Deficiencies of coagulation-inhibiting and fibrinolytic proteins in outpatients with deep-vein thrombosis BACKGROUND. Isolated deficiencies of antithrombin III, protein C, protein S, and plasminogen have been implicated as a cause of deep-vein thrombosis. It is assumed that patients with recurrent, familial, or juvenile thrombosis are very likely to have such a deficiency. METHODS. We studied the prevalence of isolated deficiencies of these proteins in 277 consecutive outpatients with venographically proved acute deep-vein thrombosis, as compared with 138 age-matched and sex-matched controls without deep-vein thrombosis, and calculated the positive predictive value of a history of recurrent, familial, or juvenile venous thromboembolism for the presence of such a deficiency. RESULTS. The overall prevalence of deficiencies of any of these proteins in the patients with venous thrombosis was 8.3 percent (23 of 277 patients) (95 percent confidence interval, 5.4 to 12.4), as compared with 2.2 percent in the controls (3 of 138 subjects) (95 percent confidence interval, 0.5 to 6.1; P less than 0.05 for the comparison between groups). The positive predictive values for the presence of an isolated protein deficiency in patients with recurrent, familial, or juvenile deep-vein thrombosis, defined as the proportion of patients with the clinical finding who had a deficiency of one or more of the proteins, were 9, 16, and 12 percent, respectively. CONCLUSIONS. The cause of acute venous thrombosis in most outpatients (91.7 percent) cannot be explained by abnormalities of coagulation-inhibiting and fibrinolytic proteins. The information obtained from the medical history concerning recurrent or familial venous thrombosis or the onset of the disease at a young age is not useful for the identification of patients with protein deficiencies. 