Heparin-Induced Thrombocytopenia — SCE Dermatology MCQ

Correct answer: D — Heparin-induced thrombocytopenia type II (HIT)

The correct answer is D, Heparin-induced thrombocytopenia type II (HIT). The timing (day 5 after starting heparin for DVT treatment), the platelet count fall from 250 to 80 x10^9/L (a drop of over 50 percent), and the new paradoxical thrombotic skin findings (livedo reticularis, digital ischaemia) are the classic triad for HIT. HIT is caused by IgG antibodies against platelet factor 4-heparin complexes, which activate platelets and cause a hypercoagulable, prothrombotic state rather than bleeding, explaining why thrombocytopenia coexists with new thrombosis rather than haemorrhage. Onset typically occurs 5 to 10 days after first heparin exposure, matching this stem exactly, and the diagnosis is supported using the 4Ts score with anti-PF4/heparin antibody testing. Why the other options are wrong: C. DIC: DIC causes consumptive coagulopathy with prolonged clotting times, low fibrinogen and raised D-dimer alongside bleeding and thrombosis together; there is no trigger here (sepsis, malignancy, obstetric event) and the isolated platelet drop with a discrete thrombotic skin picture fits HIT far better. A. Warfarin necrosis: this occurs with warfarin (not heparin) due to transient protein C deficiency, typically within days of starting warfarin without adequate heparin cover, causing skin necrosis rather than digital ischaemia with thrombocytopenia; this patient's treatment is heparin-based, not warfarin. E. Cryoglobulinaemia: causes livedo and skin ischaemia related to cold-precipitable immune complexes, usually in the context of hepatitis C or lymphoproliferative disease, with no expected temporal link to heparin exposure or platelet fall pattern described. B. Antiphospholipid syndrome: causes recurrent thrombosis and livedo reticularis but is a chronic autoimmune condition, not one that presents acutely 5 days after heparin initiation with an acute platelet drop of this magnitude. Key point: A greater than 50 percent platelet fall with new thrombosis 5 to 10 days after heparin exposure is diagnostic of HIT until proven otherwise, and all heparin must be stopped immediately.

Reference: British Society for Haematology guideline on heparin-induced thrombocytopenia, summarised in Kings Thrombosis Centre 'The Management of Heparin-Induced Thrombocytopenia (HIT)', 2023: https://kingsthrombosiscentre.org.uk/wp-content/uploads/2023/03/The_management_of_heparin_induced_thrombocytopenia-1.pdf