Comparison of estimated glomerular filtration rate with estimated creatinine clearance in the dosing of drugs requiring adjustments in elderly patients with declining renal function

Background: The National Kidney Foundation's practice guidelines for chronic kidney disease recommend using the Modification of Diet in Renal Disease (MDRD) equation for calculating the estimated glomerular filtration rate (GFR). Objective: The purpose of this article was to compare the use of this estimated GFR with estimated creatinine clearance (CrCl) calculated using the Cockcroft-Gault equation (CG(CrCl)-Eq) in the dosing of drugs requiring adjustments in elderly patients with declining renal function. Existing as well as new serum creatinine assay standards were used. Methods: A PubMed literature search for all English-language articles published before November 2007 was conducted using the terms estimated glomerular filtration, GFR, modified diet in renal disease, MDRD, creatinine clearance, CrCl, drug dosing adjustment, renal impairment, human, and elderly. Mathematical comparisons of the age, race, and sex factors for these 2 equations (CGCrCl-Eq and MDRD) were performed, as well as a Simulation of resulting values from these equations using various combinations of age, weight, and sex factors. Results: None of the articles identified found that the use of the MDRD equation in the elderly was better than the CG(CrCl)-Eq for estimating renal drug elimination. Substantial mathematical differences are inherent in these 2 prediction equations that make any clinical comparison quite difficult. Implementation of new creatinine assay standards will. further confuse the use of these 2 equations for estimating GFR and for dosage adjustments in elderly patients with renal insufficiency. Conclusions: Although an MDRD equation may be useful for estimating GFR, the CG(CrCl)-Eq should still be used for drug, dosage adjustments. The CG(CrCl)-Eq may require a slight "adjustment factor" to be applied using serum creatinine values measured by newly established assay procedures.