CLINICAL OBSERVATIONS, INTERVENTIONS, AND THERAPEUTIC TRIALS The Incidence of Venous Thromboembolism in Asymptomatic Carriers of a Deficiency of Antithrombin, Protein C, or Protein S: A Prospective Cohort Study

an annual incidence of 1.5% (95% confidence interval [CI], 0.7 to 2.8) for the 3 deficiencies combined. Five of these events occurred spontaneously, resulting in an annual incidence of spontaneous venous thromboembolism of 0.8% (95% CI, 0.3 to 1.9). For antithrombin, protein C, and protein S deficiencies separately, this figure was 1.6%, 1.0%, and 0.4%, respectively. Thirty-four subjects experienced a total of 40 risk periods during which 4 venous thromboembolic events occurred (10% per risk period). We conclude that the use of continuous anticoagulant prophylaxis seems not warranted in asymptomatic individuals with a deficiency of antithrombin, protein C, or protein S. During risk periods for venous thromboembolism, adequate anticoagulant prophylaxis is necessary. r 1999 by The American Society of Hematology.