CREATININE-CLEARANCE ESTIMATES FOR PREDICTING GENTAMICIN PHARMACOKINETIC VALUES IN OBESE PATIENTS

The Cockcroft-Gault and Salazar-Corcoran equations were compared with respect to prediction of gentamicin pharmacokinetic values in obese an d nonobese patients, and the results were used to formulate guidelines for calculating initial gentamicin dosages in obese patients. Creatin ine clearance (CL(cr)) was estimated by applying the Cockcroft-Gault e quation using total body weight (TBW), ideal body weight (IBW), and do sage weight (DW) and with Salazar-Corcoran equations using fat-free bo dy mass (FBM) in 100 obese and 100 nonobese patients. Gentamicin pharm acokinetic values (k, CL, and t(1/2)) were estimated by using CL(cr) e stimated by each method and standardized to a body surface area of 1.7 3 sq m. Actual pharmacokinetic values were determined by using steady- state gentamicin concentrations and a modified Sawchuk-Zaske equation; these values were compared with the predicted values. In the obese pa tients, pharmacokinetic values predicted from standardized CL(cr) by t he Cockcroft-Gault equation using estimated DW were not significantly biased, compared with actual values; most predictions produced by the other methods were significantly biased. Predictions produced by the D W method were generally more precise than those resulting from the oth er methods. In nonobese patients, k values estimated by the Cockcroft- Gault equation using IBW were not significantly biased, while values o btained with all other methods were biased. All methods were biased wh en predicting CL and t(1/2) in nonobese patients. Significant correlat ions existed between standardized estimates of CL(cr) (by all methods) and pharmacokinetic values in both groups. Predictions of gentamicin k, CL, and t(1/2) were best overall when CL(cr) was estimated by the C ockcroft-Gault equation using DW, rather than by other methods. Use of DW is recommended for calculating initial gentamicin regimens in obes e patients, but dosage adjustments should be based on serum drug conce ntrations.