Use of dried plasma spots to monitor rifampicin concentrations in resource-constrained settings

Tuberculosis remains to be the leading cause of mortality amongst infectious diseases with rifampicin being the cornerstone of treatment. However, subtherapeutic concentrations of rifampicin can contribute to treatment failure and drug resistance. Therapeutic drug monitoring (TDM) is essential but challenging in resource-limited settings. This study evaluated the analytical technique of rifampicin quantification using high-performance liquid chromatography (HPLC) from dried plasma spots (DPS), followed by a clinical validation comparing DPS and plasma samples to quantify rifampicin in patients with TB.An HPLC (with ultraviolet detector) assay was developed and validated to estimate rifampicin from DPS (prepared from Whatman^® Grade 1 filter paper). Clinical validation involved 61 patients with TB, where the association and bias of rifampicin concentration between DPS and its respective plasma samples were estimated using weighted Deming regression and Bland-Altman analysis.The DPS assay demonstrated an acceptable overall bias (range: -10.2% to 8.5%) and precision (intra-day and inter-day coefficient of variation < 7%) at different concentrations. Stability of rifampicin in DPS samples was established for up to 12 days at room temperature. Clinical validation confirmed the need for a correction equation to predict plasma rifampicin concentration from the DPS concentration. The predictive model had a mean bias of 0.27 µg/mL, sensitivity of 93.8%, and specificity of 91.1% for identifying therapeutic concentrations (8-24 µg/mL).DPS using Whatman^® Grade 1 filter paper is a reliable and cost-effective alternative sampling strategy for TDM of rifampicin, particularly in resource-limited settings..