Recognized by a falling platelet count (platelet count falls by >50% to <150×10⁹/L) in a patient receiving unfractionated or (much more rarely) low-molecular-weight heparin, with or without previous exposure to heparin.

Thrombotic complications (venous and arterial) occur in 20–50%. Bleeding is rare.

Probability of diagnosis can be assessed by 4T score (Table ).

Diagnosis proved by the presence of heparin-dependent antibodies.

Sepsis

Post-transfusion purpura

Thrombocytopenia caused by other drugs

Seek urgent advice from a haematologist

Stop heparin

Use alternative anticoagulant therapy with direct-acting thombin inhibitor (e.g. bivalirudin) or heparinoid (danaparoid) if needed

The platelet count typically recovers within 4–14 days after stopping heparin

The risk of thrombosis persists for up to several weeks