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Thrombocytosis — RACP Adult Medicine MCQ

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HardHaematologyThrombocytosisRACP Adult Medicine

A 45-year-old man is found to have a platelet count of 850 × 10⁹/L on routine blood tests. He is asymptomatic. His haemoglobin and white cell count are normal. Peripheral blood film confirms thrombocytosis with no dysplastic features. What is the most important initial investigation?

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Correct answer: C — BCR-ABL1 fusion gene (Philadelphia chromosome)

While JAK2 testing is important in suspected MPN, the first step in evaluating unexplained thrombocytosis is to exclude CML by testing for BCR-ABL1 (Philadelphia chromosome). CML can present with isolated thrombocytosis and misdiagnosis as ET has significant treatment implications. If BCR-ABL1 is negative, proceed with JAK2 V617F, and if negative, CALR and MPL mutation testing. Reactive causes should also be excluded.

Reference: eTG – 2025 – Haematology; BSH – 2021 – ET Guidelines