Drug induced immune mediated thrombocytopenia

Classically caused by quinidine with antibodies that bind to complexes of drug, or drug metabolites, bound to platelet glycoproteins, typically, glycoprotein IIb/IIIa (fibrinogen receptor), glycoprotein Ib/IX (von Willebrand factor receptor), or both.

Antibody coated platelets are cleared from the circulation by macrophages that can recognize the Fc tail of the drug dependent antibodies.

Because platelets bear thousands of copies of IIb/IIIa and Ib/IX glycoproteins the antibodies can cause severe thrombocytopenia in 85-90% of cases, with a nadir of less than 20,000 per cubic millimeter.

If thrombocytopenia is not severe it is unlikely to be due to an immune mediated process.

While drug induced thrombocytopenia is usually severe it may not be in thrombocytopenia caused by carbimazole which has platelet levels about 60,000 per cubic millimeter and in heparin induced thrombocytopenia.

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