Estimation of glomerular filtration rate in the elderly:: a comparison of creatinine-based formulae with serum cystatin C

Objectives. To estimate the prevalence of decreased kidney function in an elderly population and to evaluate the impact of using alternative markers of glomerular filtration rate (GFR), focusing on serum cystatin C (Cys C) and the Modification of Diet in Renal Disease (MDRD) Study prediction equation. Design and methods. In a cross-sectional community-based survey renal function was assessed by serum creatinine (SCreat), Cys C and GFR predicted by the Cockcroft-Gault (CG) and the MDRD Study formulae. Associations with age, gender and proteinuria were analysed by linear models. Subjects. A total of 1246 elderly residents in Lieto, Finland, 64-100 years of age. Results. The prevalence of moderately or severely decreased renal function, estimated by the MDRD Study equation, was 35.7%; the CG formula yielded 58.6%. The profile of Cys C performance, including variation across age groups and level of health status, showed greater similarity to GFR estimated using the MDRD Study equation than to SCreat alone, or GFR estimated using the CG formula. Discordance between high Cys C levels and only mildly decreased GFR estimates was observed in subjects with functional limitations. Microalbuminuria was associated with Cys C levels only (P =0.047). Conclusion. Prevalence estimates of decreased renal function amongst the elderly vary considerably depending on prediction formula used. Variation in creatinine metabolism amongst elderly comorbid patients and the critical dependence on the SCreat assay and exact calibration, make the use of creatinine-based formulae to predict GFR questionable in geriatric clinical practice. In this setting, Cys C is a promising alternative.