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 (eGFRMDRD ); cystatin C level, age, sex, and race equation (eGFRCysC,age,sex,race ); cystatin C–only equation (eGFRCysC ); 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 (eGFRCr,CysC,age,sex,race ). Results Differences in stage 3 or 4 CKD prevalence estimates between eGFRCysC,age,sex,race , eGFRCysC , and increased cystatin C, separately, and eGFRMDRD were greater at higher BMI levels. Specifically, compared with estimates derived using eGFRMDRD 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 eGFRCysC,age,sex,race ( P trend = 0.005); 0.1%, 0.6%, and 2.2% greater for eGFRCysC ( 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 eGFRCr,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.