Combined creatinine-cystatin C CKD-EPI equation significantly underestimates measured glomerular filtration rate in people with type 2 diabetes mellitus

Aim: To evaluate the accuracy of creatinine and cystatin C (cysC) equations to estimate glomerular filtration rate (GFR) in type 2 diabetes mellitus (DM) patients and healthy adults. Methods: Case-control study including 84 patients with type 2 DM and 100 healthy adults with measured GFR (mGFR) >= 60 mL/min/1.73 m(2). GFR was measured by Cr-51-EDTA and estimated (eGFR) by the following equations using creatinine, cysC or both markers: Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI), Caucasian Asian Pediatrics and Adults (CAPA), CKD-EPI creatinine-cystatin C (CKDEPI-CC), and CKD-EPI cystatin C (CKDEPIcysC). Agreement was evaluated by Bland & Altman analysis. Results: Healthy individuals were 66% females, aged 38 +/- 14 years; they presented mGFR 112 +/- 19 mL/min/1.73 m(2) and eGFR by CKD-EPI, CKDEPI-CC, CKDEPIcysC and CAPA equations, respectively, 108 +/- 17, 102 +/- 15, 97 +/- 16 and 93 +/- 16 mL/min/1.73 m(2). DM group were 50% females, aged 59 +/- 19 years and presented mGFR 104 +/- 27 and eGFR 87 +/- 19, 80 +/- 18, 74 +/- 20 and 73 +/- 18 mL/min/1.73 m(2), respectively. All equations significantly underestimated mGFR, excepting creatinine-based CKD-EPI in the healthy group. The performance was considerably worse for GFRs above 120 mL/min/1.73 m(2). Conclusion: In both healthy and type 2 DM patients, cystatin C-based equations, including the combined CKD-EPI creatinine-cystatin equation, failed to improve the accuracy of GFR estimation, especially for normal and high normal GFR values.