Differential Estimation of CKD Using Creatinine-Versus Cystatin C-Based Estimating Equations by Category of Body Mass Index

Backgound: Adiposity is associated with cystatin C. Cystatin C-based glomerular filtration rate (GFR) equations may result in overestimation of chronic kidney disease (CKD) prevalence at greater body mass index (BMI) levels. Study Design: Cross-sectional. Setting & Participants: 6,709 US adult Third National Health and Nutrition Examination Survey participants. Factor: BMI. Outcome: Absolute percentage of difference in prevalence of stage 3 or 4 CKD between creatinine and cystatin C-based estimating equations by level of BMI. Measurements: Normal weight, overweight, and obesity were defined as BMI of 18.5 to less than 25.0, 25 to less than 30.0, and 30 kg/m(2) or greater, respectively. Stage 3 or 4 CKD (estimated glomerular filtration rate [eGFR], 15 to 59 mL/min/1.73 m(2)) was defined using the 4-variable creatinine-based Modification of Diet in Renal Disease Study equation (eGFR(MDRD)); cystatin C level, age, sex, and race equation (eGFR(Cysc,age,sex,race)); cystatin C-only equation (eGFR(Cysc)); cystatin C level of 1.12 mg/L or greater (increased cystatin C); and an equation incorporating serum creatinine level, cystatin C level, age, sex, and race (eGFR(Cr,CysC,age,sex,race)). Results: Differences in stage 3 or 4 CKD prevalence estimates between eGFR(CysC,age,sex, race), eGFR(Cysc), and increased cystatin C, separately, and eGFR(MDRD) were greater at higher BMI levels. Specifically, compared with estimates derived using eGFR(MDRD) for normal-weight, overweight, and obese participants, estimated prevalences of stage 3 or 4 CKD were 2.1%, 3.0%, and 6.5% greater when estimated by using eGFR(Cysc,age,sex,race) (P trend = 0.005); 0.1 %, 0.6%, and 2.2% greater for eGFR(Cysc) (P trend = 0.03); 2.9%, 5.2%, and 9.5% greater for increased cystatin C (P trend < 0.001); and -0.1 %, -0.4%, and 0.0% greater for eGFR(Cr,CysC,age,sex,race), respectively (P trend = 0.7). Limitations: No gold-standard measure of GFR was available. Conclusions: BMI may influence the estimated prevalence of stage 3 or 4 CKD when cystatin C-based equations are used. Am J Kidney Dis 53:993-1001. (C) 2009 by the National Kidney Foundation, Inc.Inc