[Estimation of the glomerular filtration rate in elderly inpatients: comparison of four methods]

High variability has been observed in the estimation of the glomerular filtration rate (GFR) in older patients, according to the formula used and no single formula has been recommended to date. The aim of this study was to quantify the precision and accuracy of the GFR estimated by means of three formulas and the measurement of cystatin C.

This prospective study was conducted in an acute care geriatric unit. Participants were patients, aged 70 years and over, having a possible 24-hour urine collection. The GFR was estimated using the Cockroft-Gault (CG), the Modification of Diet in Renal Disease (MDRD), and the Creatinine Clearance (Cl-Cr) formulas. The serum level of cystatin C was also measured.

Eighty-one patients were included in the study. CG formula underestimated the GFR by a mean difference of 8.65 ml/min, compared with MDRD formula. Cl-Cr underestimated the GFR by a mean difference of 7.56 ml/min, compared with CG formula, and by a mean difference of 16.79 ml/min, compared with the MDRD formula. The degree of discrepancy between CG and Cl-Cr estimates, and between Cl-Cr and MDRD estimates decreased as the estimated GFR approached normal values. MDRD best matched the measurement of cystatin C, followed by CG and Cl-Cr (Kappa coefficient=0.43, 0.22 and 0.16, respectively).

Our study confirms the high variability of GFR in older patients and particularly in those with abnormal renal function, depending on the formula used. Serum cystatine C level and MDRD formula appear to be the most concordant estimates of GFR in this population.