Gentamicin therapeutic level monitoring was studied in 82 Thai patients initially treated with traditional physician-determined dosage regimen method. The number of patients treated by IM, IV infusion and IV push were 49, 27 and 6 respectively. The majority of the patients had subtherapeutic trough concentrations. The percentage of patients with therapeutic peak and trough concentrations were found lower in impaired renal function (Scr ≥ 1.10) group than patients with normal renal function. In addition, the percentage of patients with overtherapeutic trough concentrations were much higher. The result indicated that these patients required special care in drug level monitoring. The dosage regimen of forty-four patients were adjusted using pharmacokinetic (Sawchuk-Zaske) method and the adjusted drug was given by IV infusion in all cases. Patients who initially received gentamicin by IV infusion method will give a better prediction when the adjustment of the dosage regimen was required as seen from the higher percentage of patients whose peak and/or trough concentrations were within the therapeutic range when compared with patients who initially received gentamicin by IM or IV push. Gentamicin when administered with pharmacokinetic calculated dosage regimen resulted in higher percentage of patients whose serum gentamicin concentrations were within the therapeutic range as compared to gentamicin when administered with traditional physician dosage regimen. Pharmacokinetic parameters were required for Sawchuk-Zaske equations used to predict serum gentamicin concentrations and/or suitable dosage regimen. These parameters should be properly chosen. When serum gentamicin concentrations were known, the best result will be obtained if both the elimination rate constant (Kel) and volume of distribution (Vd) were calculated from at least two known serum concentrations. If serum gentamicin concentrations were not known, the elimination rate constant calculated from equation pregenerated from Thai population should give a better result than the elimination rate constant which calculated from published equation pregenerated from foreign population.
