New insights in glomerular filtration rate formulas and chronic kidney disease classification

Pharmacovigilance of nephrotoxic drugs in neonates: the Pottel method for acute kidney injury detection in ELBW neonates

BACKGROUND

Extremely low birth weight (ELBW) neonates (birth weight ≤ 1000 g) are at high risk to develop drug-induced acute kidney injury (AKI). However, we lack a pragmatic detection tool to capture their time-dependent (patho)physiologic serum creatinine (Scr) patterns. Pottel et al. suggested rescaling Scr by dividing Scr with the mean Scr value of the age- and sex-specific reference population. We explored if this Pottel method can detect drug-related nephrotoxicity in ELBW neonates.

METHODS

A previously reported dataset on Scr changes in ELBW neonates exposed to ibuprofen, amikacin, or vancomycin was updated to calculate Pottel scores for every available Scr value in the first 28 postnatal days. We hereby used previously published postnatal age-specific 50th centile values in an ELBW population. Linear mixed models were applied, analyzing Pottel scores as response variable and continuous time (day), drug exposure, and interaction thereof in the explanatory model.

RESULTS

Serum creatinine (n = 3231) observations in 201 ELBW neonates were collected. A statistically significant rise of Pottel scores was observed with ibuprofen starting from postnatal day 4. In addition, a cumulative effect of treatment with mean Pottel scores on day 0 of 1.020 and on day 3 during treatment of 1.106 (95% CI 1.068-1.145, p < 0.001) was observed, corrected for effect of antibiotics. Antibiotic administrations showed a small but statistically significant difference up to postnatal day 5.

CONCLUSIONS

As rescaled Scr biomarker, the Pottel method showed a clear association with ibuprofen-exposed ELBW neonates, suggesting its applicability as a pragmatic bedside alternative tool to assess nephrotoxicity.

Data mining of reference intervals for serum creatinine: an improvement in glomerular filtration rate estimating equations based on Q-values

OBJECTIVES

Glomerular filtration rate (GFR) estimating equations based on rescaled serum creatinine (SCr/Q) have shown better performance, where Q represents the median SCr for age- and sex-specific healthy populations. However, there remains a scarcity of investigations in China to determine this value. We aimed to develop Chinese age- and sex-specific reference intervals (RIs) and Q-values for SCr and to validate the equations incorporating new Q-values.

METHODS

We included 117,345 adults from five centers for establishing RIs and Q-values, and 3,692 participants with reference GFR (rGFR, 99mTc-DTPA renal dynamic imaging measurement) for validation. Appropriate age partitioning was determined using the decision tree method. Lower and upper reference limits and medians were calculated using the refineR algorithm, and Q-values were determined accordingly. We evaluated the full age spectrum (FAS) and European Kidney Function Consortium (EKFC) equations incorporating different Q-values considering bias, precision (interquartile range, IQR), and accuracy (percentage of estimates within ±20 % [P20] and ±30 % [P30] of rGFR).

RESULTS

RIs for males were: 18-79 years, 55.53-92.50 μmol/L; ≥80 years, 54.41-96.43 μmol/L. RIs for females were: 18-59 years, 40.42-69.73 μmol/L; 60-79 years, 41.16-73.69 μmol/L; ≥80 years, 46.50-73.20 μmol/L. Q-values were set at 73.82 μmol/L (0.84 mg/dL) for males and 53.80 μmol/L (0.61 mg/dL) for females. After validation, we found that the adjusted equations exhibit less bias, improved precision and accuracy, and increased agreement of GFR categories.

CONCLUSIONS

We determined Chinese age- and sex-specific RIs and Q-values for SCr. The adjustable Q-values provide an effective alternative to obtain valid equations for estimating GFR.

Evaluating the diagnostic value of rescaled β-trace protein in combination with serum creatinine and serum cystatin C in older adults

BACKGROUND

Beta trace protein (BTP) is a novel renal biomarker that has emerged as potential alternative or addition to serum creatinine (Scr) and serum cystatin C (ScysC). We analyzed BTP's diagnostic ability to detect impaired kidney function by rescaling it and we tested whether rescaling BTP allowed us to expand the Full-Age-Spectrum (FAS)-equation to BTP.

METHODS

566 participants aged ≥70 years with measured glomerular filtration rate (mGFR), Scr, ScysC and BTP from the population-based Berlin Initiative Study (BIS) were considered. We developed a single and combined FAS-equation using rescaled BTP (BTP/0.60) and calculated its sensitivity (S) and specificity (Sp) to identify kidney disease using a fixed (60 mL/min/1.73 m²) and age-dependent threshold for mGFR.

RESULTS

Rescaled BTP shared the same reference interval with rescaled Scr and ScysC and showed acceptable diagnostic performance (S = 73.1%, Sp = 86.5%), comparable to Scr (S = 71.0%, Sp = 90.5%) and ScysC (S = 80.7%, Sp = 92.9%). Rescaled BTP can be used in the FAS-equation with comparable performance as Scr and ScysC, but the Scr/ScysC/BTP-combined FAS-eq. (P10 = 57.8%, P30 = 96.6%) did not outperform the Scr/ScysC-combined FAS-eq. (P10 = 57.1%, P30 = 96.3%).

CONCLUSIONS

Rescaled BTP is a valid alternative to Scr or ScysC to diagnose kidney function. The FAS-concept can be applied to BTP or the combination of BTP, Scr and ScysC.

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.

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.

A new equation to estimate the glomerular filtration rate in children, adolescents and young adults

BACKGROUND

A new estimated glomerular filtration rate (eGFR) equation, designed for isotope dilution mass spectrometry-standardized serum creatinine (Scr), is presented for use in children, adolescent boys and girls and young adults.

METHODS

The new equation, eGFR = 107.3/(Scr/Q), is based on the concept of normalized Scr: Q is the normalization value and is considered as the Scr concentration for the average healthy child, adolescent or young adult of a specific height (L) and is modeled as a height-dependent polynomial of the fourth degree.

RESULTS

The well-known Schwartz equation [eGFR = kL/Scr, k = 0.413 (Schwartz) or k = 0.373 (Schwartz-Lyon)] for children between 1 and 14 years can be seen as a special case of the new equation for which the Q-polynomial is simplified to a linear equation: Q = 0.0035 × L (cm). The new eGFR equation has been validated in a data set of n = 750 children, adolescents and young adults aged 10-25, against the true GFR (inulin method), and outperforms the selected (but most used) creatinine-based eGFR equations for children, mainly in the healthy GFR region.

CONCLUSIONS

The new Q(height)-eGFR equation serves as an excellent screening tool for kidney disease in 1-25-year-old children, adolescents and young adults.

Demystifying ethnic/sex differences in kidney function: is the difference in (estimating) glomerular filtration rate or in serum creatinine concentration?

BACKGROUND

The recent evaluation of the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation for estimating the glomerular filtration rate (GFR) in multiple ethnicities has raised the question on how well this equation performs for African-American and Asian subjects. There is no doubt that serum creatinine (Scr) concentration differs between ethnicities and sexes. We show that creatinine-based equations for white populations may be inaccurate for estimating GFR in other ethnic/gender groups, especially in populations from Asia.

METHODS

This study presents a mathematical analysis of the CKD-EPI-equation complemented with a literature review of median and reference values for IDMS-standardized Scr-concentrations for multiple ethnicities.

RESULTS

The study shows that at equal eGFR-CKD-EPI-values, the ratio of Scr between females and males equals 0.79 and between other ethnicities/sexes and white males is constant too. From this information, it is possible to calculate mean Scr-values that correspond very well with literature values directly obtained from Scr-distributions in healthy white males and females and in black males, but the discrepancy is larger for other populations.

CONCLUSIONS

Our results confirm the criticism that has been raised for using the CKD-EPI-equation for these ethnicities. An alternative eGFR-model is proposed based on a population-normalized Scr that needs further validation.

A simple height-independent equation for estimating glomerular filtration rate in children

BACKGROUND

The chronic kidney disease (CKD) classification system for children is similar to the CKD classification system for adults, using estimated glomerular filtration rate (eGFR) combined with fixed cut-off values of 60, 30, and 15 ml/min/1.73 m(2) for CKD stages III, IV, and V, respectively. To estimate GFR in children, eGFR-equations are used that require serum creatinine (Scr), but also height information, which is normally not available in clinical laboratory databases.

METHODS

This retrospective study is based on data from two different databases, one that has previously been used to develop the Flanders Metadata equation for children and one database including 353 children who underwent (51)Cr-EDTA GFR, serum creatinine, height, and weight measurements.

RESULTS

A height-independent eGFR equation based on the concept of a population-normalized Scr, presented before for adults, is extended to children: eGFR = 107.3/(Scr/Q), with Q the median Scr for healthy children of a particular age. This equation is validated against direct measurements of GFR, and against the updated Schwartz and Flanders Metadata equation.

CONCLUSIONS

The new simple height-independent equation performs very well and should make (mass) screening of kidney function in children easier.

Results of the recalibration of creatinine measurement with the modular Beckman Coulter Jaffe creatinine method

BACKGROUND

Serum creatinine is important for detecting the beginning of a decline in kidney function. The Beckman Coulter Jaffe reagents for measuring creatinine have been standardized to the internationally accepted reference method: isotope dilution mass spectrometry (IDMS). The impact of this recalibration on the Beckman Coulter modular or cup (stat) Jaffe method is studied.

METHODS

Recalibrated Jaffe (calibrator set points IDMS traceable) and classic National Institute of Standards and Technology (NIST) creatinine methods (traceable to NIST 914a) were compared with an enzymatic method (Sentinel, traceable to NIST SRM 967). All measurements were performed on Synchron DxC 800 systems. Imprecision of the routine methods was studied using the Clinical and Laboratory Standards Institute (CLSI) protocols and laboratory quality survey. Thirteen plasma pools, with concentrations < 354 mmol/L, were analyzed with a gas chromatography isotope dilution mass spectrometry (GC-IDMS) method and routine methods. Total error of 8.2% based on biological variability, set on the GC-IDMS values and acceptance criteria (bias < 5%, imprecision < 8% at concentrations ≥ 88.4 mmol/L and a maximum 10% increase in the relative error of estimated glomerular filtration rate (eGFR) of the National Kidney Disease Educational Program (NKDEP) were used for evaluating analytical performance of the routine methods studied.

RESULTS

After recalibration of the Jaffe method, median bias (mmol/L) decreased from 12.4 (95% CI: 9.1-21.2) to 7.3 (95% CI: 1.5-10.5). Imprecision of the Jaffe method is in agreement with the claim of the manufacturer, namely < 9 mmol/L or < 3% below or above 292 mmol/L. Below creatinine of 88.4 mmol/L, imprecision of the recalibrated Jaffe and enzymatic methods is between 4.1% and 6.9%, and 5.0% and 7.1%, respectively, and significantly different (p = 0.02 for both the Jaffe and enzymatic methods) from the goal, based on biological variability, of 2.7%. For the adult pools, all recalibrated Jaffe and enzymatic results fall within the total error of 8.2%. In the pediatric samples, one-third of the recalibrated Jaffe and three of the six enzymatic results fall within this total error goal.

CONCLUSIONS

Recalibration significantly reduced bias of the Jaffe method. For pediatric samples, recalibrated Jaffe results do not comply with either the imprecision goal or the total error based on biological variability. Adult recalibrated Jaffe results are in compliance with the goals based on biological variability and with the acceptance criteria from the NKDEP.

Reference intervals for serum creatinine with enzymatic assay and evaluation of four equations to estimate glomerular filtration rate in a healthy Chinese adult population

BACKGROUND

With the wide usage of enzymatic assays to determine serum creatinine (Scr) in China, reference interval (RI) needs to be established. At the same time, the performance of Scr based equations to calculate estimated glomerular filtration rate (eGFR) in healthy Chinese adults has not been extensively investigated.

METHODS

This study has strictly followed the International Federation of Clinical Chemistry (IFCC) recommendations and the Clinical Laboratory Standards Institute (CLSI) C28-A2 document. A total number of 778 healthy Chinese adults (male 433, female 345) were enrolled in this study.

RESULTS

By nonparametric method, RIs for males were: <60 years, 58-93 μmol/l and >60 years, 54-109 μmol/l; RIs for females were: <60 years, 42-69 μmol/l and >60 years, 43-83 μmol/l. Modification of Diet in Renal Disease (MDRD) equation and MDRD for Japanese made the percentage of eGFR >90 ml/min/1.73 m(2) 31.7% and 4.9%; percentage of eGFR 60-89 ml/min/1.73 m(2) 65.2% and 64.0%, respectively. Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation and CKD-EPI for Asian made the percentage of eGFR >90 ml/min/1.73 m(2) 89.2% and 94.0%; percentage of eGFR 60-89 ml/min/1.73 m(2) 10.3% and 5.7%, respectively.

CONCLUSION

Scr RIs of healthy Chinese adults were found to be lower than those of Caucasians. Two MDRD equations underestimated GFR, while two CKD-EPI equations seemed to estimate a reasonable distribution of eGFR in healthy Chinese adult populations.