Estimating drug-free period using a graphical method: an alternative way to monitor extended-interval dosing of gentamicin therapy

OBJECTIVES: Published nomograms to monitor extended-interval dosing (EID) gentamicin therapy were based on a fixed dose of 5 or 7 mg/kg. However, the average dose used for EID gentamicin regimen in our setting was about 3 mg/kg per day. We developed a new method of monitoring based on the duration of drug-free period (DFP) in a 24-h dosing interval.

METHODS: Hospitalised adult patients on EID gentamicin were selected. We considered a DFP of between 2 and 8 h as appropriate. Data from two blood samples (2 and 6 h postdose) from each patient were used to estimate the duration of DFP (i.e. DFP method 1). DFP was also calculated for the same patient using an empirically estimated elimination rate constant (Ke ) and the same 6 h postdose concentration value (DFP method 2). Correlation between the two methods was made. An alternative graphical method to estimate DFP was attempted.

KEY FINDINGS: Correlation between Ke and age was favourable (r = -0.453; P = 0.001). Ke derived from this empirical relationship was used to estimate DFP method 2. DFP method 1 correlated well with DFP method 2 (r = 0.742; P