Estimating renal function in older people: a comparison of three formulas

Background: estimation of the glomerular filtration rate (GFR) at the bedside is important because renal insufficiency is related to increased mortality and morbidity. A discrepancy between the Cockroft-Gault (CG) and the Modification of Diet in Renal Disease (MDRD) formulas has been observed in older people. Objective: to compare the GFR of inpatients aged 65 or older estimated using the CG and two of the MDRD formulas. Setting: acute care geriatrics and internal medicine wards. Subjects and methods: data come from the Gruppo Italiano di Farmacovigilanza nell'Anziano (GIFA). To quantify the agreement between the formulas, we used the 95% limits of agreement, the kappa statistic and a graphic approach to evaluate the influence of potential confounders on the magnitude of the difference in the GFR estimates. Results: we studied 7,747 persons [51.1% women, mean age 77.8 (SD 7.2)]. The mean GFR estimated using the CG, MDRD1 and MDRD2 formulas was 51.2 ml/min (21.3), 54.9 ml/min (19.8) and 64.7 ml/min (24.2), respectively. At the individual level, the MDRD formulas can yield estimates that differ by more than 50% compared with the CG formula. The formulas showed a moderate agreement in diagnosing moderate renal insufficiency and a fair agreement in diagnosing severe renal insufficiency. The magnitude of the difference in GFR estimates was influenced by age and weight. Conclusions: the CG and MDRD formulas have a good average agreement, but at the individual level, they can give estimates that differ substantially, and cannot be used interchangeably to measure renal function in elderly people.