Audit of therapeutic drug monitoring of dosing of vancomycin on an intensive care unit

Introduction

Vancomycin is used to treat infections in critically ill patients. Although serum levels are often measured, the relationship between high plasma levels and renal and ototoxicity is not proven [1] Doses of 1 g achieve peaks of around 25-40 µg/ml and toxicity is unlikely in this range. Troughs of between 5-10 µg/ml are thought to be necessary to ensure efficacy [2]. Due to age and renal function, patients on the general ICU at St George's are routinely prescribed 1 g once a day at 6.00 pm and levels are taken the next morning which permits reporting before the next dose is due. The next dose will only be administered if the level is less than 10 µg/ml. There is no data supporting this practice.

Method

Data was collected over a 6 month period for patients who received once a day dosing of vancomycin for at least 3 days. A pharmacokinetic computer program used the measured plasma vancomycin level at 12 hours post dose to predict the level at 24 hours post dose. Renal function and whether a further dose was administered at 24 hours were recorded.

Results

Twenty-one patients (13 male: 8 female with an average age of 65 years) were audited. Ten patients had some degree of renal impairment (Cr > 110 mmol/l). One hundred and seventy-one doses were audited and results are shown in Table 1.

Table

All levels predicted within 5-10 µg/ml at 24 hours were above 10 µg/ml at 12 hours, resulting in only one of the 23 doses being given. All levels within 5-10 µg/ml at 12 hours were sub-therapeutic at 24 hours.

Conclusion

When monitoring vancomycin levels at 12 hours the target concentration should be above 10 µg/ml to prevent sub-therapeutic plasma level occurring in the later part of the dosing period.