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TPMT Azathioprine Dosing — SCE Rheumatology MCQ

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ModerateRheumatology PharmacologyTPMT Azathioprine DosingSCE Rheumatology

A 42-year-old woman with systemic lupus erythematosus is to start azathioprine. Her baseline full blood count, renal function and liver biochemistry are normal, and she is not taking allopurinol or febuxostat. TPMT testing shows heterozygous intermediate enzyme activity. What is the most appropriate initial dosing approach?

Educational content. Not a substitute for clinical judgement or local policy.

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

Heterozygous TPMT variants usually produce intermediate enzyme activity, increasing exposure to active thioguanine nucleotides and the risk of azathioprine-induced myelosuppression. UK NHS pharmacogenomic guidance recommends starting at approximately 50% of the conventional dose, with subsequent titration according to clinical response and haematological tolerance. Full blood count monitoring remains essential because TPMT status does not predict every episode of toxicity. A standard dose is therefore inappropriate. Intermediate activity is not an absolute contraindication; avoidance is generally reserved for absent or severely deficient TPMT activity. A 25% dose is associated principally with the major xanthine-oxidase interaction when azathioprine must be co-prescribed with allopurinol. Repeating TPMT after starting treatment does not replace pre-treatment-guided dose reduction or ongoing blood monitoring.

Reference: NHS Genomics Education Programme, GeNotes: Patient requiring thiopurine methyltransferase genotyping for thiopurine treatment, reviewed 12 January 2026. https://www.genomicseducation.hee.nhs.uk/genotes/in-the-clinic/presentation-patient-requiring-thiopurine-methyltransferase-genotyping-for-thiopurine-treatment/