Heparin-Induced Thrombocytopenia type II (HIT type II)

Heparin-Induced Thrombocytopenia type II (HIT type II) is a rare but serious side effect of heparin therapy associated with a high risk of developing blood clots or thrombosis (the process by which blood clots are formed). HIT type II can occur in up to 3% of patients receiving heparin.

Heparin is a widely used anticoagulant for the treatment and prevention of blood clots. It is often used in patients for treating deep vein thrombosis and pulmonary embolism. Heparin is also given for preventing thrombosis in patients at particular risk, including those undergoing major surgery. A few patients receiving heparin develop HIT type II which is a particular immune response to the drug.

Heparin tends to stick to a protein in the blood called platelet factor 4 and HIT type II occurs when antibodies are formed against this complex. The binding of antibodies to this complex activates platelets, which leads to a reduction in the number of platelets in the blood or thrombocytopenia. Activated platelets also cause the blood to coagulate and promote thrombosis. The patient with HIT type II is, therefore, at high risk of developing blood clots, and complications caused by blood clots becoming detached and lodging at another point in the blood system.

Complications of HIT type II include venous thrombosis, pulmonary embolism, strokes, heart attacks and death. The mortality rate from HIT type II has been reported as about 20%.