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Azathioprine TPMT Dosing — SCE Rheumatology MCQ

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EasyRheumatology PharmacologyAzathioprine TPMT DosingSCE Rheumatology

An adult with systemic lupus erythematosus is about to start azathioprine for maintenance treatment. Baseline full blood count, renal function and hepatic function are normal, and the patient is not taking allopurinol. TPMT genotyping shows one functional and one non-functional allele, predicting intermediate TPMT activity. What is the most appropriate initial dosing strategy?

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Correct answer: A — Start at approximately 50% of the usual dose and titrate according to response and toxicity with frequent full blood count monitoring

A TPMT heterozygote is an intermediate metaboliser and is at increased risk of excessive thioguanine-nucleotide exposure and azathioprine-induced myelosuppression. A reduced initial dose—commonly about 50% of the usual dose—should therefore be used, followed by cautious titration according to efficacy, blood counts and other toxicity. TPMT testing does not replace regular full blood count monitoring. Heterozygosity is not an absolute contraindication; avoidance or a much larger reduction is generally reserved for patients with little or no TPMT activity. A 25% dose is not the standard adjustment for isolated TPMT heterozygosity and is more familiar as the azathioprine adjustment required with concomitant allopurinol. Full-dose or increased-dose initiation would expose this patient to an avoidable risk of severe cytopenia.

Reference: Matic M, et al. Dutch Pharmacogenetics Working Group guideline for the gene-drug interaction between TPMT/NUDT15 and thiopurines. 2025. https://www.nice.org.uk/guidance/NG129/chapter/recommendations