Utilization and associated factors of TPMT testing among Australian adults receiving thiopurines: A national retrospective data‐linkage study

Introduction Thiopurine drugs are metabolized by thiopurine methyltransferase (TPMT) and low TPMT activity can result in severe adverse drug reactions. Therefore, TPMT testing is recommended for individuals receiving thiopurines to reduce the risk of toxicity. Objectives The objectives of this study were to assess the rate of TPMT testing among individuals receiving thiopurines and explore factors associated with undergoing TPMT testing in Australia. Methods This retrospective cohort study utilized administrative data from the Pharmaceutical Benefits Scheme (PBS), Medicare Benefits Schedule (MBS), and the 2021 Census, accessed via the Person Level Integrated Data Asset (PLIDA) at the Australian Bureau of Statistics (ABS) DataLab. Individuals receiving thiopurines aged 18 years or above were identified using PBS data and exposure to TPMT testing was determined using MBS data. Multivariate logistic regression was performed to identify factors associated with TPMT testing. Results A total of 62,574 prevalent thiopurine users were identified between 2020 and 2022. Of these, 20,327 (32.5%) underwent TPMT testing (2011–2022). The most significant factor associated with TPMT testing was having at least one thiopurine medication prescribed by a medical specialist (adjusted odds ratio [aOR] 2.12, 95% confidence interval [CI] 2.02–2.22), compared to having medication solely prescribed by primary care physicians (PCPs). Other significant factors associated with TPMT testing included speaking a non‐English language at home (aOR 1.29, 95% CI 1.22–1.36), having no chronic health conditions (aOR 1.18, 95% CI 1.13–1.24), not requiring assistance with core activities (aOR 1.16, 95% CI 1.08–1.23), and having a higher educational attainment (aOR 1.11, 95% CI 1.06–1.11). Compared to living in major cities, individuals living in remote areas were significantly less likely to undergo testing (aOR 0.49, 95% CI 0.39–0.60). Conclusion Our study highlights the low utilization of TPMT testing in Australia and suggests the need for targeted interventions to address disparities and improve TPMT testing.