Rapid gentamicin elimination in obstetric patients.

Wide interpatient variation in gentamicin elimination rates and dosage requirements was demonstrated in 67 obstetric patients with normal serum creatinine levels. A poor correlation occurred between the elimination rate of gentamicin and creatinine clearance (r = .23). A higher correlation was found between the drug's elimination rate and distribution volume (r = -.62), which suggests that the changing fluid status associated with pregnancy affects the kinetic parameters of gentamicin. Gentamicin dosage regimens were systematically calculated for each patient from serum concentration-time data to obtain desired serum levels. These individualized regimens demonstrated considerable variability, ranging from 3.0 to 11.6 mg/kg/day. Clinical ototoxicity and nephrotoxicity were not observed. Because of the wide interpatient variation, these increased dosages are not routinely suggested unless serum concentrations are previously measured and each patient's regimen is calculated. These results emphasize the need to measure serum concentrations and make necessary dosage adjustments to ensure therapeutic levels in obstetric patients.