13. Circulating Anticoagulants
Circulating anticoagulants inhibit clotting factors, causing excess hemorrhage. Autoantibodies such as acquired inhibitors to factor VIII may be idiopathic or secondary to hereditary hemophilia, the postpartum state, other autoimmune disorders, malignancies, or drugs. Patients present with bleeding as a syndrome of acquired hemophilia, with low F VIII: c levels and demonstrable F VIII inhibitors. Drugs implicated include antibiotics, psychotropics, fludarabine, and interferon [67]. Antibody activity resolves with cessation of the drug or with the use of immunosuppressive agents. An acquired inhibitor to factor XIII, which cross-links and stabilizes fibrin, has been associated with isoniazid [68].
Lupus anticoagulants and antiphospholipid antibodies may be induced by drugs such as chlorpromazine, hydralazine, phenytoin, quinine, and procainamide [69, 70]. In these cases, there is an association with hypercoagulability as opposed to bleeding.