Reliability of equations to estimate glomerular filtration rate in the very old

Few studies have investigated the reliability of formulas estimating renal function in very old people.

We studied 154 elderly people (mean age: 82 yrs). Serum creatinine (SC) was measured by the Jaffé method, and creatinine clearance (CrCl) with 24-h urine collection. Agreement was measured with the average ratio estimated/measured CrCl, and precision with the 95% agreement intervals (95% AI). We calculated the proportion of residents correctly classified as having renal insufficiency (accuracy).

The Cockcroft-Gault (CG) and Modification of Diet in Renal Disease 1 (MDRD1) formulas showed good average agreement with measured CrCl (0.95 and 1.016, respectively); the MDRD2 formula was more biased. Results were consistent in women, whereas the MDRD1 was more biased in men (average ratio: 1.196). The 95% AI showed that all formulas can yield results as low as 50% or as high as 200% of measured CrCl. The proportion of people with CrCl<60 ml/min misclassified by the CG, MDRD1, and MDRD2 formulas as having normal renal function was 21.4%, 27.0%, and 38.8%, respectively. These results were consistent across the various subgroups, especially in subjects with normal SC.

The clinical usefulness of formulas commonly used to estimate CrCl was limited, regardless of subjects' characteristics.