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Purpose: Appropriate initial vancomycin dosing in obese patients remains controversial. The purpose of this study was to evaluate vancomycin dosing requirements in obese patients to achieve serum trough concentrations of 15-20 mg/L.
Methods: A retrospective case-control study was conducted at two academic medical centers. Obese [Total body weight (TBW) ≥101 kg] and non-obese (TBW <101 kg) patients were included based on age ≥18 years, ≥48 hours of vancomycin therapy, baseline serum creatinine <2.0 mg/dL, and ≥1 vancomycin trough level. Exclusion criteria included cystic fibrosis, unstable renal function, renal replacement therapy, or IV contrast dye within 7 days of starting vancomycin. Total daily dose (TDD) requirements were evaluated based on TBW and adjusted dosing weight (ADW), defined as ideal body weight (IBW)+0.4(TBW-IBW).
Results: A total of 116 obese (mean ± SD 122.0 ± 20.1 kg, range 101.0 to 205.0 kg) and 115 non-obese patients (72.8 ± 12.9 kg, range 44.5 to 99.1 kg) were evaluated. Eighty-six obese patients (74%) and 78 non-obese patients (68%) had documented steady-state trough concentrations of 15-20 mg/L (median trough concentrations 17.4 and 17.2 mg/L, respectively). Among patients achieving desired trough concentrations, median TDD in obese and non-obese patients were 3,000 mg/day and 2,250 mg/day, respectively. Obese and non-obese patients required means of 26.6 ± 9.9 vs. 34.4 ± 14.4 mg/kg/day, respectively, based on TBW (p<0.0001). Among obese patients, mean TDD based on ADW was similar to the TBW-based dose in non-obese patients (35.6 ± 12.6 vs. 34.4 ± 14.4 mg/kg/day, respectively; p=0.545). However, substantial variability was observed in the TDD based on ADW in the obese group.
Conclusion: To achieve trough concentrations of 15-20 mg/L, obese patients dosed on ADW required TDD similar to non-obese patients dosed on TBW. Obese patients should be initially dosed based upon the recommended 30-40 mg/kg/day using ADW with follow-up monitoring.