thrombosis 80 fold. Homozygous or double heterozygous protein C deficiency is extremely rare and is associated with neonatal purpura fulminans. Decreased protein C levels occur during oral anticoagulant therapy, liver disease, vitamin K deficiency, cirrhosis, DIC and during an acute thrombotic event. PROTEIN C ANTIGEN Method: Enzyme immunoassay The patient should be off oral anticoagulant therapy for at least a week and should be tested at least one month after resolution of the thrombus. The protein C antigen determination is recommended when the protein C activity falls below 65% provided the patient is not on oral anticoagulant therapy and does not have vitamin K deficiency, liver disease, DIC or on-going thrombosis. The antigen determination is used to classify the type of hereditary protein C deficiency. Type I deficiency has a decreased activity and antigen. Type II deficiency has a low protein C activity and a normal or elevated antigen level. PROTEIN S ACTIVITY Method: APTT based clotting assay Protein S is a vitamin K dependent protein synthesized in the liver. Protein S acts as a cofactor for activated protein C, enhancing the anticoagulant function of activated protein C by increasing its affinity for phospholipid membranes. 31 Congenital protein S deficiency is inherited as autosomal dominant. Clinically it is very similar to protein C deficiency described above. Patients have an increased risk of venous thrombosis. It is estimated that approximately 3% of patients with venous thrombophilia have protein S deficiency. Acquired protein S deficiency occurs in oral anticoagulant therapy, liver disease, nephrotic syndrome, DIC, ongoing thrombosis, inflammation and pregnancy. There is a marked decrease in pregnancy. Since there is increased risk of thrombosis due to the pregnancy, one needs to be careful in evaluating for hereditary protein S deficiency during pregnancy. Trimester Range Activity First 43-111% Second 26-80% Third 11-55% As a clot based assay, the protein S activity level may be influenced by other clotting proteins and conditions. Elevated factor VIII activity (>250%) may falsely lower apparent protein S activity. The presence of a lupus inhibitor or a heparin concentration greater than 0.6 units/mL may lead to falsely elevated protein S activity. Direct thrombin inhibitors, such as argatroban and lepirudin, will falsely prolong clotting times and invalidate this assay. The free protein S antigen assay is recommended over the protein S clottable assay to screen for protein S deficiency. PROTEIN S ANTIGEN (FREE) Method: Latex immunoassay (LIA) As was true for protein S activity, the reference range for free protein S and total protein S are decreased during pregnancy. This needs to be taken into consideration when evaluating a possible hereditary protein S deficiency in a pregnant woman. Trimester Range of Free Protein S Antigen Range of Total Protein S Antigen First 55-132% 64-127% Second 48-134% 48-120% Third 38-84% 47-91% Protein S is a vitamin K dependent cofactor of protein C. Protein S exists in two forms in plasma, free form and complexed to C4b binding protein (C4b-BP). The free form represents approximately 40% of the total protein S. This free form acts as the cofactor for activated protein C and is the only form that has anticoagulant activity. The free protein S level should correlate with the protein S activity. In inflammation, there is an increase in C4b-BP that results in an increase of bound protein S. This leads to a decrease of free protein S antigen and corresponding decrease in protein S activity. PROTEIN S ANTIGEN (TOTAL) Method: Latex immunoassay (LIA) Total protein S antigen determination is only used to classify type of protein S deficiency when the protein S activity and free protein S antigen are decreased. Patients with type I deficiency have decreased total protein S and free protein S. Type II deficiency has normal total protein S and decreased free protein S. There is a rare form described where there is normal total and free protein S antigen with decreased protein S activity. The majority of the inherited protein S deficiency are type I. 32 PROTHROMBIN (FACTOR II) DNA SCREEN Method: DNA based PCR assay Prothrombin (factor II) is one of