A simple method to estimate the initial dose of gentamicin.

A simple equation to estimate the initial dosing interval of gentamicin was compared with the "rule of eights" equation in 81 adult patients with stable renal function. The Sawchuk-Zaske method was employed as the standard with which dosing intervals estimated by the simple equation and the rule of eights method were compared. The actual mean peak gentamicin concentration was not significantly different from that predicted by the Sawchuk-Zaske method. The actual mean trough concentration was significantly higher than the predicted concentration, but trough concentrations within the usual therapeutic range of 0.5-2 microgram/ml were achieved in more than 90% of patients. The mean dosing interval calculated by Sawchuk-Zaske was not significantly different from that calculated by the equation, but was longer than the one calculated by the rule of eights. A peak gentamicin concentration within the usual therapeutic range was generally achieved with a total daily dose of 3-5 mg/kg of actual body weight. Until gentamicin concentrations are known, an easily remembered equation can be used to estimate the initial dosing interval for gentamicin. When used together with a standard total daily dose of 3-5 mg/kg total body weight, usual therapeutic concentrations are achieved in the majority of patients.