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/m² or greater, respectively. Stage 3 or 4 CKD (estimated glomerular filtration rate [eGFR], 15 to 59 mL/min/1.73 m²) 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.