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1. Proper timing of serum sampling is critical.
The trough sample should be obtained 30 minutes prior to the dose. Measure the peak level 60 minutes after completion of the IV infusion to avoid the distributive phase. 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, therefore ask all lab and nursing personnel to 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. Vancomycin accumulation
Recent data have shown that prolonged treatment with vancomycin (>10 days) may result in a decline in the drug’s clearance despite stable renal function. Given this risk of decreased elimination, close monitoring of serum levels is advisable even in patients with normal and stable renal function.
3. Caveats about using a 1-compartment model
Vancomycin has a relatively long distribution phase and is usually characterized with a 2-compartment model. However, after the 1 to 2 hour distribution phase, it collapses to a 1-compartment model. Therefore, if peak serum levels are drawn and targeted for at least one hour after the infusion, a 1-compartment model works quite well. It is important to note that your chosen model parameters must fit within this framework.
The volume of distribution in the default 1-compartment vancomycin model corresponds to the one-hour post-infusion peak. Therefore, the commonly cited vancomycin Vd of 0.7-0.9 L/kg would be inappropriate, hence the default value of 0.5 L/kg. The same is true of the Kel equation. In this model, the Kel is the slope of the elimination and distribution phase starting at 1-hour after the infusion. Much of what is cited in the literature refers to the terminal elimination phase only. If you wish to use these literature values in your model, then draw and target your peak levels for at least two hours post-infusion.
Please keep these points in mind when you are drawing serum levels, or if you desire to create your own vancomycin model.
4. 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.