Abnormal bleeding can result from disorders of the coagulation system (see Hemostasis), of platelets, or of blood vessels. Disorders of coagulation can be acquired or hereditary. The major causes of acquired coagulation disorders are vitamin K deficiency (see Vitamin Deficiency, Dependency, and Toxicity: Vitamin K Deficiency), liver disease, disseminated intravascular coagulation, and development of circulating anticoagulants. Severe liver disease (eg, cirrhosis, fulminant hepatitis, acute fatty liver of pregnancy) may disturb hemostasis by impairing clotting factor synthesis. Because all coagulation factors are made in the liver, both the PT and PTT are elevated in severe liver disorders. (PT results are typically reported as INR.) Occasionally, decompensated liver disease also causes excessive fibrinolysis and bleeding due to decreased hepatic synthesis of α₂-antiplasmin.

The most common hereditary disorder of hemostasis is von Willebrand disease (see Thrombocytopenia and Platelet Dysfunction: Von Willebrand Disease) The most common hereditary coagulation disorders are the hemophilias.

Last full review/revision June 2009 by Joel L. Moake, MD