thrombosis. The presence of a glutamine instead of an arginine residue removes a site in factor V that is normally cleaved by activated protein C, and is associated with in vitro resistance to activated protein C. Presence of this polymorphism substantially increases the lifetime risk of venous thrombosis. This DNA-based test detects the underlying defect, but does not evaluate the current state of the patient’s blood coagulation system. 27 FACTOR ASSAYS (II, V, VII, VIII, IX, X, XI, or XII) Method: One-stage clotting based tests. These assays are based on the ability of dilute patient plasma to correct the clotting time of a specific factor deficient plasma as measured by the prothrombin time (II, V, VII, or X) or APTT (VIII, IX, XI, or XII). Congenital deficiencies of factors II, V, VII, or X are rare and all are inherited as autosomal recessives. All will be decreased in liver disease. Factor II, VII, IX, and X are decreased in vitamin K deficiency and during oral anticoagulant therapy. In these situations the factor VII is decreased first and then followed in order by IX, X, and II. The rate of decrease is determined by the in vivo half-life of the proteins. Isolated factor deficiencies for factors VIII, IX, XI, or XII will present with a borderline to marked prolongation of the APTT and all other screen procedures are normal. Direct thrombin inhibitors, such as argatroban and lepirudin, will falsely prolong clotting times and falsely decrease the factor concentrations for these assays. FACTOR INHIBITOR ASSAY Method: Bethesda assay. Acquired factor inhibitors circulate as antibodies that neutralize the procoagulant activity of a specific coagulation factor. This results in an acquired deficiency state with an associated risk of bleeding. Inhibitors have been described against all the clotting factor proteins. A factor inhibitor should be suspected when a prolonged prothrombin time and/or APTT does not correct on a 1:1 mix and a lupus inhibitor has been excluded. In the inhibitor assay, dilutions of patient plasma are incubated with pooled normal plasma for two hours at 37o C. At the end of the incubation period the factor activity in the mixture is measured. One inhibitor unit is defined as the amount of inhibitor that inactivates 50% of the factor in the normal plasma pool. Factor VIII inhibitors are time and temperature dependent which necessitates the incubation period of two hours. This is not true for the other clotting factors; an incubation period of thirty minutes is usually sufficient to express the full effect of these inhibitors. Factor VIII inhibitors associated with hemophilia A are the most common. There are two types of inhibitors. In a type I inhibitor there is complete inactivation of the factor. The Bethseda assay can be used to accurately quantitate the inhibitor level. The majority of the hemophilia A patients with anti-factor VIII antibodies fit into this category. Approximately 15% of all hemophilia A patients and 95% severe hemophiliacs will develop inhibitors. An inhibitor is suspected when the usual therapy does not have the expected results or bleeding has become more significant. Type II inhibitors show a partial, competitive type of inhibition, which precludes accurate measurement of the inhibitor level using the Bethseda assay. Many non-hemophilic inhibitors are of type II. The spontaneous development of a factor VIII inhibitor in a non-hemophilic patient is rare. Clinically, these patients may present with bleeding similar to hemophilia A, such as gastrointestinal bleeding, joint or muscle bleeding, excessive bruising, hematuria, post operative or post trauma bleeding. This bleeding may lead to life threatening central nervous system or retropharyngeal hemorrhage. This disorder may occur in autoimmune disease, lymphoproliferative malignancy, non-hematologic malignancy, drugs, dermatological disorders and post partum and in older men and women with no other underlying disease. Factor V inhibitors have been described post surgery, post transfusion, with infections, insect bite and after interoperative use of bovine topical thrombin which contains small amounts of bovine factor V. This inhibitor is associated with clinical bleeding. 28 Approximately 3% of hemophilia B patients develop factor IX inhibitors. This occurs most often in severe