function later in life. The aim of this study was to investigate early life variables in relation to adult kidney function, and compare these to the influence of later life variables.

METHODS

Detailed information was collected prospectively regarding 1,142 babies, born in 1947 in Newcastle upon Tyne. At the age of 63-64 years, 335 participants had serum creatinine successfully measured and completed a lifestyle questionnaire. These measurements were used to calculate their estimated glomerular filtration rate (eGFR).

RESULTS

Body mass index (BMI) and being female were significantly negatively associated with eGFR. Birth weight was significantly positively associated with eGFR. In sex-specific analyses, BMI and cigarette smoking remained significant for males (n = 154), with a near significant association for birth weight, whereas none of the variables remained significant for females (n = 181).

CONCLUSIONS

The findings suggest that sex, size at birth and BMI may be important variables influencing adult kidney function. However, as only a small amount of variance in eGFR was explained by these variables, additional longitudinal studies would be beneficial for assessing lifecourse influences on kidney function.

Subcutaneous interstitial pressure and volume characteristics in renal impairment associated with edema

The kidneys and the interstitial compartment play a vital role in body fluid regulation. The latter may be significantly altered in renal dysfunction, but experimental studies are lacking. To help define this we measured the subcutaneous interstitial pressure, bioimpedance volumes, and edema characteristics in 10 healthy subjects and 21 patients with obvious edema and chronic kidney disease (CKD). Interstitial edema was quantified by the time taken for a medial malleolar thumb pit to refill and termed the edema refill time. Interstitial pressure was significantly raised in CKD compared to healthy subjects. Total body water (TBW), extracellular fluid volume (ECFV), interstitial fluid volume, the ratio of the ECFV to the TBW, and segmental extracellular fluid volume were raised in CKD. The ratio of the ECFV to the TBW and the interstitial fluid volume were the best predictors of interstitial pressure. Significantly higher interstitial pressures were noted in edema of 2 weeks or less duration. A significant nonlinear relationship defined interstitial pressure and interstitial fluid volume. Edema refill time was significantly inversely related to interstitial pressure, interstitial compartment volumes, and edema vintage. Elevated interstitial pressure in CKD with obvious edema is a combined function of accumulated interstitial compartment fluid volumes, edema vintage, and tissue mechanical properties. The edema refill time may represent an important parameter in the clinical assessment of edema, providing additional information about interstitial pathophysiology in patients with CKD and fluid retention.

Glomerular filtration rate in prospective living kidney donors

PURPOSE

Measurements of glomerular filtration rate (GFR) are frequently interpreted assuming a linear variation with age. Nonlinear relationships may give a better representation of the changes associated with normal ageing.

METHODS

This was a retrospective study of 904 subjects (468 women, 436 men; age range 18-84 years) undergoing assessment as prospective living kidney donors. GFR was evaluated from (51)Cr-EDTA plasma clearance using blood samples taken at 2, 3 and 4 h. The slope-intercept GFR was corrected for body surface area (BSA) using the Haycock formula and for the fast exponential using the Brochner-Mortensen equation. The relationship between age, gender and GFR was examined using best-fit curve analysis. Nonlinear relationships with age were explored using fractional polynomials.

RESULTS

There was no gender difference in BSA-corrected GFR over five decades of age (P = 0.40). However, female donors with a body mass index >30 kg/m(2) had a statistically significantly lower GFR than nonobese women (P < 0.01). The best-fit relationship between age and GFR was nonlinear and described using a fractional polynomial model of degree 1 (GFR = 103.9-0.0061 × Age(2) mL/min/1.73 m(2)) with a root mean standard error of 12.9 mL/min/1.73 m(2). The residual variance for this model was significantly smaller than for the best-fit linear model (P = 0.006).

CONCLUSIONS

GFR measurements in prospective living kidney donors are best corrected for age using a nonlinear relationship.

Reference values for glomerular filtration rate in healthy Brazilian adults

BACKGROUND

Glomerular filtration rate (GFR) is the best index of renal function, but age, gender and ethnicity can putatively affect its values. The aim of this study was to establish reference values for GFR in healthy Brazilian subjects while taking these factors into account.

METHODS

In this cross-sectional study, GFR was measured by the 51Cr-EDTA single-injection method. GFR reference values were developed according to CLSI Guidelines for Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory (CLSI C28 protocol).

RESULTS

The age range of the 285 healthy individuals was 19 to 70  years, 57% were females, and GFR was 106 ± 18 mL/min/1.73 m(2). There was no difference between male and female GFRs (108 ± 18 vs. 104 ± 18 mL/min/1.73 m(2) respectively, P = 0.134), and reference values were therefore developed from the pooled sample. GFR values were lower in subjects aged ≥45 years as compared with those younger than 45 years (98 ± 15 vs.112 ± 18 mL/min/1.73 m(2), P < 0.001). Based on mean ± 2 SD, GFR reference values were 76 to 148 mL/min/1.73 m(2) for subjects younger than 45 years and 68 to 128 mL/min/1.73 m(2) for individuals older than 45 years, irrespective of gender.

CONCLUSION

The age-adjusted reference intervals reported may be reliably adopted to evaluate kidney function, since they are based on recommended standards.

Dialysis dose scaled to body surface area and size-adjusted, sex-specific patient mortality

BACKGROUND AND OBJECTIVES

When hemodialysis dose is scaled to body water (V), women typically receive a greater dose than men, but their survival is not better given a similar dose. This study sought to determine whether rescaling dose to body surface area (SA) might reveal different associations among dose, sex, and mortality.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Single-pool Kt/V (spKt/V), equilibrated Kt/V, and standard Kt/V (stdKt/V) were computed using urea kinetic modeling on a prevalent cohort of 7229 patients undergoing thrice-weekly hemodialysis. Data were obtained from the Centers for Medicare & Medicaid Services 2008 ESRD Clinical Performance Measures Project. SA-normalized stdKt/V (SAN-stdKt/V) was calculated as stdKt/V × ratio of anthropometric volume to SA/17.5. Patients were grouped into sex-specific dose quintiles (reference: quintile 1 for men). Adjusted hazard ratios (HRs) for 1-year mortality were calculated using Cox regression.

RESULTS

spKt/V was higher in women (1.7 ± 0.3) than in men (1.5 ± 0.2; P<0.001), but SAN-stdKt/V was lower (women: 2.3 ± 0.2; men: 2.5 ± 0.3; P<0.001). For both sexes, mortality decreased as spKt/V increased, until spKt/V was 1.6-1.7 (quintile 4 for men: HR, 0.62; quintile 3 for women: HR, 0.64); no benefit was observed with higher spKt/V. HR for mortality decreased further at higher SAN-stdKt/V in both sexes (quintile 5 for men: HR, 0.69; quintile 5 for women: HR, 0.60).

CONCLUSIONS

SA-based dialysis dose results in dose-mortality relationships substantially different from those with volume-based dosing. SAN-stdKt/V analyses suggest women may be relatively underdosed when treated by V-based dosing. SAN-stdKt/V as a measure for dialysis dose may warrant further study.

Estimation of extracellular fluid volume in children

BACKGROUND

Many equations have been developed to estimate various body fluid volumes from height and weight, but few have been developed for children. The aim of this study was to compare four height/weight formulae for estimating extracellular fluid volume (eECV) in children against measured extracellular fluid volume (mECV).

METHODS

The mECV was obtained from plasma Cr-51-EDTA clearance data used for routine measurement of glomerular filtration rate (GFR) in two groups of children (n=182 and 69, respectively). eECV obtained using the formulae of Abraham et al. (Clin J Am Assoc Nephrol 6:741-747, 2011) and Friis-Hansen (Pediatrics 28:169-181, 1961) were compared with mECV in both patient groups. The formulae of Bird et al. (J Nucl Med 44:1037-1043, 2003) and of Peters (Nucl Med Commun 32:375-380, 2011) were originally based on groups 1 and 2, respectively, so the eECV from them was compared with the mECV in groups 2 and 1, respectively.

RESULTS

The eECV from the Friis-Hansen formula underestimated the mECV in larger children. Biases (mean differences between eECV and mECV) from the Bird (0.146 l) and Peters (0.029 l) formulae were not significantly different from zero, but those from the Abraham formula was higher than zero (0.694 and 0.588 l in groups 1 and 2; p<0.001). Precisions (standard deviations of the biases) of these three formulae were similar, ranging from 0.731 l (Peters) to 0.878 l (Abraham, group 2; p>0.1).

CONCLUSION

The formulae of Bird, Peters and Abraham have similar precisions. The higher bias of the Abraham formula is probably due to the higher values of mECV on which their formula was based. The Friis-Hansen formula no longer has a place.