Protein S deficiency increases risk of thrombosis. At present, we have information on 63 Norwegian individuals with hereditary protein S deficiency belonging to 25 different families. 42 of the individuals have experienced at least one thromboembolic episode, and seven a cerebral infarction before the age of 70 years. The amount of free protein S in plasma is dependent on variation of the acute phase protein C4b-binding protein (C4bBP). Acute phase response with increased C4bBP induces free protein S deficiency, and increases risk of thrombosis. In patients with protein S deficiency, warfarin may reduce free protein S to critically low levels, and thus explain why, in some patients, recurrent thrombosis occurs during warfarin treatment. In this situation, warfarin should be replaced by heparin.
