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.