John W. Kusek

Using Standardized Serum Creatinine Values in the Modification of Diet in Renal Disease Study Equation for Estimating Glomerular Filtration Rate

Context Guidelines recommend that laboratories estimate glomerular filtration rate (GFR) with equations that use serum creatinine level, age, sex, and ethnicity. Standardizing creatinine measurements across clinical laboratories should reduce variability in estimated GFR. Contribution Using standardized creatinine assays, the authors calibrated serum creatinine levels in 1628 patients whose GFR had been measured by urinary clearance of 125I-iothalamate. They used these data to derive new equations for estimating GFR and to measure their accuracy. The equations were inaccurate only when kidney function was near-normal. Cautions There was no independent sample of patients for measuring accuracy. Implications By using this equation and a standardized creatinine assay, different laboratories can report estimated GFR more uniformly and accurately. The Editors Chronic kidney disease is a recently recognized public health problem. Current guidelines define chronic kidney disease as kidney damage or a glomerular filtration rate (GFR) less than 60 mL/min per 1.73 m2 for 3 months or more, regardless of cause (13). Kidney damage is usually ascertained from markers, such as albuminuria. The GFR can be estimated from serum creatinine concentration and demographic and clinical variables, such as age, sex, ethnicity, and body size. The normal mean value for GFR in healthy young men and women is approximately 130 mL/min per 1.73 m2 and 120 mL/min per 1.73 m2, respectively, and declines by approximately 1 mL/min per 1.73 m2 per year after 40 years of age (4). To facilitate detection of chronic kidney disease, guidelines recommend that clinical laboratories compute and report estimated GFR by using estimating equations, such as equations derived from the Modification of Diet in Renal Disease (MDRD) Study (13, 510). The original MDRD Study equation was developed by using 1628 patients with predominantly nondiabetic kidney disease. It was based on 6 variables: age; sex; ethnicity; and serum levels of creatinine, urea, and albumin (11). Subsequently, a 4-variable equation consisting of age, sex, ethnicity, and serum creatinine levels was proposed to simplify clinical use (3, 12). This equation is now widely accepted, and many clinical laboratories are using it to report GFR estimates. Extensive evaluation of the MDRD Study equation shows good performance in populations with lower levels of GFR but variable performance in those with higher levels (1332). Variability among clinical laboratories in calibration of serum creatinine assays (33, 34) introduces error in GFR estimates, especially at high levels of GFR (35), and may account in part for the poorer performance in this range (13, 14, 16, 1821, 27, 30). The National Kidney Disease Education Program (NKDEP) has initiated a creatinine standardization program to improve and normalize serum creatinine results used in estimating equations (36). The MDRD Study equation has now been reexpressed for use with a standardized serum creatinine assay (37), allowing GFR estimates to be reported in clinical practice by using standardized serum creatinine and overcoming this limitation to the current use of GFR estimating equations. The purpose of this report is to describe the performance of the reexpressed 4-variable MDRD Study equation and compare it with the performance of the reexpressed 6-variable MDRD equation and the CockcroftGault equation (38), with particular attention to the level of GFR. This information should facilitate implementation of reporting and interpreting estimated GFR in clinical practice. Methods Laboratory Methods Urinary clearances of 125I-iothalamate after subcutaneous infusion were determined at clinical centers participating in the MDRD Study. Serum and urine 125I-iothalamate were assayed in a central laboratory. All serum creatinine values reported in this study are traceable to primary reference material at the National Institute of Standards and Technology (NIST), with assigned values based on isotope-dilution mass spectrometry. The serum creatinine samples from the MDRD Study were originally assayed from 1988 to 1994 in a central laboratory with the Beckman Synchron CX3 (Global Medical Instrumentation, Inc., Ramsey, Minnesota) by using a kinetic alkaline picrate method. Samples were reassayed in 2004 with the same instrument. The Beckman assay was calibrated to the Roche/Hitachi P module Creatinase Plus enzymatic assay (Roche Diagnostics, Basel, Switzerland), traceable to an isotope-dilution mass spectrometry assay at NIST (37, 39). On the basis of these results, the 4-variable and 6-variable MDRD Study equations were reexpressed for use with standardized serum creatinine assay. The CockcroftGault equation was not reexpressed because the original serum creatinine samples were not available for calibration to standardized serum creatinine assay. Derivation and Validation of the MDRD Study Equation The MDRD Study was a multicenter, randomized clinical trial of the effects of reduced dietary protein intake and strict blood pressure control on the progression of chronic kidney disease (40). The derivation of the MDRD Study equation has been described previously (11). Briefly, the equation was developed from data from 1628 patients enrolled during the baseline period. The GFR was computed as urinary clearance of 125I-iothalamate. Creatinine clearance was computed from creatinine excretion in a 24-hour urine collection and a single measurement of serum creatinine. Glomerular filtration rate and creatinine clearance were expressed per 1.73 m2 of body surface area. Ethnicity was assigned by study personnel, without explicit criteria, probably by examination of skin color. The MDRD Study equation was developed by using multiple linear regression to determine a set of variables that jointly estimated GFR in a random sample of 1070 patients (development data set). The regressions were performed on log-transformed data to reduce variability in differences between estimated and measured GFR at higher levels. Several equations were developed, and the performance of these equations was compared in the remaining sample of 558 patients (validation data set). To improve the accuracy of the final equations, the regression coefficients derived from the development data set were updated on the basis of data from all 1628 patients (11). Estimation of GFR Glomerular filtration rate was estimated by using the following 4 equations: the reexpressed 4-variable MDRD Study equation (GFR= 175standardized Scr 1.154age0.2031.212 [if black]0.742 [if female]), the reexpressed 6-variable MDRD Study equation (GFR= 161.5standardized Scr 0.999age0.176SUN0.17albumin0.3181.18 [if black]0.762 [if female]), the CockcroftGault equation adjusted for body surface area (Ccr= [140age]weight0.85 [if female]1.73/72 standardized ScrBSA), and the CockcroftGault equation adjusted for body surface area and corrected for the bias in the MDRD Study sample (Ccr= 0.8[140age]weight0.85 [if female]1.73/72 standardized ScrBSA). In these equations, GFR and creatinine clearance (Ccr) are expressed as mL/min per 1.73 m2, serum creatinine and urea nitrogen (SUN) are expressed as mg/dL, albumin is expressed as g/dL, weight is expressed as kg, age is expressed as years, and body surface area (BSA) is expressed as m2. Correction for bias improves performance of the CockcroftGault equation because it adjusts for systematic differences between studies, such as differences in the measures of kidney function (GFR in the MDRD Study and creatinine clearance in the study by Cockcroft and Gault), the serum creatinine assays, and the study samples. Hence, the bias correction for the CockcroftGault equation provided here reexpresses that equation for the estimation of GFR for use with standardized creatinine in study samples similar to that in the MDRD Study. Measures of Performance Measures of performance include bias (median difference of measured minus estimated GFR and measured GFR) and percentage bias (percentage of bias divided by measured GFR), precision (interquartile range of the difference between estimated and measured GFR, and percentage of variance in log-measured GFR explained by the regression model [R2 values]), and accuracy (percentage of estimates within 30% of the measured values). In the overall data set, bias is expected to be close to 0 for equations derived in the MDRD Study database, including the 4-variable and 6-variable equations and the CockcroftGault equation adjusted for bias. The bootstrap method (based on percentiles, with 2000 bootstrap samples) was used to estimate 95% CIs for interquartile ranges and R2 values. Confidence intervals for the percentage of estimates within 30% of measured values were computed by using the normal approximation to the binomial or exact binomial probabilities, as appropriate. We also computed sensitivity, specificity, positive and negative predictive value of estimated GFR less than 60 mL/min per 1.73 m2, and receiver-operating characteristic (ROC) curves by using measured GFR less than 60 mL/min per 1.73 m2 as the criterion standard. Areas under the ROC curves were compared by using the method of DeLong and colleagues (41). R, version 2 (Free Software Foundation, Inc., Boston, Massachusetts), and SAS, version 9.1 (SAS Institute, Inc., Cary, North Carolina), were used for statistical analysis. We used the lowess function in R to plot smoothed functions in the figures. Role of the Funding Source The study was funded by grants from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) as part of a cooperative agreement that gives the NIDDK substantial involvement in the design of the study and in the collection, analysis, and interpretation of the data. The NIDDK was not required to approve publication of the finished manuscript. The institutional review boards of all participating institutions approved the study. Results Clinical characteristics of

The Effects of Dietary Protein Restriction and Blood-Pressure Control on the Progression of Chronic Renal Disease

Background Restricting protein intake and controlling hypertension delay the progression of renal disease in animals. We tested these interventions in 840 patients with various chronic renal diseases. Methods In study 1, 585 patients with glomerular filtration rates of 25 to 55 ml per minute per 1.73 m2 of body-surface area were randomly assigned to a usual-protein diet or a low-protein diet (1.3 or 0.58 g of protein per kilogram of body weight per day) and to a usual- or a low-blood-pressure group (mean arterial pressure, 107 or 92 mm Hg). In study 2, 255 patients with glomerular filtration rates of 13 to 24 ml per minute per 1.73 m2 were randomly assigned to the low-protein diet (0.58 g per kilogram per day) or a very-low-protein diet (0.28 g per kilogram per day) with a keto acid-amino acid supplement, and a usual- or a low-blood-pressure group (same values as those in study 1). An 18-to-45-month follow-up was planned, with monthly evaluations of the patients. Results The mean follow-up was 2.2 years. ...

Estimating Glomerular Filtration Rate from Serum Creatinine and Cystatin C

BACKGROUND Estimates of glomerular filtration rate (GFR) that are based on serum creatinine are routinely used; however, they are imprecise, potentially leading to the overdiagnosis of chronic kidney disease. Cystatin C is an alternative filtration marker for estimating GFR. METHODS Using cross-sectional analyses, we developed estimating equations based on cystatin C alone and in combination with creatinine in diverse populations totaling 5352 participants from 13 studies. These equations were then validated in 1119 participants from 5 different studies in which GFR had been measured. Cystatin and creatinine assays were traceable to primary reference materials. RESULTS Mean measured GFRs were 68 and 70 ml per minute per 1.73 m(2) of body-surface area in the development and validation data sets, respectively. In the validation data set, the creatinine-cystatin C equation performed better than equations that used creatinine or cystatin C alone. Bias was similar among the three equations, with a median difference between measured and estimated GFR of 3.9 ml per minute per 1.73 m(2) with the combined equation, as compared with 3.7 and 3.4 ml per minute per 1.73 m(2) with the creatinine equation and the cystatin C equation (P=0.07 and P=0.05), respectively. Precision was improved with the combined equation (interquartile range of the difference, 13.4 vs. 15.4 and 16.4 ml per minute per 1.73 m(2), respectively [P=0.001 and P<0.001]), and the results were more accurate (percentage of estimates that were >30% of measured GFR, 8.5 vs. 12.8 and 14.1, respectively [P<0.001 for both comparisons]). In participants whose estimated GFR based on creatinine was 45 to 74 ml per minute per 1.73 m(2), the combined equation improved the classification of measured GFR as either less than 60 ml per minute per 1.73 m(2) or greater than or equal to 60 ml per minute per 1.73 m(2) (net reclassification index, 19.4% [P<0.001]) and correctly reclassified 16.9% of those with an estimated GFR of 45 to 59 ml per minute per 1.73 m(2) as having a GFR of 60 ml or higher per minute per 1.73 m(2). CONCLUSIONS The combined creatinine-cystatin C equation performed better than equations based on either of these markers alone and may be useful as a confirmatory test for chronic kidney disease. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases.).

FGF23 induces left ventricular hypertrophy

Chronic kidney disease (CKD) is a public health epidemic that increases risk of death due to cardiovascular disease. Left ventricular hypertrophy (LVH) is an important mechanism of cardiovascular disease in individuals with CKD. Elevated levels of FGF23 have been linked to greater risks of LVH and mortality in patients with CKD, but whether these risks represent causal effects of FGF23 is unknown. Here, we report that elevated FGF23 levels are independently associated with LVH in a large, racially diverse CKD cohort. FGF23 caused pathological hypertrophy of isolated rat cardiomyocytes via FGF receptor-dependent activation of the calcineurin-NFAT signaling pathway, but this effect was independent of klotho, the coreceptor for FGF23 in the kidney and parathyroid glands. Intramyocardial or intravenous injection of FGF23 in wild-type mice resulted in LVH, and klotho-deficient mice demonstrated elevated FGF23 levels and LVH. In an established animal model of CKD, treatment with an FGF-receptor blocker attenuated LVH, although no change in blood pressure was observed. These results unveil a klotho-independent, causal role for FGF23 in the pathogenesis of LVH and suggest that chronically elevated FGF23 levels contribute directly to high rates of LVH and mortality in individuals with CKD.

Estimating GFR Using Serum Cystatin C Alone and in Combination With Serum Creatinine: A Pooled Analysis of 3,418 Individuals With CKD

BACKGROUND Serum cystatin C was proposed as a potential replacement for serum creatinine in glomerular filtration rate (GFR) estimation. We report the development and evaluation of GFR-estimating equations using serum cystatin C alone and serum cystatin C, serum creatinine, or both with demographic variables. STUDY DESIGN Test of diagnostic accuracy. SETTING & PARTICIPANTS Participants screened for 3 chronic kidney disease (CKD) studies in the United States (n = 2,980) and a clinical population in Paris, France (n = 438). REFERENCE TEST Measured GFR (mGFR). INDEX TEST Estimated GFR using the 4 new equations based on serum cystatin C alone, serum cystatin C, serum creatinine, or both with age, sex, and race. New equations were developed by using linear regression with log GFR as the outcome in two thirds of data from US studies. Internal validation was performed in the remaining one third of data from US CKD studies; external validation was performed in the Paris study. MEASUREMENTS GFR was measured by using urinary clearance of iodine-125-iothalamate in the US studies and chromium-51-EDTA in the Paris study. Serum cystatin C was measured by using Dade-Behring assay, standardized serum creatinine values were used. RESULTS Mean mGFR, serum creatinine, and serum cystatin C values were 48 mL/min/1.73 m(2) (5th to 95th percentile, 15 to 95), 2.1 mg/dL, and 1.8 mg/L, respectively. For the new equations, coefficients for age, sex, and race were significant in the equation with serum cystatin C, but 2- to 4-fold smaller than in the equation with serum creatinine. Measures of performance in new equations were consistent across the development and internal and external validation data sets. Percentages of estimated GFR within 30% of mGFR for equations based on serum cystatin C alone, serum cystatin C, serum creatinine, or both levels with age, sex, and race were 81%, 83%, 85%, and 89%, respectively. The equation using serum cystatin C level alone yields estimates with small biases in age, sex, and race subgroups, which are improved in equations including these variables. LIMITATIONS Study population composed mainly of patients with CKD. CONCLUSIONS Serum cystatin C level alone provides GFR estimates that are nearly as accurate as serum creatinine level adjusted for age, sex, and race, thus providing an alternative GFR estimate that is not linked to muscle mass. An equation including serum cystatin C level in combination with serum creatinine level, age, sex, and race provides the most accurate estimates.

Fibroblast growth factor 23 and risks of mortality and end-stage renal disease in patients with chronic kidney disease.

CONTEXT A high level of the phosphate-regulating hormone fibroblast growth factor 23 (FGF-23) is associated with mortality in patients with end-stage renal disease, but little is known about its relationship with adverse outcomes in the much larger population of patients with earlier stages of chronic kidney disease. OBJECTIVE To evaluate FGF-23 as a risk factor for adverse outcomes in patients with chronic kidney disease. DESIGN, SETTING, AND PARTICIPANTS A prospective study of 3879 participants with chronic kidney disease stages 2 through 4 who enrolled in the Chronic Renal Insufficiency Cohort between June 2003 and September 2008. MAIN OUTCOME MEASURES All-cause mortality and end-stage renal disease. RESULTS At study enrollment, the mean (SD) estimated glomerular filtration rate (GFR) was 42.8 (13.5) mL/min/1.73 m(2), and the median FGF-23 level was 145.5 RU/mL (interquartile range [IQR], 96-239 reference unit [RU]/mL). During a median follow-up of 3.5 years (IQR, 2.5-4.4 years), 266 participants died (20.3/1000 person-years) and 410 reached end-stage renal disease (33.0/1000 person-years). In adjusted analyses, higher levels of FGF-23 were independently associated with a greater risk of death (hazard ratio [HR], per SD of natural log-transformed FGF-23, 1.5; 95% confidence interval [CI], 1.3-1.7). Mortality risk increased by quartile of FGF-23: the HR was 1.3 (95% CI, 0.8-2.2) for the second quartile, 2.0 (95% CI, 1.2-3.3) for the third quartile, and 3.0 (95% CI, 1.8-5.1) for the fourth quartile. Elevated fibroblast growth factor 23 was independently associated with significantly higher risk of end-stage renal disease among participants with an estimated GFR between 30 and 44 mL/min/1.73 m(2) (HR, 1.3 per SD of FGF-23 natural log-transformed FGF-23; 95% CI, 1.04-1.6) and 45 mL/min/1.73 m(2) or higher (HR, 1.7; 95% CI, 1.1-2.4), but not less than 30 mL/min/1.73 m(2). CONCLUSION Elevated FGF-23 is an independent risk factor for end-stage renal disease in patients with relatively preserved kidney function and for mortality across the spectrum of chronic kidney disease.

Effect of clopidogrel on early failure of arteriovenous fistulas for hemodialysis: a randomized controlled trial.

CONTEXT The arteriovenous fistula is the preferred type of vascular access for hemodialysis because of lower thrombosis and infection rates and lower health care expenditures compared with synthetic grafts or central venous catheters. Early failure of fistulas due to thrombosis or inadequate maturation is a barrier to increasing the prevalence of fistulas among patients treated with hemodialysis. Small, inconclusive trials have suggested that antiplatelet agents may reduce thrombosis of new fistulas. OBJECTIVE To determine whether clopidogrel reduces early failure of hemodialysis fistulas. DESIGN, SETTING, AND PARTICIPANTS Randomized, double-blind, placebo-controlled trial conducted at 9 US centers composed of academic and community nephrology practices in 2003-2007. Eight hundred seventy-seven participants with end-stage renal disease or advanced chronic kidney disease were followed up until 150 to 180 days after fistula creation or 30 days after initiation of dialysis, whichever occurred later. INTERVENTION Participants were randomly assigned to receive clopidogrel (300-mg loading dose followed by daily dose of 75 mg; n = 441) or placebo (n = 436) for 6 weeks starting within 1 day after fistula creation. MAIN OUTCOME MEASURES The primary outcome was fistula thrombosis, determined by physical examination at 6 weeks. The secondary outcome was failure of the fistula to become suitable for dialysis. Suitability was defined as use of the fistula at a dialysis machine blood pump rate of 300 mL/min or more during 8 of 12 dialysis sessions. RESULTS Enrollment was stopped after 877 participants were randomized based on a stopping rule for intervention efficacy. Fistula thrombosis occurred in 53 (12.2%) participants assigned to clopidogrel compared with 84 (19.5%) participants assigned to placebo (relative risk, 0.63; 95% confidence interval, 0.46-0.97; P = .018). Failure to attain suitability for dialysis did not differ between the clopidogrel and placebo groups (61.8% vs 59.5%, respectively; relative risk, 1.05; 95% confidence interval, 0.94-1.17; P = .40). CONCLUSION Clopidogrel reduces the frequency of early thrombosis of new arteriovenous fistulas but does not increase the proportion of fistulas that become suitable for dialysis. Trial Registration clinicaltrials.gov Identifier: NCT00067119.

Serum creatinine levels in the US population: Third National Health and Nutrition Examination Survey

This report describes the distribution of serum creatinine levels by sex, age, and ethnic group in a representative sample of the US population. Serum creatinine level was evaluated in the third National Health and Nutrition Examination Survey (NHANES III) in 18,723 participants aged 12 years and older who were examined between 1988 and 1994. Differences in mean serum creatinine levels were compared for subgroups defined by sex, age, and ethnicity (non-Hispanic white, non-Hispanic black, and Mexican-American). The mean serum creatinine value was 0.96 mg/dL for women in the United States and 1.16 mg/dL for men. Overall mean creatinine levels were highest in non-Hispanic blacks (women, 1.01 mg/dL; men, 1.25 mg/dL), lower in non-Hispanic whites (women, 0.97 mg/dL; men, 1.16 mg/dL), and lowest in Mexican-Americans (women, 0.86 mg/dL; men, 1.07 mg/dL). Mean serum creatinine levels increased with age among both men and women in all three ethnic groups, with total US mean levels ranging from 0.88 to 1.10 mg/dL in women and 1.00 to 1.29 mg/dL in men. The highest mean creatinine level was seen in non-Hispanic black men aged 60+ years. In the total US population, creatinine levels of 1.5 mg/dL or greater were seen in 9.74% of men and 1.78% of women. Overall, among the US noninstitutionalized population, 10.9 million people are estimated to have creatinine values of 1.5 mg/dL or greater, 3.0 million have values of 1.7 mg/dL or greater, and 0.8 million have serum creatinine levels of 2.0 mg/dL or greater. Mean serum creatinine values are higher in men, non-Hispanic blacks, and older persons and are lower in Mexican-Americans. In the absence of information on glomerular filtration rate (GFR) or lean body mass, it is not clear to what extent the variability by sex, ethnicity, and age reflects normal physiological differences rather than the presence of kidney disease. Until this information is known, the use of a single cutpoint to define elevated serum creatinine values may be misleading.

Intensive blood-pressure control in hypertensive chronic kidney disease.

BACKGROUND In observational studies, the relationship between blood pressure and end-stage renal disease (ESRD) is direct and progressive. The burden of hypertension-related chronic kidney disease and ESRD is especially high among black patients. Yet few trials have tested whether intensive blood-pressure control retards the progression of chronic kidney disease among black patients. METHODS We randomly assigned 1094 black patients with hypertensive chronic kidney disease to receive either intensive or standard blood-pressure control. After completing the trial phase, patients were invited to enroll in a cohort phase in which the blood-pressure target was less than 130/80 mm Hg. The primary clinical outcome in the cohort phase was the progression of chronic kidney disease, which was defined as a doubling of the serum creatinine level, a diagnosis of ESRD, or death. Follow-up ranged from 8.8 to 12.2 years. RESULTS During the trial phase, the mean blood pressure was 130/78 mm Hg in the intensive-control group and 141/86 mm Hg in the standard-control group. During the cohort phase, corresponding mean blood pressures were 131/78 mm Hg and 134/78 mm Hg. In both phases, there was no significant between-group difference in the risk of the primary outcome (hazard ratio in the intensive-control group, 0.91; P=0.27). However, the effects differed according to the baseline level of proteinuria (P=0.02 for interaction), with a potential benefit in patients with a protein-to-creatinine ratio of more than 0.22 (hazard ratio, 0.73; P=0.01). CONCLUSIONS In overall analyses, intensive blood-pressure control had no effect on kidney disease progression. However, there may be differential effects of intensive blood-pressure control in patients with and those without baseline proteinuria. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases, the National Center on Minority Health and Health Disparities, and others.)

Factors other than glomerular filtration rate affect serum cystatin C levels.

Cystatin C is an endogenous glomerular filtration marker hence its serum level is affected by the glomerular filtration rate (GFR). To study what other factors might affect it blood level we performed a cross-sectional analysis of 3418 patients which included a pooled dataset of clinical trial participants and a clinical population with chronic kidney disease. The serum cystatin C and creatinine levels were related to clinical and biochemical parameters and errors-in-variables models were used to account for errors in GFR measurements. The GFR was measured as the urinary clearance of 125I-iothalamate and 51Cr-EDTA. Cystatin C was determined at a single laboratory while creatinine was standardized to reference methods and these were 2.1+/-1.1 mg/dL and 1.8+/-0.8 mg/L, respectively. After adjustment for GFR, cystatin C was 4.3% lower for every 20 years of age, 9.2% lower for female gender but only 1.9% lower in blacks. Diabetes was associated with 8.5% higher levels of cystatin C and 3.9% lower levels of creatinine. Higher C-reactive protein and white blood cell count and lower serum albumin were associated with higher levels of cystatin C and lower levels of creatinine. Adjustment for age, gender and race had a greater effect on the association of factors with creatinine than cystatin C. Hence, we found that cystatin C is affected by factors other than GFR which should be considered when the GFR is estimated using serum levels of cystatin C.