1. Proper timing of serum sampling is critical.
The trough sample should be obtained 30 minutes prior to the dose. Measure the peak level 15 to 30 minutes after completion of the IV infusion to avoid the distributive phase. Measure the peak level 90 minutes after an IM injection. Drawing the peak too soon will result in inaccurate analysis.
Drawing at exactly the right time is not as important as having the lab note the exact times that the samples were drawn. Also, have the nurse note the exact times that the sample infusion was started and when it ended. Please be aware of the widespread policy of nursing personnel to record a dose as having been given exactly as ordered if it is given within 30 minutes of the recorded time. This will lead to significant errors in analysis, have everyone record the exact times.
This issue cannot be stressed enough. Inaccurate recording of drug administration times and lab draw times are the greatest source of calculation error, having a greater effect than pharmacy preparation error or lab assay error.
2. Bayesian cautions
|In general the Bayesian approach to the determination of individual drug-dosage requirements performs better than other approaches. However, outlying patients in a population (ie, those patients whose pharmacokinetic parameters lie outside of the 95th percentile of the population) may be put at risk. As is always the case, the computerized algorithms outlined below can only assist in the decision-making process and should never become a substitute for rational thought or informed judgment.
|Although a single trough level can be evaluated with the Bayesian approach, more accurate results are obtained when both peak and trough levels are measured.
|Steady-state levels are required for accurate Bayesian analysis.
3. Extended interval method cautions
Extended interval dosing is not recommended in:
|patients with CrCl less than 30
|greater than 20% BSA burns
|history of hearing loss or vestibular dysfunction
|gram positive infections (when the aminoglycoside is used for synergy)