Many times a person will not have symptoms associated with the presence of an inhibitors. In these cases, inhibitors are often discovered by laboratory testing during regular physician visits. For those who are symptomatic, an inhibitors is usually suspected when bleeding does not stop after being treatment with factor concentrate. Once an inhibitor is suspected, confirmation is made using a blood test called the "Bethesda inhibitor assay." This test measures the presence and amount of antibodies directed against a coagulation factor in a person's blood after being exposed to factor.

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