Comparison of creatinine and cystatin C based eGFR in the estimation of glomerular filtration rate in Indigenous Australians: The eGFR Study

Background The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation that combines creatinine and cystatin C is superior to equations that include either measure alone in estimating glomerular filtration rate (GFR). However, whether cystatin C can provide any additional benefits in estimating GFR for Indigenous Australians, a population at high risk of end-stage kidney disease (ESKD) is unknown. Methods Using a cross-sectional analysis from the eGFR Study of 654 Indigenous Australians at high risk of ESKD, eGFR was calculated using the CKD-EPI equations for serum creatinine (eGFRcr), cystatin C (eGFRcysC) and combined creatinine and cystatin C (eGFRcysC + cr). Reference GFR (mGFR) was determined using a non-isotopic iohexol plasma disappearance technique over 4 h. Performance of each equation to mGFR was assessed by calculating bias, % bias, precision and accuracy for the total population, and according to age, sex, kidney disease, diabetes, obesity and c-reactive protein. Results Data were available for 542 participants (38% men, mean [sd] age 45 [14] years). Bias was significantly greater for eGFRcysC (15.0 mL/min/1.73 m²; 95% CI 13.3–16.4, p < 0.001) and eGFRcysC + cr (10.3; 8.8–11.5, p < 0.001) compared to eGFRcr (5.4; 3.0–7.2). Accuracy was lower for eGFRcysC (80.3%; 76.7–83.5, p < 0.001) but not for eGFRcysC + cr (91.9; 89.3–94.0, p = 0.29) compared to eGFRcr (90.0; 87.2–92.4). Precision was comparable for all equations. The performance of eGFRcysC deteriorated across increasing levels of c-reactive protein. Conclusion Cystatin C based eGFR equations may not perform well in populations with high levels of chronic inflammation. CKD-EPI eGFR based on serum creatinine remains the preferred equation in Indigenous Australians.