Objective: To report a case in which vancomycin clearance was used to determine the daptomycin dosing interval in a morbidly obese patient with renal impairment.
Case summary: A 46-year-old man (209 kg; 178 cm) failed a 42 day course of vancomycin therapy for treatment of a methicillin-resistant Staphylococcus aureus-infected wound and cellulitis. The median trough vancomycin concentration was 12.6 microg/mL (range 7.3-24.1) through his course of therapy. Estimation of creatinine clearance (Cl(cr)) was confounded in this clinical scenario, given the patient's weight and a lack of valid equations in this patient population. Daptomycin was administered empirically at 6 mg/kg dosed every 48 hours based on estimated clearance from measured vancomycin concentrations. Steady-state plasma concentrations of daptomycin were determined, and the daptomycin half-life in this patient was more accurately estimated using vancomycin clearance as a surrogate. In addition, a 4 mg/kg dose of daptomycin would have been sufficient based on plasma concentrations. The patient demonstrated rapid clinical improvement and remained free of cellulitis for 6 months after completion of daptomycin and a 12 week course of trimethoprim/sulfamethoxazole.
Discussion: The dosing interval of daptomycin is adjusted based on Cl(cr). However, estimation of Cl(cr) is difficult in morbidly obese patients with renal impairment, given a lack of valid equations. In this clinical scenario, vancomycin concentrations were used to estimate Cl(cr) and served as a surrogate measure to determine the daptomycin dosing interval. Measured daptomycin concentrations validated this approach and confirmed the inadequacy of commonly used Cl(cr) equations.
Conclusions: In this clinical scenario, vancomycin concentrations more accurately estimated Cl(cr), thereby facilitating determination of the daptomycin dosing interval.