Hemostasis Reference Laboratory

Factor VIII Inhibitor Screen (Bethesda Assay)

• Transaction Code: 3220-02

Inhibitor to Factor VIII is screened for by mixing the test plasma with a known amount of Factor VIII. After a 2 hour incubation period at 37°C, the residual Factor VIII activity is determined in a factor VIII assay. By comparing the difference in the Factor VIII activity of the patient incubation mixture and a control mixture, the absence or presence of a Factor VIII inhibitor can be demonstrated. Some antibodies will only prolong the PTT after incubation (Ewing and Kasper, 1982).

Factor VIII inhibitors, IgG antibodies directed against factor VIII, can occur in alloimmunized patients with congenital factor VIII deficiency (Hemophilia A) or as autoantibodies. The latter are associated with pregnancy, autoimmune disease, or drugs but most often occur spontaneously, particularly among elderly persons.

• Sample Required: 0.5 ml frozen plasma x4 or (2) 3.2% sodium citrated tubes, 5 ml each.
• See Sample Requirements when ordered with other tests.
• Normal Test Value: None detected (values reported in Bethesda units)
• Analytic Time: Run about twice a month; available sooner if pre-approved