30th August 2024

Heparin induced thrombocytopaenia

Heparin-induced thrombocytopenia (HIT) is classified into two types: HIT type 1 and HIT type 2. HIT type 1 is a non-immune condition where heparin directly causes platelet aggregation and is less severe than HIT type 2.

HIT type 1 occurs in about 10% of patients within 72 hours of starting heparin treatment. It involves a modest drop in platelet count, rarely below 100 (normal 150-400 *109/L) and is not linked to an increased risk of thrombosis. Symptoms improve once heparin is discontinued.

HIT type 2 is an immune-mediated reaction occurring within 10 days of starting heparin. Platelet factor 4 (PF4) forms immune complexes with heparin, accumulating on platelets and activating them. This triggers the release of more PF4, creating a positive feedback loop. Procoagulant microparticles are then released, leading to excessive thrombin formation, consumptive thrombocytopenia, and a high risk of thrombosis. These characteristics make HIT type 2 a significant clinical concern.