It is unclear which renal function equation, employing an isotope dilution mass spectrometry (IDMS)-aligned creatinine assay, best predicts gentamicin clearance.

The performances of the Cockcroft-Gault, Modification of Diet in Renal Disease (MDRD) Study and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations for predicting gentamicin clearances were assessed retrospectively in 240 patients treated with gentamicin during 2011–2012, when the local creatinine assay was IDMS-aligned. Comparisons were based on the percentage within 30% of gentamicin clearance (P₃₀) and the root-mean-square error (RMSE) of each equation. Gentamicin clearance was calculated from plasma concentrations using a one-compartment model.

The Cockcroft-Gault equation and the CKD-EPI equation corrected for individual body surface area (BSA) were associated with the highest P₃₀ (69% and 67%, respectively) and lowest RMSE (39 and 36 mL/min, respectively) in the 240 patients. Correction for individual BSA improved the performances of the MDRD Study and CKD-EPI equations in patients with body mass indices <18.5 or ≥30 kg/m². The equations systematically underestimated gentamicin clearance as gentamicin clearance increased, with performance being inferior with gentamicin clearance ≥90 versus <90 mL/min.

The CKD-EPI equation corrected for individual BSA, and the Cockcroft-Gault equation, provided the best estimates of gentamicin clearance. The CKD-EPI and MDRD Study equations should be corrected for individual BSA at the extremes of body size, if used for guiding gentamicin therapy. The performances of the equations were inferior in patients with higher values of gentamicin clearance.