Assessment of an online calculator’s vancomycin dosing and exposure appropriateness in persons who inject drugs with methicillin‐resistant Staphylococcus aureus bloodstream infections

What is known and objective While the gold standard for calculating AUC involves two steady‐state concentrations, online calculators can empirically estimate AUC and other pharmacokinetic (PK) parameters. In patients with potentially altered PK, such as persons who inject drugs (PWID), the reliability of these predictions is unclear. Our objectives were to characterize the PK of vancomycin in PWID with methicillin‐resistant Staphylococcus aureus (MRSA) bloodstream infections (BSI) and to assess the impact of these PK parameters on dosing regimens when compared to regimens generated by an online calculator. Methods This descriptive pilot study included a retrospective chart review of 48 inpatient PWID with MRSA BSI from 30 April 2018 through 31 August 2020. Demographic and clinical data along with vancomycin dosing and serum concentrations were collected. Patient‐specific PK parameters were used to calculate the AUC of each empiric regimen compared with the originally predicted AUC. Results and discussion The study population had a median volume of distribution of 0.74 L/kg, clearance of 0.081 L/kg/h, elimination rate constant of 0.110/h and half‐life of 6.3 h. The online calculator empirically predicted 6 subtherapeutic and 42 appropriate AUC values with its recommended empiric dosing regimens. Using the actual patient‐specific PK parameters, the empiric vancomycin regimens actually resulted in 21 (43.75%) underexposures, 24 (50%) appropriate exposures and 3 (6.25%) overexposures. What is new and conclusions In PWID, empiric vancomycin dosing strategies suggested by an online calculator frequently resulted in lower‐than‐predicted vancomycin exposures. These findings suggest that PWID with MRSA BSI may require higher and/or more frequent vancomycin doses than those empirically recommended by the population‐based methods of an online calculator. This study population had a median volume of distribution of 0.74 L/kg, clearance of 0.081 L/kg/h, elimination rate constant of 0.110/h and half‐life of 6.3 h, which led to 43.75% of empiric vancomycin regimens resulting in underexposures. These findings suggest that this patient population may require higher and/or more frequent vancomycin doses than those empirically recommended by the population‐based methods of an online calculator.