Thiopurine safety screening — ESEGH MCQ

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EasyGI PharmacologyThiopurine safety screeningESEGH

A 25-year-old man with Crohn's colitis is being considered for azathioprine after repeated courses of corticosteroids. His full blood count and liver biochemistry are normal, and he has not previously received a thiopurine. Which investigation is most appropriate before starting azathioprine?

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Correct answer: C — Thiopurine methyltransferase activity measurement

The correct answer is C. NICE recommends measuring thiopurine methyltransferase (TPMT) activity before offering azathioprine or mercaptopurine. Thiopurines should be avoided when TPMT activity is absent or very low, while a reduced starting dose should be considered for intermediate activity. A normal baseline full blood count does not exclude inherited susceptibility to myelotoxicity, and regular blood-count monitoring remains essential even with normal TPMT activity. NUDT15 variants also confer increased risk of early leucopenia and alopecia; the azathioprine SmPC states that NUDT15 genotyping may be considered, but this does not replace TPMT assessment. Serum amylase is not a routine pretreatment screen and is measured if pancreatitis is suspected. Faecal elastase assesses pancreatic exocrine insufficiency, HLA-B27 supports assessment of spondyloarthritis, and CT colonography has no role in thiopurine safety screening.

Reference: National Institute for Health and Care Excellence. Crohn’s disease: management (NG129), recommendation 1.2.8. 2019. https://www.nice.org.uk/guidance/NG129/chapter/recommendations