Estimating aminoglycoside clearance and creatinine clearance in underweight patients

An adjustment factor (AF) was developed and evaluated to determine the best method for estimating aminoglycoside clearance (CL(amino)) and creatinine clearance (CL(cr)) in underweight patients. This study was a retrospective, multicenter, chart analysis of aminoglycoside pharmacokinetic data obtained between January 2000 and August 2006 at the University of Southern California University Hospital and Cedars-Sinai Medical Center. Adult patients were included in this study if they had received inpatient aminoglycoside therapy, were at least 60 inches tall, and were at least 10% below their ideal body weight (IBW). CL(cr) and CL(amino) were estimated and compared to actual CL(amino) using the Cockcroft-Gault equation with actual serum creatinine (SCr) (CG(SCr)), Cockcroft-Gault equation with SCr rounded to 1 mg/dL (CG(rnd)), and Cockcroft-Gault equation multiplied by an AF (CG(AF)). Results An AF of 0.69 was determined from 52 patients and tested in 53 separate patients. The CG(AF) method was more precise and less biased than the CG(SCr) equation; the CG(rnd) equation was less biased than the CG(SCr) equation; the CG(AF) method was more precise and less biased than the CG(rnd) equation, but this difference was not statistically significant. In underweight patients with an SCr concentration of > or = 1 mg/dL, the CG(AF) method had less bias compared with the CG(SCr) equation. Both the CG(rnd) and CG(AF) methods of predicting CL(amino) in underweight patients were superior to the CG(SCr) equation. The CG(AF) method was more accurate in patients exhibiting greater differences between IBW and actual body weight.