Aminoglycoside monitoring: timing of peak levels is critical.

Recommendations for optimal therapeutic peak concentrations of aminoglycosides are often not differentiated with respect to duration of infusion and timing of peak sample thereafter. To document the relevance of the timing, 139 dose intervals were analyzed in 58 patients during administration of gentamicin, amikacin, and netilmicin. Serum concentrations measured immediately after 30-min infusions were compared with concentrations obtained 90 min later (2 h values). The ratio of 30 min/2 h concentrations showed considerable variability. This ratio was less than 1.5 in 15% of the dose intervals analyzed and greater than 3 in 8% of the intervals. The poor correlation between concentrations measured at 30 min and at 2 h was documented by the coefficients of variation of 0.82, 0.30, and 0.67 for gentamicin, amikacin, and netilmicin, respectively. This variability was not explained by interindividual differences, renal function, or drug half-life. However, the initial decrease in concentrations was significantly lower in patients with impaired renal function (p less than 0.001). These data suggest that timing is critical for the sampling of serum to determine peak levels in patients and the definition of optimal therapeutic concentrations.