Monitoring of gentamicin serum concentrations in obstetrics and gynaecology patients in Namibia

Background Therapeutic drug monitoring is frequently used to optimize the gentamicin dose. Objective The study investigated whether a 240 mg once daily standard dose achieves the recommended target serum gentamicin concentrations. Setting The prospective, observational study took place in the 2 major public hospitals in Namibia. Method Twenty-nine female patients receiving a standard dose (240 mg gentamicin once daily) participated in the study. Two blood samples were withdrawn to estimate gentamicin pharmacokinetic parameters. Serum creatinine was used to calculate creatinine clearance with the Cockcroft–Gault formula (CL_cr), and estimate glomerular filtration rate (eGFR) by the Modified Diet in Renal Disease (MDRD) and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equations. Main outcome measure The outcome measure was the proportion of patients receiving 240 mg gentamicin once daily having C_max values above 15 mg/L. Results Total body weight (TBW) and body mass index were highly variable: 43–115 kg, and 17.3–41.3 kg/m², respectively. The gentamicin dose normalized for TBW (adjusted body weight for obese patients) was relatively low, i.e. 4.2 ± 0.8 mg/kg (mean SD). Gentamicin C_max was 14.4 ± 4.7 mg/L; only 9 patients (31%) had a C_max > 15 g/mL. eGFR (MDRD-4) correlated well with CL_cr, but eGFR (EPI-CKD) formula showed systematic deviations from CLcr. Conclusions (1) a standard 240 mg dose results in gentamicin C_max values below 15 mg/L in the majority of the patients, (2) eGFR formulas to estimate kidney function will have to be evaluated for their usefulness in the Namibian patient population.