typically due to the presence of factor V Leiden. Based on the prolongation of the clotting time by APC, it can be determined whether the patient is normal, likely factor V Leiden heterozygote or likely factor V Leiden homozygote. The assay has shown close correlation with the factor V DNA screen. This assay is not affected by oral anticoagulant therapy, heparin therapy or the presence of a lupus inhibitor. Direct thrombin inhibitors, such as argatroban and lepirudin, will falsely prolong the clotting times and invalidate this assay. ANTITHROMBIN ACTIVITY Method: Chromogenic substrate, back titration method. Samples for antithrombin III activity should be obtained at least a week after resolution of the acute thrombotic event and a minimum of 3-4 days post heparin therapy. Direct thrombin inhibitors, such as argatroban and lepirudin, will falsely increase and invalidate this assay. Congenital antithrombin deficiency is inherited as autosomal dominant. Patients usually have less than 65% antithrombin activity. The disorder occurs in 2-3 % of patients presenting with venous thrombophilia. Acquired antithrombin deficiency occurs in acute thrombosis, liver disease, kidney disease, nephrotic syndrome, sepsis, DIC, post-operatively and during asparaginase or heparin therapy. Once heparin therapy is stopped, the antithrombin activity should return to baseline levels within 72 hours. Elevated antithrombin III activity has no known clinical significance. 26 CHROMOGENIC FACTOR VIII ASSAY Method: Chromogenic substrate Based on the study by Koster et al. a baseline factor VIII activity of >150% is a risk factor for venous thrombosis. Providing that the sample is drawn when there is no inflammation or acute clinical illness, no aerobic exercise in the past several to 24 hours and no use of estrogen therapy, a baseline value of >150% is a significant risk factor for individual subjects. This finding is very common: approximately 20-25% of patient with thrombophilia have an elevated factor VIII. Factor VIII levels are most appropriately considered in subjects after venous thrombotic events if they have a strong family history. D-DIMER ASSAY - QUANTITATIVE (cross-linked fibrin degradation products) Method: LIA assay for clinical samples and ELISA for research (batched) samples. Cross-linked fibrin degradation products are formed by enzymatic degradation of factor XIII cross-linked fibrin. D-dimers are elevated in pulmonary embolus, deep venous thrombosis, arterial thrombosis, malignancy, sepsis, cirrhosis, post-operatively and during disseminated intravascular coagulation. The level of D-dimer is influenced by the severity of the thrombotic episode, rate of D-dimer production and the time the specimen was drawn after the thrombotic episode. A normal level of D-dimer by this method helps rule out the presence of evolving deep venous thrombosis or pulmonary embolus. It has been used as a screening test when large proximal deep venous thrombosis is suspected. DIRECT THROMBIN INHIBITOR ASSAY Method: Clot-based dilute thrombin time. Heparin-induced thrombocytopenia (HIT) is a serious autoimmune complication of heparin therapy. Heparin anticoagulation is not an option, as continued heparin therapy would fuel the underlying autoimmune reaction. Direct thrombin inhibitors, such as argatroban or lepirudin, are effective alternatives for anticoagulation in patients with acute HIT. These direct thrombin inhibitors are generally followed with the activated partial thromboplastin time (aPTT). In the setting of HIT with a lupus inhibitor, the aPTT may not be an accurate indication of direct thrombin inhibitor effect due to prolongation of the aPTT by the lupus inhibitor. The thrombin time reagent contains no phospholipid and thus has no interference from a lupus inhibitor. This assay uses a 1:4 dilution of the patient’s plasma into normal plasma to minimize interfering substances and standardize the therapeutic range. FACTOR V DNA SCREEN (FACTOR V LEIDEN) Method: DNA based PCR assay. The Gln-506 (1691G->A, R506Q, factor V Leiden) polymorphism in the factor V gene is present in approximately 3-5% of the general population and in about 20-40% of patients with a history of unexplained recurrent venous