Analysis of various creatinine clearance formulas in predicting gentamicin elimination in patients with low serum creatinine.

Predicted gentamicin elimination rate constants (kelS) using creatinine clearance (Clcr) estimates from seven equations were compared with kelS calculated from steady-state serum gentamicin concentrations in 186 hospitalized patients. In predicting kel, the equations varied significantly in precision (mean absolute percentage error), and were particularly imprecise among patients with serum creatinine values of 71 mumol/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 mumol/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 mumol/L, had significantly greater precision and less bias than several of the equations. We recommend one of these two methods for predicting gentamicin kel in patients with low serum creatinine values.