A multicentric evaluation of IDMS-traceable creatinine enzymatic assays

The global burden of chronic kidney disease: estimates, variability and pitfalls

Chronic kidney disease (CKD) is currently defined by abnormalities of kidney structure or function assessed using a matrix of variables - including glomerular filtration rate (GFR), thresholds of albuminuria and duration of injury - and is considered by many to be a common disorder globally. However, estimates of CKD prevalence vary widely, both within and between countries. The reasons for these variations are manifold, and include true regional differences in CKD prevalence, vagaries of using estimated GFR (eGFR) for identifying CKD, issues relating to the use of set GFR thresholds to define CKD in elderly populations, and concerns regarding the use of one-off testing for assessment of eGFR or albuminuria to define the prevalence of CKD in large-scale epidemiological studies. Although CKD is common, the suggestion that its prevalence is increasing in many countries might not be correct. Here, we discuss the possible origins of differences in estimates of CKD prevalence, and present possible solutions for tackling the factors responsible for the reported variations in GFR measurements. The strategies we discuss include approaches to improve testing methodologies for more accurate assessment of GFR, to improve awareness of factors that can alter GFR readouts, and to more accurately stage CKD in certain populations, including the elderly.

[Inadequacy of the African-American ethnic factor to estimate glomerular filtration rate in an African general population: Results from Côte d'Ivoire]

Glomerular filtration rate (GFR) estimation is fundamental in clinical nephrology. It is usually estimated from equations based on serum creatinine. An ethnic factor is currently recommended for the black population for the two most used equations, i.e. the Chronic Kidney Disease-Epidemiology Collaboration (CKD-EPI) and Modification of Diet in Renal Disease (MDRD) study equations. However, these factors were determined from African-American subjects. Therefore, their use in the African subject (non-American) remained questionable. To date, no data are available in West Africa for the adequacy of these ethnic coefficients, as compared with a measurement of GFR by a reference method. One hundred and twenty subjects of the general population, with no nephrologic history, were included in the study (60 women and 60 men). GFR was determined by a reference method, i.e. the plasma clearance of iohexol. The performance (bias, standard deviation, accuracy within 30%) of both CKD-EPI and MDRD study equations were assessed with and without the ethnic factors. GFR measurements (mGFR) according iohexol reference method were 100±19mL/min/1.73m2. The MDRD study equation without any ethnic factor underestimates mGFR by -9±16mL/min/1.73m2, whereas the MDRD study equation with the ethnic factor overestimates mGFR by +10±18mL/min/1.73m2. Regarding the CKD-EPI equation, bias and accuracy within 30% are significantly better without than with the ethnic factor. Indeed, bias is 16±2mL/min/1.73m2 and 18±17mL/min/1.73m2 and accuracy is 93% and 76%, without and with the ethnic factor, respectively (P<0.0001). We show for the first time in African population that the performance of CKD-EPI and MDRD study equations is significantly better, in a general population, without the "African-American" ethnic factor. The "African-American" ethnic factor should not be applied in West Africa.

Iohexol plasma clearance for measuring glomerular filtration rate in clinical practice and research: a review. Part 1: How to measure glomerular filtration rate with iohexol?

While there is general agreement on the necessity to measure glomerular filtration rate (GFR) in many clinical situations, there is less agreement on the best method to achieve this purpose. As the gold standard method for GFR determination, urinary (or renal) clearance of inulin, fades into the background due to inconvenience and high cost, a diversity of filtration markers and protocols compete to replace it. In this review, we suggest that iohexol, a non-ionic contrast agent, is most suited to replace inulin as the marker of choice for GFR determination. Iohexol comes very close to fulfilling all requirements for an ideal GFR marker in terms of low extra-renal excretion, low protein binding and in being neither secreted nor reabsorbed by the kidney. In addition, iohexol is virtually non-toxic and carries a low cost. As iohexol is stable in plasma, administration and sample analysis can be separated in both space and time, allowing access to GFR determination across different settings. An external proficiency programme operated by Equalis AB, Sweden, exists for iohexol, facilitating interlaboratory comparison of results. Plasma clearance measurement is the protocol of choice as it combines a reliable GFR determination with convenience for the patient. Single-sample protocols dominate, but multiple-sample protocols may be more accurate in specific situations. In low GFRs one or more late samples should be included to improve accuracy. In patients with large oedema or ascites, urinary clearance protocols should be employed. In conclusion, plasma clearance of iohexol may well be the best candidate for a common GFR determination method.

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).

Cystatin C standardization decreases assay variation and improves assessment of glomerular filtration rate

BACKGROUND

Cystatin C is increasingly used in glomerular filtration rate (GFR) estimation equations. The dependence of cystatin C results upon the analytical method has been a major source of controversy.

METHODS

Cystatin C was measured with non-standardized turbidimetric Roche Generation 1 and standardized nephelometric Siemens assays in 3666 and additionally with standardized Roche Generation 2 and Siemens in 567 blood samples of the Berlin Initiative Study. Cystatin C-based GFR was assessed with CKD-EPIcys (Chronic Kidney Disease Epidemiology) and CAPA (Caucasian, Asian, Pediatric, Adult) equations and the impact of the assays on GFR estimation was determined. Equation performance compared to measured GFR was evaluated.

RESULTS

Concordance of Roche Gen2 and Siemens was high with median difference of 0.003 ± 0.13 mg/L (limits of agreement: -0.12 to 0.12) and Passing Bablok correlation was essentially perfect. Roche Gen1 assay showed worse concordance with Siemens: median difference was 0.08 ± 0.13 mg/L (limits of agreement: -0.18 to 0.34) and correlation was inferior. Mean difference (± SD) of estimated GFRCKD-EPIcys was 0 ± 4 mL/min/1.73 m(2) for Gen2 and Siemens compared to -5 ± 8 with Gen1. Performance of GFR estimating equations was not influenced by the choice of Siemens or Gen2 assays.

CONCLUSIONS

Standardization of Roche Gen2 assay improved accuracy of cystatin C measurement compared to Siemens. It suggests only negligible method bias and results in equal performance of both assays when estimating GFR indicating that successful calibration has led to major progress in cystatin C analysis.

An estimated glomerular filtration rate equation for the full age spectrum

BACKGROUND

Glomerular filtration rate (GFR) is accepted as the best indicator of kidney function and is commonly estimated from serum creatinine (SCr)-based equations. Separate equations have been developed for children (Schwartz equation), younger and middle-age adults [Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation] and older adults [Berlin Initiative Study 1 (BIS1) equation], and these equations lack continuity with ageing. We developed and validated an equation for estimating the glomerular filtration rate that can be used across the full age spectrum (FAS).

METHODS

The new FAS equation is based on normalized serum creatinine (SCr/Q), where Q is the median SCr from healthy populations to account for age and sex. Coefficients for the equation are mathematically obtained by requiring continuity during the paediatric-adult and adult-elderly transition. Research studies containing a total of 6870 healthy and kidney-diseased white individuals, including 735 children, <18 years of age, 4371 adults, between 18 and 70 years of age, and 1764 older adults, ≥70 years of age with measured GFR (inulin, iohexol and iothalamate clearance) and isotope dilution mass spectrometry-equivalent SCr, were used for the validation. Bias, precision and accuracy (P30) were evaluated.

RESULTS

The FAS equation was less biased [-1.7 (95% CI -3.4, -0.2) versus 6.0 (4.5, 7.5)] and more accurate [87.5% (85.1, 89.9) versus 83.8% (81.1, 86.5)] than the Schwartz equation for children and adolescents; less biased [5.0 (4.5, 5.5) versus 6.3 (5.9, 6.8)] and as accurate [81.6% (80.4, 82.7) versus 81.9% (80.7, 83.0)] as the CKD-EPI equation for young and middle-age adults; and less biased [-1.1 (-1.6, -0.6) versus 5.6 (5.1, 6.2)] and more accurate [86.1% (84.4, 87.7) versus 81.8% (79.7, 84.0)] than CKD-EPI for older adults.

CONCLUSIONS

The FAS equation has improved validity and continuity across the full age-spectrum and overcomes the problem of implausible eGFR changes in patients which would otherwise occur when switching between more age-specific equations.

Impact of sleep disturbances on kidney function decline in the elderly

While sleep disturbances are frequent in renal disease patients, no studies have examined prospectively the associations between sleep disturbances and kidney function decline in community-dwelling elderly subjects.

Glomerular filtration rates (eGFRs) were estimated at baseline and at 11-year follow-up. A glomerular filtration decline over the follow-up period was defined as a percentage decline greater than or equal to the cut-off value of the highest tertile of kidney function decline (22%) in 1105 subjects. Excessive daytime sleepiness (EDS) and insomnia complaints were self-rated at baseline. Restless legs syndrome (RLS) and its age at onset were assessed at study end-point. An ambulatory polysomnography recording was performed during the follow-up in 277 subjects. Apnoea-hypopnoea index (AHI), periodic limb movements during sleep (PLMS) and total sleep time were analysed.

An increased risk of eGFR decline was associated with EDS (OR 1.67, 95% CI 1.18–2.34) and RLS (OR 1.98, 95% CI 1.18–3.30) independently of potential confounders including cardiovascular risk factors. Among insomnia complaints, a borderline association with eGFR decline was found for early morning awakening only. High AHI (≥30 events·h−1) and short total sleep time (<6 h), but not PLMS were linked to eGFR decline in crude associations, but only AHI remained significantly associated after multi-adjustments.

EDS, RLS and AHI constitute independent risk factors for kidney glomerular function decline.

[Dosing adjustment and renal function: Which equation(s)?]

While the CKD-EPI (for Chronic Kidney Disease Epidemiology) equation is now implemented worldwide, utilization of the Cockcroft formula is still advocated by some physicians for drug dosage adjustment. Justifications for this recommendation are that the Cockcroft formula was preferentially used to determine dose adjustments according to renal function during the development of many drugs, better predicts drugs-related adverse events and decreases the risk of drug overexposure in the elderly. In this opinion paper, we discuss the weaknesses of the rationale supporting the Cockcroft formula and endorse the French HAS (Haute Autorité de santé) recommendation regarding the preferential use of the CKD-EPI equation. When glomerular filtration rate (GFR) is estimated in order to adjust drug dosage, the CKD-EPI value should be re-expressed for the individual body surface area (BSA). Given the difficulty to accurately estimate GFR in the elderly and in individuals with extra-normal BSA, we recommend to prescribe in priority monitorable drugs in those populations or to determine their "true" GFR using a direct measurement method.

Development and validation of an ultra-high performance liquid chromatography-tandem mass spectrometry method to measure creatinine in human urine

Despite decades of creatinine measurement in biological fluids using a large variety of analytical methods, an accurate determination of this compound remains challenging. Especially with the novel trend to assess biomarkers on large sample sets preserved in biobanks, a simple and fast method that could cope with both a high sample throughput and a low volume of sample is still of interest. In answer to these challenges, a fast and accurate ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method was developed to measure creatinine in small volumes of human urine. In this method, urine samples are simply diluted with a basic mobile phase and injected directly under positive electrospray ionization (ESI) conditions, without further purification steps. The combination of an important diluting factor (10(4) times) due to the use of a very sensitive triple quadrupole mass spectrometer (XEVO TQ) and the addition of creatinine-d3 as internal standard completely eliminates matrix effects coming from the urine. The method was validated in-house in 2012 according to the EMA guideline on bioanalytical method validation using Certified Reference samples from the German External Quality Assessment Scheme (G-Equas) proficiency test. All obtained results for accuracy and recovery are within the authorized tolerance ranges defined by G-Equas. The method is linear between 0 and 5 g/L, with LOD and LOQ of 5 × 10(-3) g/L and 10(-2) g/L, respectively. The repeatability (CV(r) = 1.03-2.07%) and intra-laboratory reproducibility (CV(RW) = 1.97-2.40%) satisfy the EMA 2012 guideline. The validated method was firstly applied to perform the German G-Equas proficiency test rounds 51 and 53, in 2013 and 2014, respectively. The obtained results were again all within the accepted tolerance ranges and very close to the reference values defined by the organizers of the proficiency test scheme, demonstrating an excellent accuracy of the developed method. The method was finally applied to measure the creatinine concentration in 210 urine samples, coming from 190 patients with a chronic kidney disease (CKD) and 20 healthy subjects. The obtained creatinine concentrations (ranging from 0.12 g/L up to 3.84 g/L) were compared, by means of a Passing Bablok regression, with the creatinine contents obtained for the same samples measured using a traditional compensated Jaffé method. The UHPLC-MS/MS method described in this paper can be used to normalize the concentration of biomarkers in urine for the extent of dilution.

Routine serum creatinine measurements: how well do we perform?

BACKGROUND

The first aim of the study was to investigate the accuracy and intra-laboratory variation of serum creatinine measurements in clinical laboratories in Flanders. The second purpose was to check the effect of this variation in serum creatinine concentration results on the calculated estimated glomerular filtration rate (eGFR) and the impact on classification of patients into a chronic kidney disease (CKD) stage.

METHODS

26 routine instruments were included, representing 13 different types of analyzers from 6 manufacturers and covering all current methodologies (Jaffe, compensated Jaffe, enzymatic liquid and dry chemistry methods). Target values of five serum pools (creatinine concentrations ranging from 35 to 934 μmol/L) were assigned by the gold standard method (ID-GC/MS).

RESULTS

Intra-run CV (%) (n = 5) and bias (%) from the target values were higher for low creatinine concentrations. Especially Jaffe and enzymatic dry chemistry methods showed a higher error. The calculated eGFR values corresponding with the reported creatinine concentration ranges resulted in a different CKD classification in 47% of cases.

CONCLUSIONS

Although most creatinine assays claim to be traceable to the gold standard (ID-GC/MS), large inter-assay differences still exist. The inaccuracy in the lower concentration range is of particular concern and may lead to clinical misinterpretation when the creatinine-based eGFR of the patient is used for CKD staging. Further research to improve harmonization between methods is required.

How to assess kidney function in outpatient clinics

BACKGROUND

In 2002, a new definition and classification of chronic kidney disease was published, and glomerular filtration rate < 60 ml/min/1.73 m(2) for 3 months or more was adapted to define chronic kidney disease irrespective of other signs of kidney damage.

AIMS

To discuss different ways to assess kidney function in outpatient clinics and especially in primary care.

METHODS

The PubMed database was searched for relevant articles.

RESULTS

The estimated glomerular filtration rate equations which take into account plasma creatinine, age, sex, race and body size have been developed to identify patients with chronic kidney disease formerly overlooked if the renal function had been assessed by plasma creatinine alone. Cystatin C-based equations have also been developed to enhance accuracy for individuals with whom creatinine-based estimates for kidney function are acknowledged to be less accurate.

DISCUSSION

The characteristics of the patients to whom the diagnostic test is applied can influence the sensitivity of the test. Thus, there is nowadays controversy over the best method to assess kidney function in general population.

CONCLUSION

In the overwhelming majority of patients currently treated in primary care, the CKD-EPI creatinine equation is suitable for estimating renal function. The CKD-EPIcr-cys equation would provide further reliability in individuals with a CKD-EPI creatinine eGFR of 45-59 ml/min/1.73 m(2) , but the cost of serum cystatin C analysis limits its use in everyday general practice.

Value of serum cystatin C in estimating renal function in children with non-renal solid organ transplantation

Children with non-renal solid organ transplants are surviving longer, but outcome is complicated by CKD. Accurate and frequent renal function monitoring is imperative to recognize and institute measures early to reverse, prevent, or arrest progression. This study of 59 children determined the accuracy (P30), bias, sensitivity and specificity between measured renal function by NM-GFR, and estimated GFR by three formulas: Filler (serum cystatin C), mSchwartz (serum creatinine), and CKiD (serum cystatin C, creatinine, urea, and height). Mean GFR by all formulas differed significantly from NM-GFR. Filler and mSchwartz formulas significantly increased the proportion of patients with GFR ≥ 90 mL/min/1.73 m(2) (CKD stage 1) while decreasing those with GFR 60-89 mL/min/1.73 m(2) (CKD stage 2). All formulas overestimated GFR. CKiD showed the highest P30 and lowest bias (79.7%; 6.9 mL/min/1.73 m(2) ) followed by Filler (67.7%; 19.9 mL/min/1.73 m(2) ) and Schwartz (57.6%; 26.8 mL/min/1.73 m(2) ) for all GFR values. All formulas performed best with GFR ≥ 90 mL/min/1.73 m(2) , but CKiD was the only formula to achieve 91.1% accuracy. All formulas showed high sensitivities, but low specificities at NM-GFR cutoff at 90. Thus, GFR estimated by CKiD followed by Filler formula is an adequate method to monitor renal function closely and frequently in these children.

Comparative performances of the new chronic kidney disease epidemiology equations incorporating cystatin C for use in cancer patients

AIM

In cancer patients receiving chemotherapy treatment, accurate assessment of kidney function is required. The aim of our study was to investigate whether the inclusion of cystatin C together with creatinine in prediction equations would improve the prediction of glomerular filtration rate (GFR).

METHODS

Plasma creatinine and cystatin C were analyzed in 155 patients (cancer, n = 80, kidney donors, n = 75) undergoing (99m) Technetium diethylenepentaacetic (Tc-DTPA) GFR clearance. Equations by the CKD-EPI (chronic kidney disease epidemiology) group (creatinine-, creatinine + cystatin C-, cystatin C-based, respectively) and Cockcroft-Gault were compared with Tc-DTPA GFR by difference plots, receiver operator characteristics curve analysis, root mean square error, chi-squared analysis and percentage concordance according to carboplatin dosage. Comparisons between two creatinine methodologies (enzymatic vs Jaffe) were also performed.

RESULTS

In the overall group, the combination creatinine and cystatin C equation had 69% of results within 20% of GFR (P20), a sensitivity of 86.3% and a specificity of 73.1% to detect reduced GFR at <90 mL/min/1.73 m(2), and a concordance of 78%. In contrast, the traditional Cockcroft-Gault equation had a P20 of 38.0%, with a large underestimation to predict GFR, thereby accounting for approximately 45% of dosing discordance. No obvious differences were obtained when comparing the performance of equations using the two creatinine methodologies.

CONCLUSION

The inclusion of cystatin C in the CKD-EPI equations improved the prediction of kidney function in the overall population, although probably not sufficiently for it to be favored over radioisotopic GFR for guiding chemotherapy. More research is warranted to further improve estimated GFR equations for these purposes.

Determination of Cystatin C in human serum by isotope dilution mass spectrometry using mass overlapping peptides

We propose a peptide-based isotope dilution mass spectrometry approach for Cystatin C determination in human serum samples, a clinical marker for renal status for which backup by a mass spectrometry based primary method has been missing so far. In contrast to common protocols, the isotope labelled version of the proteotypic signature peptide is designed such as keeping the isotopic difference as little as possible with respect to the peptide released from the protein. Peptides labelled in two (13)C atoms are added to the serum samples just before proteolysis. After two steps of chromatographic purification the sample is measured by selected reaction monitoring using a LC-MS/MS. Resolution of the first quadrupole is reduced to transmit the whole parent ion cluster to the collision cell for monitoring accurate isotopic distributions of the molecular fragments. Molar fractions of labelled and natural abundance peptides are directly obtained from the experimental mass spectra of the in-cell fragment ions. Thus, the natural abundance protein concentration is obtained from the fragment-ion spectrum of the sample without resorting to extra calibration runs. Applicability of the approach is demonstrated by the measurement of the serum concentration of Cystatin C in Reference Material ERM R-DA471/IFCC and real samples.

BIOLOGICAL SIGNIFICANCE

Cystatin C is used as an alternative marker instead of, or in combination with creatinine for non-invasive determination of glomerular filtration rates. Advantages advocating in favour of Cystatin C in diagnosis of chronic kidney diseases are the lower variability of its serum level and, particularly, virtual independence on sex, age and muscle mass. However, in order to capitalize, accuracy of measurement has to be in proportion with the predictive power of the marker. Though there are label-free methods available for screening purposes or high-throughput analysis, achieving high levels of reliability and accuracy in quantitative proteomics takes reference to isotope labelled materials. Present routine assays (mainly nephelometry, turbidimetry and ligand-binding assays) are known to leave improvement to be desired in that respect. Absolute quantification based on enzymatic signature-peptides provides a method principle establishing traceability to the International System of Units on the level of primary methods. The kind of technique is capable, by this way, of high accuracy value-assignment to matrix materials needed for calibration of present routine assays, where not completely replacing them. Cystatin C measurement by isotope dilution mass spectrometry is developed in this study with the aim of making available this tool to support diagnostics of kidney function in the same way.

A temporary loop ileostomy affects renal function

PURPOSE

The purpose of this study was to evaluate renal morbidity after a temporary loop ileostomy and to identify possible preoperative risk factors.

METHOD

Consecutive patients at four hospitals serving 1,520,000 inhabitants who received a temporary loop ileostomy and underwent subsequent closure were identified and retrospectively studied from 1 January 2007 until 28 February 2010. Serum creatinine levels were obtained 1 week before index surgery and 1 week before closure of the loop ileostomy. Estimated glomerular filtration rate (eGFR) was calculated using the CKD-EPI formula.

RESULTS

Three hundred eight patients with median age of 64 were identified. The indication for the loop ileostomy was colorectal cancer (226), inflammatory bowel disease (41), diverticulosis (8), and other conditions (33). Median time until closure was 161 days (3-873). There was a decrease in eGFR at time of closure (89 vs. 83; p < 0.0001), and the number of patients with renal impairment (eGFR <60) increased (7.5 vs. 21 %, p < 0.0001). Preoperative risk factors for eGFR <60 at closure were age and hypertension.

CONCLUSIONS

This study found that a loop ileostomy is associated with a reduced renal function for most patients, especially for older and hypertensive patients. This should be considered before constructing a loop ileostomy, and perhaps another stoma should be chosen if possible in patients at risk. Evaluation of medications before discharge and early and frequent postoperative follow-up could also reduce the risk of a reduced renal function.

Calibration and precision of serum creatinine and plasma cystatin C measurement: impact on the estimation of glomerular filtration rate

Serum creatinine (SCr) is the main variable for estimating glomerular filtration rate (GFR). Due to inter-assay differences, the prevalence of chronic kidney disease (CKD) varies according to the assay used, and calibration standardization is necessary. For SCr, isotope dilution mass spectrometry (IDMS) is the gold standard. Systematic differences are observed between Jaffe and enzymatic methods. Manufacturers subtract 0.30 mg/dl from Jaffe results to match enzymatic results ('compensated Jaffe method'). The analytical performance of enzymatic methods is superior to that of Jaffe methods. In the original Modification of Diet in Renal Disease (MDRD) equation, SCr was measured by a Jaffe Beckman assay, which was later recalibrated. A limitation of this equation was an underestimation of GFR in the high range. The Chronic Kidney Disease Epidemiology (CKD-EPI) consortium proposed an equation using calibrated and IDMS traceable SCr. The gain in performance was due to improving the bias whereas the precision was comparable. The CKD-EPI equation performs better at high GFR levels (GFR >60 ml/min/1.73 m(2)). Analytical limitations have led to the recommendation to give a grade (>60 ml/min/1.73 m(2)) rather than an absolute value with the MDRD equation. By using both enzymatic and calibrated methods, this cutoff-grade could be increased to 90 ml/min/1.73 m(2) (with MDRD) and 120 ml/min/1.73 m(2) (with CKD-EPI). The superiority of the CKD-EPI equation over MDRD is analytical, but the precision gain is limited. IDMS traceable enzymatic methods have been used in the development of the Lund-Malmö (in CKD populations) and Berlin Initiative Study equations (in the elderly). The analytical errors for cystatin C are grossly comparable to issues found with SCr. Standardization is available since 2011. A reference method for cystatin C is still lacking. Equations based on standardized cystatin C or cystatin C and creatinine have been proposed. The better performance of these equations (especially the combined CKD-EPI equation) has been demonstrated.

Height-independent estimation of glomerular filtration rate in children: an alternative to the Schwartz equation

OBJECTIVE

To compare the diagnostic performance of 2 height-independent equations used to calculate estimated glomerular filtration rate (eGFR), those of Pottel (eGFR-Pottel) and the British Columbia Children's Hospital (BCCH) (eGFR-BCCH), with the commonly used Schwartz equation (eGFR-Schwartz).

STUDY DESIGN

We externally validated eGFR-Pottel and eGFR-BCCH in a well-characterized pediatric patient population (n = 152) and compared their diagnostic performance with that of eGFR-Schwartz using Bland-Altman analysis. All patients underwent glomerular filtration rate measurement using the gold standard single-injection inulin clearance method (GFR-inulin).

RESULTS

Median GFR-inulin was 92.0 mL/min/1.73 m² (IQR, 76.1-107.4 mL/min/1.73 m²). Compared with GFR-inulin, the mean bias for eGFR-Schwartz was -10.1 mL/min/1.73 m(2) (95% limits of agreement [LOA], -77.5 to 57.2 mL/min/1.73 m(2)), compared with -12.3 mL/min/1.73 m² (95% LOA, -72.6 to 47.9 mL/min/1.73 m(2)) for eGFR-Pottel and -22.1 mL/min/1.73 m² (95% LOA, -105.0 to 60.8 mL/min/1.73 m(2)) for eGFR-BCCH. eGFR-Pottel showed comparable accuracy to eGFR-Schwartz, with 77% and 76% of estimates within 30% of GFR-inulin, respectively. eGFR-BCCH was less accurate than eGFR-Schwartz (66% of estimates within 30% of GFR-inulin; P < .01).

CONCLUSION

The performance of eGFR-Pottel is superior to that of eGFR-BCCH and comparable with that of eGFR-Schwartz. eGFR-Pottel is a valid alternative to eGFR-Schwartz in children and could be reported by the laboratory if height data are not available.

Enzymatic creatinine assays allow estimation of glomerular filtration rate in stages 1 and 2 chronic kidney disease using CKD-EPI equation

The National Kidney Disease Education Program group demonstrated that MDRD equation is sensitive to creatinine measurement error, particularly at higher glomerular filtration rates. Thus, MDRD-based eGFR above 60 mL/min/1.73 m² should not be reported numerically. However, little is known about the impact of analytical error on CKD-EPI-based estimates. This study aimed at assessing the impact of analytical characteristics (bias and imprecision) of 12 enzymatic and 4 compensated Jaffe previously characterized creatinine assays on MDRD and CKD-EPI eGFR. In a simulation study, the impact of analytical error was assessed on a hospital population of 24084 patients. Ability using each assay to correctly classify patients according to chronic kidney disease (CKD) stages was evaluated. For eGFR between 60 and 90 mL/min/1.73 m², both equations were sensitive to analytical error. Compensated Jaffe assays displayed high bias in this range and led to poorer sensitivity/specificity for classification according to CKD stages than enzymatic assays. As compared to MDRD equation, CKD-EPI equation decreases impact of analytical error in creatinine measurement above 90 mL/min/1.73 m². Compensated Jaffe creatinine assays lead to important errors in eGFR and should be avoided. Accurate enzymatic assays allow estimation of eGFR until 90 mL/min/1.73 m² with MDRD and 120 mL/min/1.73 m² with CKD-EPI equation.

MDRD versus CKD-EPI equation to estimate glomerular filtration rate in kidney transplant recipients

BACKGROUND

The new Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine-based equation was developed to address the systematic underestimation of the glomerular filtration rate (GFR) by the Modification of Diet in Renal Disease (MDRD) Study equation in patients with a relatively well-preserved kidney function. The performance of the new equation for kidney transplant recipients is discussed.

METHODS

We analyzed the performances of the CKD-EPI equation in comparison with the MDRD Study equation in 825 stable kidney transplant recipients. Bias, precision, and accuracy within 30% of true GFR were determined. GFR was measured by urinary clearance of inulin (n=488) and plasma clearance of Cr-EDTA (n=337).

RESULTS

Mean measured GFR (mGFR) was 50±19 mL/min/1.73 m. On the whole cohort, bias was significantly lower for MDRD Study equation compared with CKD-EPI creatinine. This superiority translates into a better accuracy (80% and 74% for the MDRD and CKD-EPI creatinine, respectively). The best performance of the MDRD Study equation is confirmed both in the subgroups of patients with mGFR <60 mL/min/1.73 m and between 60 and 90 mL/min/1.73 m. For mGFR >90 mL/min/1.73 m, there were no significant differences between the two equations in terms of performance.

CONCLUSIONS

The CKD-EPI creatinine equation does not offer a better GFR prediction in renal transplant patients compared with the MDRD Study equation, even in the earlier CKD stages.

Interaction of chronic total occlusion and chronic kidney disease in patients undergoing primary percutaneous coronary intervention for acute ST-elevation myocardial infarction

Chronic total occlusion (CTO) in a non-infarct-related artery and chronic kidney failure (CKD) are associated with worse outcomes after primary percutaneous coronary intervention (PCI). The aim of this study was to investigate the interaction of CTO and CKD in patients who underwent primary PCI for acute ST-segment elevation myocardial infarction (STEMI). Patients with STEMIs with or without CKD, defined as an estimated glomerular filtration rate <60 ml/min/1.73 m(2), were categorized into those with single-vessel disease and those with multivessel disease with or without CTO. The primary outcomes were the incidence of 30-day and 1-year mortality. Among 1,873 consecutive patients with STEMIs included between 2006 and 2011, 336 (18%) had CKD. The prevalence of CTO in a non-infarct-related artery was 13% in patients with CKD compared with 7% in those without CKD (p = 0.0003). There was a significant interaction between CKD and CTO on 30-day mortality (p = 0.018) and 1-year mortality (p = 0.013). Independent predictors of late mortality in patients with CKD were previous myocardial infarction (hazard ratio [HR] 1.71, 95% confidence interval [CI] 1.01 to 2.79), age >75 years (HR 1.86, 95% CI 1.19 to 2.95), a left ventricular ejection fraction after primary PCI <40% (HR 2.20, 95% CI 1.36 to 3.63), left main culprit artery (HR 4.46, 95% CI 1.64 to 10.25), and shock (HR 7.44, 95% CI 4.56 to 12.31), but multivessel disease with CTO was not a predictor. In contrast, multivessel disease with CTO was an independent predictor of mortality in patients without CKD (HR 3.30, 95% CI 1.70 to 6.17). In conclusion, in patients with STEMIs who underwent primary PCI, with preexisting CKD, the prevalence of CTO in a non-infarct-related artery was twice as great. In these patients, the clinical impact of CTO seems to be overshadowed by the presence of CKD.

Validation of a new standardized cystatin C turbidimetric assay: evaluation of the three novel CKD-EPI equations in hypertensive patients

OBJECTIVES

Analytical and clinical performances of the new standardized cystatin C particle-enhanced turbidimetric immunoassay (PETIA) using DiaSys reagents on Olympus AU2700® analyzer were evaluated.

DESIGN AND METHODS

We have studied imprecision, linearity, limit of detection and limit of quantification of this new immunoassay. Method comparison was assessed in relation to results generated by the standardized Siemens-particle-enhanced nephelometric immunoassay (PENIA). In order to evaluate the clinical relevance of this assay, estimated glomerular filtration rate (GFR) was calculated using MDRD, CKD-EPI creatinine, CKD-EPI cystatin C 2012 and CKD-EPI creatinine-cystatin C 2012 equations and compared to GFR measured using urinary clearance of (99m)Tc-DTPA in 100 hypertensive patients.

RESULTS

Cystatin C measurements using DiaSys reagents have reliable analytical performances and are comparable to the standardized Siemens-PENIA method (bias of 0.01 mg/L). The mean measured GFR was 90.0±29.7 mL/min/1.73 m². Bias and accuracy of the three CKD-EPI equations were better than the MDRD. Both CKD-EPI creatinine-based and cystatin C-based formulae had similar bias, precision and accuracy. The combined creatinine-cystatin C equation was significantly more accurate and precise than the CKD-EPI creatinine equation in patients with GFR above 60 mL/min/1.73 m².

CONCLUSIONS

The use of cystatin C in a combined equation with creatinine could improve the accuracy of eGFR in the reference interval.

Tubular and glomerular proteinuria in HIV-infected adults with estimated glomerular filtration rate ≥ 60 ml/min per 1.73 m2

OBJECTIVE

To assess the frequency of glomerular and tubular proteinuria in a cohort of HIV-infected patients, and to determine the factors associated with each type of injury.

DESIGN

Cross-sectional study of 1210 consecutive HIV-infected adults followed in HIV outpatient unit (Montpellier/France).

METHODS

Spot urine protein to creatinine (uPCR), albumin to creatinine (uACR) and albumin to protein (uAPR) ratios were assessed. Glomerular injury was defined as uACR at least 30 mg/g or uPCR at least 200 mg/g with uAPR at least 0.4. Tubular injury was defined as uPCR 200 mg/g or more with uAPR less than 0.4. Multivariate logistic regression identified independent factors of each type of proteinuria, in the 1158 patients with estimated glomerular filtration rate (eGFR) at least 60 ml/min per 1.73 m, using re-expressed modification of diet in renal disease equation.

RESULTS

Frequency of proteinuria was 18.2% among patients with eGFR at least 60 ml/min per 1.73 m consisting in tubular proteinuria for 50.7% of them. Factors associated with glomerular proteinuria were age [OR 1.34/10-year increment (95%CI: 1.08-1.66)], diabetes [OR 3.37 (95%CI: 1.53-7.44)], and arterial hypertension [OR 2.52 (95%CI: 1.36-4.66)]. Factors associated with tubular proteinuria were age [OR 1.43 (95%CI: 1.14-1.79)], current tenofovir use [OR 3.52 (95%CI: 1.86-6.65)], hepatitis C co-infection [OR 1.62 (95%CI: 1.00-2.65)], AIDS stage [OR 1.83 (95%CI: 1.18-2.82)], CD4 cell count less than 200 per μl [OR 2.48 (95%CI: 1.31-4.70)].

CONCLUSION

This study distinguished risk factors for tubular injury, mainly related to HIV disease and its treatment (tenofovir), and glomerular injury, linked to non HIV-related variables (age, diabetes, hypertension). Measuring uPCR, uACR and uAPR may help with the detection and specific management of early chronic kidney disease in HIV-infected patients having normal or sub-normal eGFR.

Enzymatic but not compensated Jaffe methods reach the desirable specifications of NKDEP at normal levels of creatinine. Results of the French multicentric evaluation

The French Society of Clinical Biochemistry conducted this study to compare the accuracy and performances of the best creatinine enzymatic assays and the compensated Jaffe methods from the same manufacturers. Creatinine was measured in 3 serum pools with creatinine levels of 35.9±0.9 μmol/L, 74.4±1.4 μmol/L, and 97.9±1.7 μmol/L (IDMS determination). The performances of the assays (total error that includes the contribution of bias and imprecision) were evaluated using Monte-Carlo simulations and compared against desirable NKDEP criteria. The enzymatic assays always fell within the desirable total Error of 7.6%. By contrast, this requirement was never obtained for the compensated Jaffe methods at the critical level of 74.4±1.4 μmol/L. Only the compensated Jaffe creatinine on Olympus analyzer reached this specification at 35.9±0.9 and 97.9±1.7 μmol/L levels. This study demonstrates that, despite substantial improvement regarding traceability to the IDMS reference method and precision, compensated Jaffe creatinine methods, by contrast to enzymatic ones, do not reach the desirable specifications of NKDEP at normal levels of creatinine.

Precision of estimating equations for GFR in children with a solitary functioning kidney: the KIMONO study

BACKGROUND AND OBJECTIVE

Children with a solitary functioning kidney may develop CKD. Although widely used, equations to estimate GFR are not validated in these patients. This study sought to determine the precision of common estimating equations in the KIMONO (KIdney of MONofunctional Origin) cohort.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Two creatinine-based (estimated GFR [eGFR]-Schwartz, urinary creatinine clearance), two cystatin C-based (eGFR-Zappitelli1, eGFR-CKiD [Chronic Kidney Disease in Children] 1), and two cystatin C/creatinine-based (eGFR-Zappitelli2, eGFR-CKiD2) estimates were compared with the gold standard GFR measured by inulin single injection (GFR-inulin) in 77 children with a solitary functioning kidney (time span of assembly, 2005-2012). Included patients were 1.5-19.8 years of age. Kidney Disease Outcomes Quality Initiative (K/DOQI) classification was compared between GFR-inulin and eGFR methods to analyze misclassification by estimating equations.

RESULTS

The eGFR-CKiD2 equation performed best in children with a solitary functioning kidney (mean bias, -0.9 ml/min per 1.73 m(2); 95% and 54% of values within ±30% and ±10% of GFR-inulin, respectively). Mean bias for eGFR-Schwartz was 0.4 ml/min per 1.73 m(2), with 90% and 33% of values within ±30% and ±10% of GFR-inulin, respectively. For all estimates, misclassification in K/DOQI stage ranged from 22% (eGFR-Zappitelli1) to 44% (urinary creatinine clearance) of children.

CONCLUSIONS

Use of a combined serum cystatin C/creatinine-based equation (eGFR-CKiD2) is recommended to monitor renal function in children with a solitary functioning kidney. When cystatin C is not routinely available, eGFR-Schwartz should be used. Misclassification in K/DOQI-stage remains a caveat for all equations.

GFR estimation using standardized cystatin C in kidney transplant recipients

BACKGROUND

The utility of serum cystatin C (SCysC) as a filtration marker in kidney transplantation is uncertain. We took advantage of the recent validation of a reference calibrator for SCysC and of newly developed CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equations (2012) expressed for use with standardized SCysC level to reassess the performance of SCysC as a filtration marker in kidney transplant recipients.

STUDY DESIGN

Study of diagnostic test accuracy.

SETTING & PARTICIPANTS

670 kidney transplant recipients from 3 centers undergoing glomerular filtration rate (GFR) measurements from December 2006 to November 2012.

INDEX TEST

Estimated GFR (eGFR) using the 2012 SCysC-based and serum creatinine (SCr)/SCysC-based CKD-EPI equations (eGFR(cys) and eGFR(cr-cys), respectively) and the 2009 SCr-based CKD-EPI equation (eGFR(cr)), with SCysC and SCr measured at a single laboratory between April 2011 and June 2011.

REFERENCE TEST

Measured GFR (mGFR) using urinary clearance of inulin.

RESULTS

Bias (the difference between mGFR and eGFR) was significantly smaller for eGFR(cys) and eGFR(cr-cys) versus eGFR(cr) (-2.82 and -0.54 vs +4.4 mL/min/1.73 m(2), respectively; P < 0.001). Precision (standard deviation of the mean bias) also was better for eGFR(cys) and eGFR(cr-cys) versus eGFR(cr) (12 and 11 vs 13 mL/min/1.73 m(2) [P < 0.001 for both comparisons]). Accuracy (percentage of GFR estimates within 30% of mGFR) was greater for eGFR(cys) and eGFR(cr-cys) versus eGFR(cr) (81% and 86% vs 75%, respectively [P = 0.004 and P < 0.001]). Net reclassification index with respect to mGFR of 30 mL/min/1.73 m(2) for eGFR(cr-cys) and eGFR(cys) versus eGFR(cr) was 18.8% [95% CI, 8.6%-28.9%] and 22.5% [95% CI, 10.2%-34.9%].

LIMITATIONS

Patients were exclusively of European descent; association with transplant outcome was not evaluated.

CONCLUSIONS

Our data validate the use of both the newly developed SCysC-based and SCr/SCysC-based CKD-EPI equations (2012) in kidney transplant recipients. Both equations perform better than the SCr-based CKD-EPI equation (2009).

Chronic kidney disease and automatic reporting of estimated glomerular filtration rate: new developments and revised recommendations

The publication of the Australasian Creatinine Consensus Working Group's position statements in 2005 and 2007 resulted in automatic reporting of estimated glomerular filtration rate (eGFR) with requests for serum creatinine concentration in adults, facilitated the unification of units of measurement for creatinine and eGFR, and promoted the standardisation of assays. New advancements and continuing debate led the Australasian Creatinine Consensus Working Group to reconvene in 2010. The working group recommends that the method of calculating eGFR should be changed to the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula, and that all laboratories should report eGFR values as a precise figure to at least 90 mL/min/1.73 m(2). Age-related decision points for eGFR in adults are not recommended, as although an eGFR < 60 mL/min/1.73 m(2) is very common in older people, it is nevertheless predictive of significantly increased risks of adverse clinical outcomes, and should not be considered a normal part of ageing.If using eGFR for drug dosing, body size should be considered, in addition to referring to the approved product information. For drugs with a narrow therapeutic index, therapeutic drug monitoring or a valid marker of drug effect should be used to individualise dosing. The CKD-EPI formula has been validated as a tool to estimate GFR in some populations of non-European ancestry living in Western countries. Pending publication of validation studies, the working group also recommends that Australasian laboratories continue to automatically report eGFR in Aboriginal and Torres Strait Islander peoples. The working group concluded that routine calculation of eGFR is not recommended in children and youth, or in pregnant women. Serum creatinine concentration (preferably using an enzymatic assay for paediatric patients) should remain as the standard test for kidney function in these populations.

Clinical evaluation of analytical variations in serum creatinine measurements: why laboratories should abandon Jaffe techniques

BACKGROUND

Non-equivalence in serum creatinine (SCr) measurements across Dutch laboratories and the consequences hereof on chronic kidney disease (CKD) staging were examined.

METHODS

National data from the Dutch annual external quality organization of 2009 were used. 144 participating laboratories examined 11 pairs of commutable, value-assigned SCr specimens in the range 52-262 μmol/L, using Jaffe or enzymatic techniques. Regression equations were created for each participating laboratory (by regressing values as measured by participating laboratories on the target values of the samples sent by the external quality organization); area under the curves were examined and used to rank laboratories. The 10th and 90th percentile regression equation were selected for each technique separately. To evaluate the impact of the variability in SCr measurements and its eventual clinical consequences in a real patient population, we used a cohort of 82424 patients aged 19-106 years. The SCr measurements of these 82424 patients were introduced in the 10th and 90th percentile regression equations. The newly calculated SCr values were used to calculate an estimated glomerular filtration rate (eGFR) using the 4-variable Isotope Dilution Mass Spectrometry traceable Modification of Diet in Renal Disease formula. Differences in CKD staging were examined, comparing the stratification outcomes for Jaffe and enzymatic SCr techniques.

RESULTS

Jaffe techniques overestimated SCr: 21%, 12%, 10% for SCr target values 52, 73 and 94 μmol/L, respectively. For enzymatic assay these values were 0%, -1%, -2%, respectively. eGFR using the MDRD formula and SCr measured by Jaffe techniques, staged patients in a lower CKD category. Downgrading to a lower CKD stage occurred in 1-42%, 2-37% and 12-78.9% of patients for the 10th and 90th percentile laboratories respectively in CKD categories 45-60, 60-90 and >90 ml/min/1.73 m2. Using enzymatic techniques, downgrading occurred only in 2-4% of patients.

CONCLUSIONS

Enzymatic techniques lead to less variability in SCr measurements than Jaffe techniques, and therefore result in more accurate staging of CKD. Therefore the specific enzymatic techniques are preferably used in clinical practice in order to generate more reliable GFR estimates.

Limitations of compensated Jaffe creatinine assays in cirrhotic patients

OBJECTIVES

This study aimed to evaluate the impact of two creatinine measurement methods on the Model for End Stage Liver Disease (MELD) score and glomerular filtration rate estimation (eGFR) in cirrhotic patients. We focused on ID-MS traceable method such as compensated Jaffe (cJafCreat) and enzymatic (EnzCreat) methods.

DESIGN AND METHODS

Potential protein-related interferences in creatinine determination were evaluated using dialysates spiked with albumin. MELD score, CKD-EPI formula creatinine-based eGFR and cystatin C-based eGFR were evaluated in 100 cirrhotic patients.

RESULTS

In vitro model demonstrated that low protein levels result in an underestimation of creatinine levels using cJafCreat. In patients, cJafCreat created a negative bias of -6.1 μmol/L that led to higher eGFR and lower MELD scores.

CONCLUSIONS

cJafCreat contributes to an overestimation of renal function in cirrhotic patients and may alter cirrhosis-severity assessment. Compensated Jaffe assays should therefore be replaced by enzymatic methods.

Accuracy of GFR predictive equations in renal transplantation: validation of a new turbidimetric cystatin C assay on Architect c8000

OBJECTIVES

To evaluate the Sentinel-PETIA cystatin C on Architect c8000 analyzer.

DESIGN AND METHODS

We assessed analytical performances and clinical relevance by comparison with a reference isotopic method in kidney transplant recipients.

RESULTS

This assay exhibited reliable precision and was close to the non standardized Siemens-PENIA method. All tested equations allowed reliable assessment of GFR.

CONCLUSIONS

Cystatin C improved GFR determination at the critical level of 60 mL/min/1.73 m². New formulas might be necessary after IFCC standardization.