Amikacin pharmacokinetics in morbidly obese patients.

The pharmacokinetics of single-dose amikacin sulfate was studied in seven morbidly obese patients to determine the fraction of fat weight (FW) that, when added to ideal body weight (IBW), will normalize the volume of distribution. Seven patients (mean total body weight of 166.5 kg) were each given a 1,250-g intravenous injection of amikacin. Amikacin serum levels over eight hours were measured by radioimmunoassay. A correction factor (CF) of 0.38 was found to normalize the patients' volume of distribution (Varea) to 0.26 liters/kg, when added to the patients' IBW. There was no significant difference between peak amikacin levels predicted using the actual Varea and using the Varea plus the CF. Predictions using the actual Varea and those using Varea estimated by ideal body weight were significantly different. In morbidly obese patients, peak amikacin serum levels can be predicted best when a volume of distribution based on IBW plus a correction factor of 38% of FW is used.