TPMT Azathioprine Dosing — SCE Rheumatology MCQ

Correct answer: B — Start at approximately 50% of the standard target dose with close blood-count monitoring

The correct answer is B. Heterozygosity for a reduced-function TPMT allele usually produces intermediate enzyme activity. Reduced TPMT-mediated inactivation increases exposure to cytotoxic thioguanine nucleotides and therefore the risk of leucopenia and other myelotoxicity. UK guidance recommends starting thiopurines at about 50% of the usual dose in this group, followed by cautious titration and close full blood count monitoring. Azathioprine is not automatically contraindicated in a TPMT heterozygote; avoidance applies principally to absent or very low TPMT activity. A full starting dose carries excessive toxicity risk, and increased monitoring does not substitute for dose reduction. A 25% dose is generally used for other circumstances, such as co-prescription with allopurinol, rather than isolated intermediate TPMT activity. TPMT testing also does not eliminate the need for ongoing haematological monitoring.

Reference: NHS England Genomics Education Programme, Patient requiring thiopurine methyltransferase genotyping for thiopurine treatment, reviewed 2026, https://www.genomicseducation.hee.nhs.uk/genotes/in-the-clinic/presentation-patient-requiring-thiopurine-methyltransferase-genotyping-for-thiopurine-treatment/