ABSTRACT Testing thiopurine methyltransferase (TPMT) enzyme activity or genotype prior to thiopurine prescribing is recommended to reduce the risk of moderate to severe—and potentially fatal—myelosuppression in poor or intermediate TPMT metabolizers. Despite this, only about one‐third of individuals prescribed thiopurines in Australia currently receive TPMT testing. The budgetary implications of expanding testing to align with guidelines remain unclear. We conducted a budget impact analysis from the Australian healthcare system perspective, comparing costs under current versus increased TPMT testing uptake. Phenotype frequencies among thiopurine users were estimated using ancestry‐stratified prescribing data from the Person Level Integrated Data Asset of the Australian Bureau of Statistics combined with published phenotype distribution by ancestry. A simulation model was developed, incorporating phenotype frequencies, phenotype‐specific hospitalization risks, and costs of testing and hospitalizations. In a hypothetical cohort of 10, 000 thiopurine users, current testing rates of 32. 5%–39. 8% identify approximately 296 poor or intermediate metabolizers, leaving 586 individuals at elevated risk undetected. Increasing testing uptake by 10 percentage points from baseline could prevent 16 hospitalizations and save AUD88, 113 in hospital costs, leading to a mean net saving of AUD42, 728 (95% CI: AUD41, 685—AUD43, 770). The number needed to test to prevent one hospitalization was approximately 63. As myelosuppression represents a serious and potentially life‐threatening adverse drug reaction, expanding TPMT testing offers a cost‐effective, high‐yield strategy to enhance patient safety and reduce preventable healthcare burden. These findings support more systematic integration of pharmacogenomic testing into routine thiopurine prescribing in Australia.
Ianni et al. (Fri,) studied this question.