Rifampin levels in daily practice: The accuracy of a single measurement

Background: Measurement of rifampin levels is not part of routine practice. However, low levels are associated with failure of tuberculosis treatment. The clinical relevance of serum levels in daily practice is unclear. The objective was to evaluate rifampin serum concentrations and factors associated with insufficient concentrations. Methods: Patients with at least one rifampin concentration drawn 3 hours after intake (C-3) between 2005 and 2014 were included. Data on demographic and clinical characteristics were collected, including side effects and dose adjustments. Two different criteria were used to define adequate concentrations (criterion 1: C(3 )and C-6 >= 3 mg/l; criterion 2: C-3 or C-6 >= 5 mg/l). Results: Of 63 patients, 66% and 76% had a sufficient level according to criterion 1 or 2, respectively. C(3 )exceeded C-6 in most patients, while a late maximum was significantly associated with diabetes mellitus (p = 0.003). A dose adjustment was made in 19% of cases, more frequently in patients with insufficient levels (p = 0.02) or with 2 side effects (p = 0.03). Conclusion: Rifampin levels varied but were mostly adequate and a single measurement at 3 hours after intake provided the required information in most cases, indicating that full AUC(0.24) measurements could be limited to specific situations.