Background Pharmacokinetic studies have suggested that total body weight (TBW) could be the optimal approach to calculate the dose of intravenous vancomycin (15–20 mg/kg TBW every 8–12 h) for target achievement (Css > 15 mg/L).¹ However, recent data concluded that the use of adjusted body weight (ABW) might be a better approach in obese patients.²

Purpose To determine which is the preferred method (TBW, ideal body weight (IBW) or ABW) to optimise vancomycin treatment in obese patients.

Material and methods Retrospective, non-interventional, observational study. Inclusion criteria: >18 years-old, body-mass-index (BMI) ≥30 kg/m², creatinine clearance ≥60 mL/min, and vancomycin TDM at steady-state. Non-obese patients were included as a control group. Patients were identified by reviewing TDM reports.

Vancomycin theoretical total daily doses (15–20 mg/kg) were calculated using TBW, IBW and ABW for each patient. They were compared with the dose used in our patients after TDM (Bayesian forecasting; PKSAbbot Software; target: Css > 15 mg/L) (TDM dose).

Dose differences greater than 10–12.5% of the TDM dose were considered unsuitable, since they could be related to clinical failure. Wilcoxon’s test analysis was performed using SPSS; (p < 0.05).

Results Forty obese patients: 35% men; 60.4 ± 12.6 years-old; BMI: 33.3 ± 2.7 kg/m².

Compared to the TDM dose and considering 15–20 mg/kg:

(i) Overdosage was observed in 72.5 (95%), 25 (47.5%) and 32.5 (72.5%) patients for TBW, IBW and ABW, respectively. Statistically significant differences were seen, with mean dose differences higher than 500 mg in the 20 mg/kg group.

(ii) Underdosage was seen in: 22.5 (5.0%), 75 (45%) and 67.5 (27.5%) respectively.

Statistically significant differences were seen with mean dose differences lower than 400 mg in the 15 mg/kg group.

No relevant differences were observed in the control group.

Conclusion Compared to the TDM dose, a high incidence of overdosage would be observed by using TBW. In our obese patient cohort, ABW might be the best approach to set the dose of intravenous vancomycin (15–20 mg/kg), as already seen in previous literature.²

References

1. Rybak M, Lomaestro B, Rotschafer JC, et al. Therapeutic monitoring of vancomycin in adult patients: a consensus review of the American Society of Health-System Pharmacists, the Infectious Diseases Society of America, and the Society of Infectious Diseases Pharmacists. Am J Health Syst Pharm 2009;66:82–98

2. Leong JV, Boro MS, Winter M, et al. Determining vancomycin clearance in an overweight and obese population. Am J Health Syst Pharm 2011;68:599–603

ReferencesNo conflict of interest.