Comparison of Drug Dosing Recommendations Based on Measured GFR and Kidney Function Estimating Equations

Background: Kidney disease alters the pharmacokinetic disposition of many medications, requiring dosage adjustment to maintain therapeutic serum concentrations. The Cockcroft-Gault (CG) equation is used for pharmacokinetic studies and drug dosage adjustments, but the Modification of Diet in Renal Disease (MDRD) Study equation is more accurate and more often reported by clinical laboratories than the CG equation. Study Design: Diagnostic test study. Settings & Participants: Pooled data set for 5,504 participants from 6 research studies and 4 clinical populations with measured glomerular filtration rate (GFR). Index Test: Estimated kidney function using the MDRD Study and CG equations incorporating actual (CG) or ideal body weight (CG(IBW)) and standardized serum creatinine concentrations. Reference Test: Measured GFR assessed by using iodine-125-iothalamate urinary clearance. Outcome: Concordance of assigned kidney function categories designated by the Food and Drug Administration (FDA) Guidance for Industry for pharmacokinetic studies and recommended dosages of 15 medications cleared by the kidneys. Results: Concordance of kidney function estimates with measured GFR for FDA-assigned kidney function categories was 78% for the MDRD Study equation compared with 73% for the CG equation (P < 0.001) and 66% for the CG(IBW) equation (P < 0.001). Concordance between the MDRD Study equation and CG and CG(IBW) equations was 78% and 75%, respectively (P < 0.001). Concordance of kidney function estimates with measured GFR for recommended drug dosages was 88% for MDRD Study equation compared with 85% for the CG equation (P < 0.001) and 82% for the CG(IBW) equation (P < 0.001), with lower concordance when dosing recommendations for drugs included narrow GFR ranges. Concordance rates between the CG and CG(IBW) equations and MDRD Study equation were 89% and 88%, respectively (P < 0.05). Limitations: Results based on simulation rather than pharmacokinetic studies. Outcome was drug dosage recommendations, rather than observed drug efficacy and safety. Conclusions: The MDRD Study equation can also be used for pharmacokinetic studies and drug dosage adjustments. As more accurate GFR-estimating equations are developed, they should be used for these purposes. Am J Kidney Dis 54:33-42. (c) 2009 by the National Kidney Foundation, Inc.