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Detecting the presence and titer of a specific factor inhibitor directed against coagulation factor VIII
Testing begins with coagulation factor VIII activity assay with dilutions to evaluate assay inhibition; if the factor VIII activity assay is decreased, an inhibitor screen will be performed to look for specific factor VIII inhibition. If specific inhibition is apparent, the titer of the inhibitor will be determined.
Factor VIII inhibitors are IgG antibodies directed against coagulation FVIII that typically result in development of potentially life-threatening hemorrhage. These antibodies may develop in 1 of 4 different patient populations:
-Patients with congenital FVIII deficiency (hemophilia A) in response to therapeutic infusions of factor VIII concentrate
-Elderly nonhemophiliac patients (not previously factor VIII deficient)
-Women in postpartum period
-Patients with other autoimmune illnesses
FACTOR VIII ACTIVITY ASSAY
Normal, full-term newborn infants or healthy premature infants usually have normal or elevated factor VIII.*
*See Pediatric Hemostasis References in Coagulation Studies in Special Instructions.
FACTOR VIII INHIBITOR SCREEN
Normally, there is no inhibitor, (ie, negative result).
If the screening assays indicate the presence of an inhibitor, it will be quantitated and reported in Bethesda (or equivalent) units.
This test is not useful for detecting the presence of inhibitors directed against other clotting factors and will not detect the presence of lupus anticoagulants.
If presence or type of inhibitor is unknown, PROCT / Prolonged Clot Time Profile or LUPPR / Lupus Anticoagulant Profile should be ordered.
Occasionally, a potent lupus-like anticoagulant may cause false-positive results for a specific factor inhibitor (eg, factor VIII or IX). See preceding caution statement.
1. Kasper CK: Treatment of factor VIII inhibitors. Prog Hemost Thromb 1989;9:57-86
2. Peerschke EI, Castellone DD, Ledford-Kraemer M, et al: Laboratory assessment of FVIII inhibitor titer. Am J Clin Pathol 2009;131(4):552-558
3. Pruthi RK, Nichols WL: Autoimmune factor VIII inhibitors. Curr Opin Hematol 1999;6(5):314-322