In an effort to achieve accuracy, oftentimes clinicians are hesitant to check first dose peak and trough levels for aminoglycosides. The tendency is to obtain peak and trough levels after the 3rd dose. The rationale behind the timing here, is to ensure that the results are steady state results and therefore reliable.
This strategy may well be appropriate for non-lifethreatening infection and for Q 8hr or Q12 hr dosing intervals. However, when dealing with serious infection, valuable time may be lost if the initial estimated dose is inadequate. This is especially true if the dosing interval is Q16 hr or more. This means 2-3 days could elapse before checking levels only to learn that the dose is inadequate. This could be critical in patients with serious infection. This type delay is likewise problematic if the initial dose is toxic.
We utilize a more cautious approach. Here, we obtain a first dose peak and trough and calculate our pharmacokinetic parameters i.e. volume of distribution and t1/2. Using the ratio of accumulation, we can then predict with a high degree of certainty, the steady state peak and trough estimates. This allows us to make adjustments on day 1 to ensure therapeutic levels at the start of therapy which can oftentimes reduce hospital stay by achieving therapeutic effects sooner. In addition, this approach reduces the likelihood of exposing the patient to elevated levels during this 2-3 day time period.