Review Question - QID 108745

The most likely diagnosis in this patient is type II heparin-induced thrombocytopenia (HIT). Type II HIT is caused by antibodies directed against the heparin-platelet factor 4 complex.

Heparin is an anticoagulant that binds to antithrombin III in order to accelerate the inactivation of multiple clotting factors (IIa, IXa, Xa, XIa, and XIIa). It is used to provide immediate anticoagulation in cases of deep venous thrombosis, pulmonary embolism, acute coronary syndrome, stroke, and disseminated intravascular coagulation. It is also used to provide anticoagulation in pregnant women because it does not cross the placenta. A major adverse event is HIT, which has two distinct types. Type I HIT is a non-immunologic process that arises within three days of heparin initiation and is caused by direct interactions between heparin and platelets, leading to transient platelet aggregation and thrombocytopenia. Type II HIT typically occurs four or more days after administration of heparin. Type II HIT is caused by the development of IgG autoantibodies directed against the heparin-platelet factor 4 complex, leading to platelet destruction.

Incorrect Answers:
Answer 1: Warfarin is an anticoagulant that decreases production of vitamin K-dependent clotting factors (eg., II, VII, IX, X, protein C, and protein S). Heparin does not affect this process.

Answer 3: Type I HIT occurs due to direct interaction between heparin and platelets, causing platelet aggregation and transient thrombocytopenia. Unlike type II HIT, this condition typically occurs within 3 days of initiating heparin and platelet levels rarely fall below 100,000/mm^3.

Answer 4: Idiopathic thrombocytopenic purpura (ITP) is caused by anti- GPIIb/IIIa IgG autoantibodies. In children, ITP typically presents as a transient decrease in platelets following a viral infection. In adults, ITP can present more chronically, especially in patients with autoimmune disorders, HIV, or lymphoproliferative disorders.

Answer 5: Thrombotic thrombocytopenic purpura (TTP) is caused by a decrease in ADAMTS13-mediated cleavage of von Willebrand factor (vWF) multimers. TTP classically presents with thrombocytopenia, renal dysfunction, and microangiopathic hemolytic anemia.

Bullet Summary:
Heparin is an anticoagulant that accelerates antithrombin III-mediated inactivation of multiple clotting factors. Heparin-induced thrombocytopenia (HIT) is a major adverse effect caused by the development of antibodies directed against heparin-platelet factor 4.