Comparison of methods for estimating gentamicin clearance and retrospective analysis of changes in clearance with emphasis on patients with normal renal function.

The subjects were 29 general medical and surgical patients with apparently normal renal function at the start of treatment with gentamicin. Nine methods of estimating gentamicin clearance (CL) were compared as means of calculating gentamicin dosage. In 23 of the 29 patients CL was measured twice during treatment. The first measurement was based on 10 blood samples taken around a dose early in treatment. CL calculated from the area under the serum concentration-time curve using all 10 samples was designated a reference method for comparison with other methods. There was considerable inter-individual variation in CL. Serum creatinine and body weight gave poor estimates of an individual's CL. A good estimate of CL was obtained from four blood samples taken immediately before and 2, 3 and 4 h after dosing. Prediction of serum levels is not a good method for checking CL estimations. A significant decrease in CL was found during treatment. This was probably due to a reduction in glomerular filtration rate.