Frederick Van Lente

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

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.).

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.

Cardiac troponins in renal insufficiency: review and clinical implications.

Patients with renal insufficiency may have increased serum troponins even in the absence of clinically suspected acute myocardial ischemia. While cardiovascular disease is the most common cause of death in patients with renal failure, we are just beginning to understand the clinical meaning of serum troponin elevations. Serum troponin T is increased more frequently than troponin I in patients with renal failure, leading clinicians to question its specificity for the diagnosis of myocardial infarction. Many large-scale trials demonstrating the utility of serum troponins in predicting adverse events and in guiding therapy and intervention in acute coronary syndromes have excluded patients with renal failure. Despite persistent uncertainty about the mechanism of elevated serum troponins in patients with reduced renal function, data from smaller groups of renal failure patients have suggested that troponin elevations are associated with added risk, including an increase in mortality. It is possible that increases in serum troponin from baseline in patients with renal insufficiency admitted to hospital with acute coronary syndrome may signify myocardial necrosis. Further studies are needed to clarify this hypothesis.

Plasma B-Type Natriuretic Peptide Levels in Ambulatory Patients With Established Chronic Symptomatic Systolic Heart Failure

Background—The diagnostic and prognostic values of plasma B-type natriuretic peptide (BNP) testing are established. However, the range of plasma BNP levels present in the setting of chronic, stable systolic heart failure (HF) is unclear. Methods and Results—We followed up 558 consecutive ambulatory patients with chronic, stable systolic HF (left ventricular ejection fraction <50%) treated at a specialized outpatient HF clinic between November 2001 and February 2003. Retrospective chart review was performed to determine clinical and functional data at the time of BNP testing (Biosite Triage). The clinical characteristics of patients with plasma BNP levels <100 pg/mL and those with ≥100 pg/mL were compared. In our cohort, 60 patients were considered asymptomatic, and their plasma BNP levels ranged from 5 to 572 pg/mL (median, 147 pg/mL). Of the remaining 498 symptomatic (NYHA functional class II–III) patients, 106 (21.3%) had plasma BNP levels in the “normal” diagnostic range (<100 pg/mL). Patients in this “normal BNP” subgroup were more likely to be younger, to be female, to have nonischemic pathogenesis, and to have better-preserved cardiac and renal function and less likely to have atrial fibrillation. Conclusions—In the ambulatory care setting, both symptomatic and asymptomatic patients with chronic, stable systolic HF may present with a wide range of plasma BNP levels. In a subset of symptomatic patients (up to 21% in our cohort), plasma BNP levels are below what would be considered “diagnostic” (<100 pg/mL).

Serum Cystatin C in the United States: The Third National Health and Nutrition Examination Survey (NHANES III)

BACKGROUND Serum cystatin C increasingly is used as a marker of glomerular filtration rate and cardiovascular risk. However, information for serum cystatin C levels in the general population, specifically across a wide age range and different ethnicities, is lacking. OBJECTIVES To determine nationally representative serum cystatin C levels, estimate the prevalence of increased cystatin C levels in the general population, and identify factors associated with increased cystatin C levels. STUDY DESIGN Cross-sectional survey. SETTING AND PARTICIPANTS A nationally representative subsample of 7,596 participants aged 12 years or older in the Third National Health and Nutrition Examination Survey conducted in 1988-1994. PREDICTORS Age, sex, race/ethnicity, risk factors for chronic kidney disease. OUTCOMES Continuous serum cystatin C levels and serum cystatin C level greater than 1.12 mg/L. MEASUREMENTS Cystatin C was measured in 2006 from stored sera by using an automated particle-enhanced nephelometric assay. RESULTS Overall median serum cystatin C level was 0.85 mg/L. Median cystatin C levels increased steeply with age and were greater in males and non-Hispanic white persons, even in a healthy subgroup of 20- to 39-year-olds. Prevalences of increased serum cystatin C levels (>1.12 mg/L) were 1%, 41%, and greater than 50% in all persons aged younger than 20 years, 60 years or older, and 80 years or older. In persons aged 60 years and older, older age, non-Hispanic white ethnicity, hypertension, current smoking, lower levels of education and high-density lipoprotein cholesterol, and increased body mass index, C-reactive protein, and triglyceride values were associated significantly with increased serum cystatin C levels. LIMITATIONS No measured glomerular filtration rate, single measurement of cystatin C, cross-sectional study design. CONCLUSIONS Serum cystatin C level is related to sex and ethnicity, even in young healthy individuals. The prevalence of increased cystatin C levels increases dramatically with age, reaching greater than 50% after the age of 80 years in both sexes and all ethnic groups.

Low Rates of Testing and Diagnostic Codes Usage in a Commercial Clinical Laboratory: Evidence for Lack of Physician Awareness of Chronic Kidney Disease

Improving outcomes for chronic kidney disease (CKD) requires early identification and recognition by physicians. There are few data on rates of testing or use of diagnostic codes for CKD. A cross-sectional analysis was performed of patients who were older than 40 yr and had one or more laboratory tests between April 1, 2002, and March 31, 2003, at a Laboratory Corporation of America regional laboratory. Objectives were to determine the frequency of testing for serum creatinine; prevalence of CKD, defined as estimated GFR <60 ml/min per 1.73 m2; and sensitivity of diagnostic codes for CKD for patients with and without risk factors for CKD and with or without cardiovascular disease (CVD). Of the 277,111 patients, 19% had serum creatinine measured, compared with 33 and 71% who had measurements of serum glucose and lipids, respectively. Patients with hypertension, diabetes, and age >60 yr were more likely to be tested for serum creatinine with odds ratio (OR; 95% confidence interval) of 2.09 (2.05 to 2.14), 1.22 (1.19 to 1.25), and 1.24 (1.22 to 1.27) respectively. Among patients tested, 30% had CKD. Sensitivity and specificity of kidney disease diagnostic codes compared with CKD defined by estimated GFR <60 ml/min per 1.73 m2 were 11 and 96%, respectively. In patients with hypertension, diabetes, age >60 years, and CVD, rates of testing and sensitivity of diagnostic codes were 53 and 14%, respectively. Low rates of testing for serum creatinine and insensitivity of diagnostic codes for CKD, even in high-risk patients, suggests inadequate physician awareness of CKD and limited utility of administrative databases for identification of patients with CKD.

Ability of Troponins to Predict Adverse Outcomes in Patients With Renal Insufficiency and Suspected Acute Coronary Syndromes: A Case-Matched Study

OBJECTIVES The purpose of this study was to investigate the utility of cardiac troponin T and troponin I for predicting outcomes in patients presenting with suspected acute coronary syndromes and renal insufficiency relative to that observed in similar patients without renal disease. BACKGROUND Cardiac troponin T and troponin I have shown promise as tools for risk stratification of patients with acute coronary syndromes. However, there is uncertainty regarding their cardiac specificity and utility in patients with renal disease. METHODS We measured troponin T, troponin I and creatine kinase MB in 51 patients presenting with suspected acute coronary syndromes and renal insufficiency and in 102 patients without evidence of renal disease matched for the same peak troponin T or I value, selected from a larger patient cohort. Blood samples were obtained at presentation to an emergency room 4 hours, 8 hours and 16 hours later. The ability of biochemical markers to predict adverse outcomes in both groups including infarction, recurrent ischemia, bypass surgery, heart failure, stroke, death or positive angiography/angioplasty during hospitalization and at six months was assessed by receiver-operator curve analysis. The performance of both troponins was compared between groups. RESULTS Thirty-five percent of patients in the renal group and 45% of patients in the nonrenal group experienced an adverse initial outcome; over 50% of patients in all groups had experienced an adverse outcome by 6 months, but these differences were not significant. The area under the curve (AUC) for the ROC curve for troponin T as predictor of initial outcomes was significantly lower in the renal group than in the nonrenal group: 0.56+/-0.07 and 0.75+/-0.07, respectively. The area under the curve was also significantly lower in the renal group compared with the nonrenal group for troponin T as predictor of six month outcomes: 0.59+/-0.07 and 0.74+/-0.07, respectively. The area under the curve was also significantly lower in the renal group compared to the nonrenal group for troponin I as predictor of both initial and six month outcomes: 0.54+/-0.06 vs. 0.71+/-0.07 and 0.53+/- 0.06 vs. 0.65+/-0.07, respectively. The sensitivity of troponin T for both initial and six month adverse outcomes was significantly lower in the renal group than in the nonrenal group at a similar level of specificity (0.87): 0.29 vs. 0.60 and 0.45 vs. 0.56, respectively. Troponin I also exhibited similar differences in sensitivity in the renal group (0.29 vs. 0.50 and 0.33 vs. 0.40, respectively). CONCLUSIONS The ability of cardiac troponin T and troponin I to predict risk for subsequent adverse outcomes in patients presenting with suspected acute coronary syndromes is reduced in the presence of renal insufficiency.

Method of Glomerular Filtration Rate Estimation Affects Prediction of Mortality Risk

Decreased kidney function, determined using a serum creatinine-based estimation of GFR, is associated with a higher risk for mortality from cardiovascular disease. Equations incorporating cystatin C improve the estimation of GFR, but whether this improves the prediction of risk for mortality is unknown. We measured cystatin C on 6942 adult participants in the Third National Health and Nutrition Examination Survey Linked Mortality File, including all participants who had high serum creatinine (>1.2 mg/dl for men; >1.0 mg/dl for women) or were older than 60 yr and 25% random sample of participants who were younger than 60 yr. We estimated GFR using equations that included standardized serum creatinine, cystatin C, or both. Participant data were linked to the National Death Index. A total of 1573 (22.7%) deaths (713 deaths from cardiovascular disease) occurred during a median of 8 yr. Lower estimated GFR based on cystatin C was strongly associated with higher risk for overall and cardiovascular mortality across the range of normal to moderately decreased estimated GFR. Creatinine-based estimates of GFR resulted in weaker associations, with the association between estimated GFR and all-cause mortality reversed at higher levels of estimated GFR. An equation using both creatinine and cystatin C (in addition to age, race, and gender) resulted in weaker associations than equations using only cystatin C (with or without age, race, and gender). In conclusion, despite better performance in terms of estimating GFR, equations based on both cystatin C and creatinine do not predict mortality as well as equations based on cystatin C alone.

Detection of Soluble Angiotensin-Converting Enzyme 2 in Heart Failure : Insights Into the Endogenous Counter-Regulatory Pathway of the Renin-Angiotensin-Aldosterone System

OBJECTIVES We sought to determine whether circulating soluble angiotensin-converting enzyme 2 (sACE2) is increased in the plasma of patients with heart failure (HF). BACKGROUND Angiotensin-converting enzyme 2 (ACE2) is an integral membrane protein that antagonizes the actions of angiotensin II and prevents the development of HF in animal models. However, because of the need for invasive cardiac tissue sampling, little is known about whether ACE2 is involved in the pathophysiology of HF in humans. METHODS We developed a sensitive and specific assay to measure sACE2 activity in human plasma and screened a heterogeneous group of patients suspected of having clinical HF. RESULTS Increasing sACE2 plasma activity strongly correlated with a clinical diagnosis of HF (p = 0.0002), worsening left ventricular ejection fraction (p < 0.0001), and increasing B-type natriuretic peptide levels (p < 0.0001). Similar to B-type natriuretic peptide, sACE2 activity reflected the severity of HF, with increasing levels associated with worsening New York Heart Association functional class (p < 0.0001). These associations were independent of other disease states and medication use. We found that sACE2 activity was increased in patients with both ischemic and nonischemic cardiomyopathies and also in patients with clinical HF but a preserved left ventricular ejection fraction. CONCLUSIONS Soluble ACE2 activity is increased in patients with HF and correlates with disease severity, suggesting that a cardioprotective arm of the renin-angiotensin-aldosterone system is active in HF.