A patient develops heparin-induced thrombocytopenia (HIT) type II after 8 days of unfractionated heparin for DVT. Platelet count drops from 240,000 to 70,000/µL with a new left lower limb arterial thrombus. What is the mechanism of this paradoxical thrombosis and what is the appropriate management?

• A Direct platelet heparin toxicity → thrombocytopenia → platelet aggregates form; manage with platelet transfusions and warfarin
• B IgG antibodies against heparin-PF4 complex bind platelet FcγRIIa (CD32) receptors → platelet activation, aggregation, and thrombosis; manage by stopping heparin and starting a non-heparin anticoagulant (argatroban or fondaparinux) ✓
• C Heparin directly inhibits thrombomodulin, reducing protein C activation and creating a hypercoagulable state; manage by stopping heparin and starting warfarin immediately
• D Heparin-PF4 antibodies activate complement, causing microangiopathic thrombosis; manage with plasmapheresis and IVIG

Explanation

HIT type II is an immune-mediated thrombocytopenia (not direct toxicity). Heparin forms a complex with platelet factor 4 (PF4), creating a neoantigen that stimulates IgG antibody production. These IgG antibodies (anti-heparin-PF4) bind the complex and the Fc portion simultaneously engages platelet FcγRIIa (CD32) receptors, causing platelet activation, release of procoagulant microparticles, and massive thrombin generation — paradoxical thrombosis occurs despite thrombocytopenia. Management: (1) Immediately discontinue ALL heparin (including flushes, heparin-coated catheters); (2) Start non-heparin anticoagulation — argatroban (IV direct thrombin inhibitor; preferred in liver dysfunction patients) or bivalirudin; fondaparinux is also used; (3) Avoid warfarin initially (risk of warfarin-induced skin necrosis via protein C drop); (4) Do NOT transfuse platelets.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.