Comparison of glomerular filtration rate estimating equations derived from creatinine and cystatin C: Validation in the Age, Gene/Environment Susceptibility-Reykjavik elderly cohort

Background. Validation studies comparing glomerular filtration rate (GFR) equations based on standardized creatinine and cystatin C assays in the elderly are needed. The Icelandic Age, Gene/Environment Susceptibility-Kidney cohort was used to compare two pairs of recently developed GFR equations, the revised Lund-Malmö creatinine equation (LMR_Cr) and the arithmetic mean of the LMR_Cr and Caucasian, Asian, Paediatric and Adult cystatin C equations (MEAN_LMRþ_CAPA), as well as the Full Age Spectrum creatinine equation (FAS_Cr) and its combination with cystatin C (FAS_Crþ_Cys), with the corresponding pair of Chronic Kidney Disease Epidemiology Collaboration equations (CKD-EPI_Cr and CKD-EPI_Crþ_Cys). Methods. A total of 805 individuals, 74-93 years of age, underwent measurement of GFR (mGFR) using plasma clearance of iohexol. Four metrics were used to compare the performance of the GFR equations: bias, precision, accuracy [including the percentage of participants with estimated GFR (eGFR) within 30% of mGFR (P₃₀)] and the ability to detect mGFR <60 mL/min/1.73 m². Results. All equations had a P₃₀ >90%. LMR_Cr and FAS_Cr yielded significantly higher precision and P₃₀ than CKD-EPI_Cr, while bias was significantly worse. LMR_Cr, FAS_Cr and CKD-EPI_Cr showed similar ability to detect mGFR <60 mL/min/1.73 m² based on the area under the receiver operating characteristic curves. MEAN_LMRþCAPA, FAS_Crþ_Cys and CKD-EPI_Crþ_Cys all exhibited consistent improvements compared with the corresponding creatinine-based equations. Conclusion. None of the creatinine-based equations was clearly superior overall in this community-dwelling elderly cohort. The addition of cystatin C improved all of the creatinine-based equations.