Aminoglycoside dosing weight correction factors for patients of various body sizes

Prior investigations have suggested the use of a dosing weight correction factor of ideal body weight (IBW) plus 40% excess body weight (EBW, where EBW = total body weight [TBW] - IBW) to determine the weight to use for aminoglycoside dosing in morbidly obese (TBW/IBW ratio, > 2) patients. Little data are available to provide dosing information for underweight or moderately obese patients. We investigated aminoglycoside pharmacokinetics in 1,708 patients receiving gentamicin and tobramycin. Patients were stratified into underaverage-weight or overweight weight categories based on both TBW/IBW ratio and body mass index (weight/height2 ratio), which has been shown to correlate with physiologic estimates of body fat. Regression analyses revealed that the TBW/IBW ratio predicts the volume of distribution. Dosing weight correction factors to give equivalent predicted peak aminoglycoside concentrations with a 2-mg/kg loading dose are 1.13 times the TBW for underweight patients and 0.43 times the EBW plus IBW for overweight patients. There were no large differences between the dosing weight correction factors derived from IBW- and body mass index-based classification systems. These data generate useful aminoglycoside dosing weight equations for both underweight and overweight patients.