Heparin-induced thrombocytopenia (HIT)

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⚠️ HIT / HITT: A potentially fatal complication of Heparin therapy caused by antibodies against the Heparin–Platelet Factor 4 (PF4) complex. These antibodies activate platelets, leading to thrombocytopenia and a paradoxical risk of thrombosis (arterial + venous). Stop all Heparin immediately if HIT is suspected.

📖 About

• Consider in patients on Heparin with falling platelets + thrombosis 🩸.
• Usually affects large vessels; may cause acute limb ischaemia 🦵.
• Occurs 5–14 days after starting Heparin ⏳.
• Associated with anti-PF4 antibodies.
• Immediate cessation of all Heparin (including flushes) is essential 🚫.

🧬 Aetiology

• HIT typically arises 5–10 days after initiation of Heparin.
• Common with unfractionated Heparin (UFH); less so with LMWH.
• Caused by Heparin-dependent IgG antibodies activating platelets via FcγIIa receptors.
• Antigen = Heparin–PF4 complex.

⚡ Clinical Features

• New venous or arterial thrombosis (e.g. DVT, PE, stroke, acute limb ischaemia).
• Cold, painful, pulseless leg 🦵 (arterial thrombosis).
• Skin necrosis at injection sites ❌.
• Can cause adrenal vein thrombosis → adrenal failure 🧠.

⚠️ Complications

• Arterial thrombosis (MI, stroke, limb ischaemia).
• Venous thrombosis (DVT, PE).
• Skin necrosis at injection sites 🩹.
• Adrenal haemorrhage/failure due to venous thrombosis.
• Severe cases: venous limb gangrene (esp. with warfarin exposure, high INR).

🧮 The 4T Score

• ≤3: Low probability ❌
• 4–5: Intermediate probability ⚠️
• ≥6: High probability ✅

🩺 Management

• Stop all Heparin immediately (including line flushes) 🚫.
• Urgent Haematology advice recommended 📞.
• Alternative anticoagulation: Argatroban, Bivalirudin, Fondaparinux. Hirudin may be used but avoid in renal failure 🚫.
• Warfarin: Avoid initially (can worsen thrombosis). Start only when platelets >150 × 10⁹/L, overlapped with a non-Heparin anticoagulant for 5 days.
• Vitamin K if patient already on Warfarin 💉.
• Platelet transfusions not needed unless actively bleeding.

🔑 Key Features Suggesting HIT

• Platelet count drop (≥50%).
• Timing: 5–10 days after Heparin start.
• New/progressive thrombosis.
• No better alternative explanation.

📚 Reference

• Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 Ts) for diagnosis of Heparin-induced thrombocytopenia. J Thromb Haemost 2006; 4: 759–65.

Cases — Heparin-Induced Thrombocytopenia (HIT)

• Case 1 — Post-Orthopaedic Surgery: A 65-year-old woman receives unfractionated heparin after a hip replacement. By day 6, her platelet count has fallen from 240 ×10⁹/L to 90 ×10⁹/L. She develops a new swollen, painful calf. Doppler confirms a popliteal DVT. Diagnosis: HIT type II with thrombosis.
• Case 2 — Medical Inpatient with Thrombocytopenia: A 72-year-old man on low-molecular-weight heparin (LMWH) for pneumonia prophylaxis shows a >50% platelet drop after 7 days (from 200 ×10⁹/L to 95 ×10⁹/L). He is otherwise well, no bleeding. HIT antibody (anti-PF4) test positive. Diagnosis: Immune-mediated HIT without thrombosis.
• Case 3 — Cardiac Surgery Patient: A 58-year-old man treated with IV unfractionated heparin during CABG develops thrombocytopenia 8 days later. He presents with chest pain; ECG shows an inferior STEMI. Coronary angiography reveals new thrombotic occlusion of the right coronary artery. Diagnosis: HIT causing arterial thrombosis (myocardial infarction).

Teaching Commentary 🧪

HIT is a paradoxical, immune-mediated reaction where heparin–PF4 complexes trigger IgG antibodies, leading to platelet activation, thrombocytopenia, and thrombosis rather than bleeding. It typically appears 5–14 days after starting heparin. The 4Ts score (Thrombocytopenia, Timing, Thrombosis, oTher causes) helps risk stratify. Confirmation is with anti-PF4/heparin ELISA or functional assays. Management: stop all heparin immediately and start a non-heparin anticoagulant (argatroban, fondaparinux, or a DOAC). Platelet transfusions are avoided unless life-threatening bleeding occurs.