Drug-induced thrombocytopenia

What it is

A low platelet count may be the work of a drug the patient takes. The best described form is heparin induced thrombocytopenia (HIT), which comes in two types that differ in frequency, onset, mechanism and outcome. Type 1 HIT comes from a direct interaction between heparin and platelets, for example heparin directly induces platelet aggregation, with no antibodies involved. Type 2 HIT comes from autoantibodies against a complex composed of platelet factor 4 (PF4) and heparin. Those autoantibodies induce thrombocytopenia, through removal of antibody-coated platelets by macrophages and the reticuloendothelial system and through consumption of platelets in thrombi, and they induce arterial and venous thrombosis as well, by an unclear mechanism that appears to involve platelet activation.

Causes and risk

Type 1 HIT is the commoner of the two, at about 10-20%. Type 2 HIT occurs in 1-5% of those given UFH and in 0.1-1% of those given LMWH. Other than heparin, some medications are frequently reported to cause drug-induced thrombocytopenia, so the drug list is checked when a low count is found.