Usefulness of vancomycin serum concentration monitoring in the critically ill patient

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Introduction

The aim of this study was to compare two methods, the serum concentration monitoring (SCM) and the nomogram of Moellering (NM), employed to calculate the dose of vancomycin (V) in the critically ill patient.

Methods

We studied 32 patients admitted to our ICU and treated with V for Staphylococcus aureus infections. The patients were randomly divided into two groups: A (16) and B (16). In group A the dose of V was calculated by the SCM (fluorescence polarization immunoassay technique) on two blood samples drawn daily just before the first administration of the drug [to determine the minimal serum concentration (C min)] and 30 min after the infusion of V has been stopped [to determine the maximal serum concentration (C max)]. In group B the daily dose of V was calculated on the basis of the NM even though the SCM was also performed but its result was not known. We fixed the therapeutic range of V between 5 and 10 mg/l for C min [1].

Results

In group A, C min of vancomycin was inside the therapeutic range in all patients: In group B it was above 10 mg/l in 4 patients (14.3 ± 2.7) and below 5 mg/l in 3 patients (3.9 ± 0.3) (Table 1). C max was 20.4 ± 3 mg/l in group A and 20.1 ± 6 mg/l in group B.

Discussion

The results obtained show that the SCM is a useful tool to maintain the serum concentration (SC) of vancomycin inside the therapeutic range, while the NM is not so effective. The possibility of maintaining the right SC of V is essential to prevent both the side effects from overdosing (oto, nephro-toxicity) and underdosing the drug (uneffective antibiotic activity, bacterial resistance), but it can be problematic in the critically ill patient whose physiopathological characteristics may interfere with the pharmacokinetics of the drug. In the 4 patients who showed a C min of vancomycin above 10 mg/l we did not find any correlation between the elevated level of V and the clinical conditions; in the 3 cases with C min below 5 mg/l we observed an increase in the volumes of distribution because of peritonitis and fluid overload.

We conclude sustaining the usefulness of the serum concentration monitoring to establish the right dose of vancomycin in the critically ill patient, and underlining that the clinical conditions characterized by the increase in the volumes of distribution represent a risk factor of underdosing the drug.

Table 1 Number of patients with the minimal serum concentration (C min) of V below, between and above the limits of the therapeutic range.