Monitoring of serum aminoglycoside levels with once-daily dosing.

There is considerable confusion as to how to monitor serum aminoglycoside levels when using once-daily dosing. At least five methods are in use in Australia. We prospectively assessed 100 consecutive once-daily courses of gentamicin or tobramycin, during which 120 pre-dose and 213 sets of immediate post-dose and six hour post-dose levels were taken. By using the six hour post-dose level we were able to compare dosage recommendations made using methods known as ALADDIN, DOSECALC and the Australian Antibiotic Guidelines nomogram (AAGN). There were statistically significant differences in the doses recommended by each method. When comparing each of the three methods, at least 25% of dosage recommendations differed by more than 80 mg per dose. Although we have not been able to determine the clinical significance of these differences, we are concerned that methods used in dosage adjustment of aminoglycosides differ so widely in their recommendations. Presumably the ALADDIN method, which utilises two post-dose levels to determine an area under aminoglycoside concentration-time curve, gives more accurate pharmacokinetic information than methods which rely on a single level. Comparative cost-effectiveness studies of different methods, although in practice difficult to perform, should be undertaken to resolve the optimal management of patients receiving aminoglycosides once-daily.