Analysis of Various Creatinine Clearance Formulas in Predicting Gentamicin Elimination in Patients With Low Serum Creatinine

Predicted gentamicin elimination rate constants (k_els) using creatinine clearance (C1_cr) estimates from seven equations were compared with k_els calculated from steady-state serum gentamicin concentrations in 186 hospitalized patients. In predicting k_el, the equations varied significantly in precision (mean absolute percentage error), and were particularly imprecise among patients with serum creatinine values of 71 μmol/L or less. Significant differences in bias (mean prediction error) were also observed. All equations using serum creatinine as an element showed improved precision, and most showed reduced bias when a minimum value of 71 μmol/L was used. The Cockcroft-Gault normalized to 72 kg and the Hull equations are among the simplest to calculate and, when using a minimum serum creatinine of 71 μmol/L, had significantly greater precision and less bias than several of the equations. We recommend one of these two methods for predicting gentamicin k_el in patients with low serum creatinine values.