To report pharmacokinetic alterations and optimal dosing of piperacillin/tazobactam in an obese patient.

A 39-year-old morbidly obese (weight 167 kg, body mass index 50 kg/m²) man was treated with piperacillin/tazobactam 3.375 g every 4 hours for recurrent cellulitis. The wound culture grew Groups A and B Streptococcus and rare Pseudomonas aeruginosa. Blood samples were obtained at steady-state from a peripheral venous catheter at 0, 0.5, 1, 2, 3, and 4 hours after the start of the infusion. Population pharmacokinetics were generated from a previously published data set. The serum concentrations of piperacillin/tazobactam obtained in the patient were compared with the 95% confidence interval from the representative population. Pharmacokinetic parameters such as maximal serum concentration, minimal serum concentration, average steady-state concentration, half-life, elimination rate constant, volume of distribution (V_d), clearance, area under the curve at steady-state, and percent of time greater than the minimum inhibitory concentration (%t>MIC) were calculated and qualitatively compared between the sample and the population.

Substantial differences were noted in both the absolute values at the times of sample collection and the overall concentration-versus-time profile of both compounds. The morbidly obese individual compared with the population demonstrated a reduced average serum steady-state concentration: 39.8 mg/L versus 123.6 mg/L, an increased V_d: 54.3 L versus 12.7 L, and an increased half-life: 1.4 hours versus 0.6 hours, respectively. The %t>MIC of piperacillin for the patient, assuming MICs of 2, 4, 8, 16, 32, 64, and 128 mg/L, was 100%. 100%, 90.9%, 55.4%. 19.9%, 0%, and 0%, respectively.

Pathogens with elevated MICs may require altered dosing schemes with piperacillin/tazobactam. Future studies are warranted to assess increased dosages, more frequent dosing intervals, or continuous infusion dosing schemes for obese individuals with serious infections.