Comparison of dosing strategies in obese patients prescribed intravenous acyclovir and evaluation of rate of acute kidney injury

There is limited guidance on the most appropriate dosing strategy for intravenous (IV) acyclovir in obese patients. The manufacturer's labelling suggests using ideal body weight (IBW); however, previous pharmacokinetic studies of obese patients have shown more rapid systemic clearance and lower area under the curve and peak concentrations compared with patients with a body mass index (BMI) < 30 kg/m2. Although pharmacokinetic data suggest that plasma concentrations of acyclovir are best predicted when using adjusted body weight (AdjBW) doses, there is concern about higher rates of acute kidney injury (AKI). This was a retrospective cohort review of adult patients with a BMI ≥ 30 kg/m2 prescribed IV acyclovir ≥ 48 hours between 1 January 2014 and 31 August 2021 at a 511-bed academic medical centre. The primary objective was to compare AdjBW with IBW dosing in obese patients who had been prescribed IV acyclovir and to determine whether AdjBW dosing results in higher rates of AKI. Ninety-four patients were included: 61 were in the IBW cohort and 33 were in the AdjBW cohort. The median BMI [IQR] for all patients was 34.7 kg/m2 [31.8–40.6]. Patients in the AdjBW cohort received a significantly higher median acyclovir dose of 800 mg/dose [IQR 700–850] compared with 600 mg/dose [IQR 500–700] for the IBW cohort (P ≤ 0.0001). No patients dosed using AdjBW developed AKI compared with eight (13.1%) in the IBW group. In this study, 8.5% of all obese patients receiving acyclovir developed AKI. Further studies are needed to confirm dosing recommendations.