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ANCA-associated vasculitis relapse — SCE Rheumatology MCQ

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HardVasculitisANCA-associated vasculitis relapseSCE Rheumatology

A patient taking azathioprine 150 mg daily for systemic vasculitis develops recurrent tophaceous gout. Allopurinol is considered essential after specialist discussion. If the combination cannot be avoided, which azathioprine adjustment is specified in the UK SmPC?

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

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Correct answer: D — Reduce azathioprine to 25% of the original dose and monitor the full blood count closely

Explanation lettering: B = shown as A · E = shown as B · D = shown as C · A = shown as D · C = shown as E

A is correct. Allopurinol inhibits xanthine oxidase and can cause profound, sometimes fatal, thiopurine myelotoxicity. The combination should be avoided where possible; if essential, the UK azathioprine SmPC specifies reduction to one quarter of the original dose—150 mg becomes 37.5 mg daily—with close haematological surveillance. B leaves a dangerously high dose and monitors the wrong outcome. C ignores the shared metabolic pathway. D reverses the interaction. E treats baseline pharmacogenetics as a substitute for serial blood counts; TPMT or NUDT15 information does not remove the acquired allopurinol interaction. Febuxostat also inhibits xanthine oxidase and is not a simple workaround.

Reference: UK azathioprine summary of product characteristics: https://www.medicines.org.uk/emc/product/11142/smpc