Unexplained thromboembolism. When to suspect antithrombin III, protein C, or protein S deficiency

Patients with recurrent or unusual thromboembolic events may have abnormalities in regulation of hemostasis because of inherited or acquired deficiencies of antithrombin III, protein C, or protein S. A working knowledge of the clotting process and a high degree of suspicion are needed to guide the clinician in this difficult diagnosis. Prompt and appropriate anticoagulant therapy can be lifesaving in these patients.