ICU · Haematology

Heparin-induced thrombocytopenia (HIT): 4T score, diagnosis, and non-heparin anticoagulation

Also known as HIT · Heparin-induced thrombocytytopenia · Type II HIT · HITTS · Heparin-induced thrombocytopenia and thrombosis syndrome · Anti-PF4 antibody

Heparin-induced thrombocytopenia (HIT) = immune-mediated (IgG antibody against platelet factor 4 [PF4] + heparin complex) platelet activation → THROMBOCYTOPENIA (paradoxically PROTHROMBOTIC — thrombosis NOT bleeding). TYPE I (non-immune — common, benign — early, mild drop — direct platelet aggregation). TYPE II (IMMUNE — the dangerous form — IgG anti-PF4/heparin → platelet activation → consumption + THROMBOSIS — 5-10 days after heparin start [or rapid if prior exposure within 30 days]). CLINICAL: platelet fall 50% from baseline (NOT necessarily <150 — relative drop), 5-10 days post-heparin, THROMBOSIS (venous — DVT/PE; arterial — limb/stroke; microvascular — skin necrosis at injection site), NO other cause. 4T SCORE (Thrombocytopenia, Timing, Thrombosis, oTher cause): ≤3 = low probability (unlikely HIT — continue heparin); 4-5 = intermediate; ≥6 = high probability (STOP heparin, start alternative). DIAGNOSIS: anti-PF4 ELISA (sensitive, fast) + serotonin release assay (SRA — confirmatory, slow). MANAGEMENT: STOP ALL HEPARIN (including LMWH, heparin flushes, heparin-coated catheters), start NON-HEPARIN anticoagulant (argatroban, bivalirudin, danaparoid, fondaparinux, DOAC), DO NOT give platelets (worsens thrombosis), DO NOT start warfarin alone (skin necrosis, venous limb gangrene — bridge with non-heparin first until platelets recover). MORTALITY: 10-20% (from thrombosis).