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Evidence‐based approaches

Rationale

The main criterion for monitoring serum drug concentration is to ensure that there is enough drug given for efficacy while avoiding concentrations associated with a significant risk of toxicity (Roberts et al. [209]).

Indications

Accurate and timely monitoring of antimicrobial assay levels is indicated in patients receiving intravenous aminoglycoside or glycopeptide antibiotics to ensure optimum therapeutic range and to minimize high serum levels, which may cause adverse side‐effects of the drugs (particular caution should be exercised in patients who have renal impairment) (Roberts et al. [209]).

Contraindications

The samples should not be taken if there has been an insufficient time lapse since dose administration.

Principles of care

The initial dosage regimen should be appropriate for the clinical condition being treated, the patient's clinical characteristics (age, weight, renal function, etc.) and any concomitant drug therapy (Thomson [241]). The timing of the sample and interpretation of the results of analysis need consideration in relation to the dose given and the timing of previous dose administration.
Serum samples can be taken at two different times: the peak or the trough. A peak sample is collected at the drug's highest therapeutic concentration within the dosing period. The peak‐level timing will vary depending on the drug; ordinarily this is 1 hour after the completion of an infusion of aminoglycoside (Tobin et al. [242], Walker and Whittlesea [249]). The trough level can be taken 18–24 hours after the last dose.
Vancomycin trough levels are measured just prior to the administration of the next dose – that is, at the lowest concentration in the dosing period (Tobin et al. [242]). Therefore, trough levels are more commonly used than peak levels because the level is more representative of how the different variables (such as drug absorption and renal function) affect the concentration within a predetermined timeframe. The results can then be used to adjust dosages to achieve the optimal response with the minimal toxicity.
Abnormally elevated serum levels may be obtained if the samples are taken from a CVAD through which the drug has been administered. This is more likely when residual drugs remain in the catheter if it has not been flushed correctly following administration of the drug or when the first 5 to 10 mL of blood are not discarded (Himberger and Himberger [87]). If a multilumen CVAD is in situ, a different lumen from the one used to administer the drug should be used to obtain the blood specimen.
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