Coagulation screening tests in patients with hemophilia typically show a normal prothrombin time (PT), normal thrombin clot time (TCT), and prolonged activated partial thromboplastin time (aPTT), reflective of the abnormality in the intrinsic pathway. However, if the factor VIII level in hemophilia A or factor IX level in hemophilia

B is greater than 30 percent of normal activity (mild hemophilia), the aPTT can be normal. The only way to distinguish hemophilia A and hemophilia B is by specific factor assays of factors VIII and IX. Inhibitors are circulating antibodies, usually IgG, that are directed against factor VIII or factor IX, whichever factor the patient is lacking. Inhibitors occur in 10 to 15 percent of patients with severe hemophilia A and in about 10 percent of patients with severe hemophilia B. Inhibitors develop in response to exposure to the "missing factor" through replacement therapy. Most of these develop early in life. An inhibitor is diagnosed in the laboratory when the plasma of a patient with hemophilia with a prolonged aPTT is "mixed" 50-50 with the plasma of a "normal" control. If the aPTT corrects to normal with this "mix," no inhibitor is present, just a factor deficiency. If, however, the mix does not correct, an inhibitor is present. Further testing can be done to determine which coagulation factor the inhibitor is directed against, although in patients with known hemophilia, it is against the missing factor. The quantity of inhibitor that is present is measured most commonly by the Bethesda inhibitor assay (BIA) and is reported in BIA units. This quantitation is important in determining what type of factor replacement therapy the patient will require.

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