P149 Routine monitoring of rifampicin levels reveals subtherapeutic levels in 76% of patients with fully sensitive TB

<h3>Introduction</h3> Rifampicin plays a key role in TB treatment, due to its bactericidal and sterilizing capacity. Literature suggests that low serum rifampicin levels are associated with treatment failure, relapse and acquired drug resistance. Yet, therapeutic drug monitoring is not routine in active TB treatment. In October 2021, our service began routine monitoring of rifampicin levels 2 weeks after treatment initiation in order to support early optimisation of dosing. Levels below 8 mg/L were considered subtherapeutic. <h3>Methods</h3> We performed a retrospective review of all rifampicin levels taken between September 2021 and May 2022, identifying 38 patients. Only those with documented exact timings of rifampicin levels were included (8 excluded). We compared our practice in the 2 years prior to routine monitoring. <h3>Results</h3> Of the 30 patients, only 7 (23.3%) had therapeutic rifampicin levels. 13 (43.4%) had levels between 4–8, and 10 (33.3%) had levels &lt;4. All but one of the rifampicin levels were taken between 1 hr 30 and 3 hrs 40 mins. Of those with subtherapeutic levels, 20 (87.0%) had their dosage increased. Of the patients who had initial levels &lt;4, all levels remained &lt;8 after one dose increase. 4 patients required two dose increases before achieving levels &gt;8. Of the 13 patients who had initial levels 4–8, 11 had their levels repeated after dose increase – 7 patients had levels &gt;8 after one dose increase. 4 patient’s levels remained subtherapeutic, requiring further dose increases. 22 (73.3%) of patients had pulmonary TB. 3 patients had smear positive TB and initial levels &lt;4 – all remained smear positive at 14 days. Of the patients who had initial levels 4–8, 4 were smear positive, 2 of which remained smear positive at 14 days. In 2020, rifampicin levels were only taken in 13 of 57 patients. Levels were taken in those with concerns around clinical response, compliance and malabsorption. 61.5% of patients had levels &lt;4 and 38.5% had levels &gt;8. <h3>Conclusions</h3> Optimisation of rifampicin dosage has the potential to improve treatment outcome and may shorten treatment duration. 76.7% of our patients had subtherapeutic rifampicin levels, which instigated early optimisation of dosing without additional side-effects.