Purpose: Oseltamivir is indicated for the treatment and prophylaxis of Influenza infections. Achieving therapeutic concentrations EARLY in the course of the infection impacts greatly on the magnitude of benefit. Oseltamivir is renally cleared and dose reductions are advised for patients with renal impairment. The purpose of this review was to determine if these dose reductions facilitate the early attainment of therapeutic concentrations. The review also examined the effect of body mass on the same outcome.

Method: Oseltamivir is administered as a pro-drug and converted to the active carboxylate moiety in the liver. Published papers that included Oseltamivir Carboxylate pharmacokinetics in patients with renal impairment and those with large body mass were reviewed. Concentrations of Oseltamivir Carboxylate achieved in the first 24h were compared with those from patients with normal renal function and body mass.

Results: Studies that informed dosage regimens for patients with mild to moderately impaired renal function focussed on attaining steady state concentrations similar to those observed in patients with normal renal function. They overlooked the importance of achieving therapeutic concentrations EARLY in the course of the infection. As a result, many patients will not attain therapeutic concentrations until too late in the infection. This is also true for patients with a large body mass.

Conclusions: Current dosing advice for Oseltamivir in patients with mild to moderate renal impairment and those with a larger body mass are likely to reduce (or even negate) its efficacy. The first dose should be 75mg for patients with normal body mass and proportionately larger when body mass is larger. Subsequent doses should be reduced in proportion to the degree of renal impairment. Timely therapeutic drug monitoring can provide invaluable dosing (and other) information to the clinician treating patients with Influenza and could improve patient outcomes.