Laboratory confirmation of a FVIII inhibitor is clinically important when a bleeding episode is not controlled despite infusion of adequate amounts of factor concentrate. For the assay, the aPTT measurement is repeated after incubating the patient's plasma with normal plasma at 37°C for 1-2 hours. If the prolonged aPTT is not corrected, the inhibitor concentration is titrated using the Bethesda method. Ideally, the Nijmegen modification of the Bethesda inhibitor assay should be used to detect an inhibitor if the mixing test result is positive. [14]
By convention, more than 0.6 Bethesda units (BU) is considered a positive result for an inhibitor. Less than 5 BU is considered a low titer of inhibitor, and more than that is a high titer. The distinction is clinically significant, as patients with low-titer inhibitors may respond to higher doses of FVIII concentrate while those with high-titer inhibitors require treatment with agents that bypass FVIII and consideration for induction of immune tolerance.
Caution is warranted when obtaining blood samples for coagulation assays from heparinized central lines because of the effect of heparin contamination on all coagulation test results. The excess heparin causes false-positive results and/or higher inhibitor titer values than are actually present in the patient, because heparin is also an inhibitor of coagulation.
One study found significant heparin contamination in 45% of all specimens obtained through implanted venous access devices. These researchers suggested that all blood samples obtained from such devices, which are usually flushed with heparin, should be treated with heparinase before performing an inhibitor assay. [29]
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Coagulation pathway.
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The hemostatic pathway. APC = activated protein C (APC); AT-III = antithrombin III; FDP = fibrin degradation products; HC-II = heparin cofactor II; HMWK = high-molecular-weight kininogen; PAI = plasminogen activator inhibitor; sc-uPA = single-chain urokinase plasminogen activator; tc-uPA = two-chain urokinase plasminogen activator; TFPI = tissue factor pathway inhibitor; tPA = tissue plasminogen activator
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Structural domains of human factor VIII. Adapted from: Stoilova-McPhie S, Villoutreix BO, Mertens K, Kemball-Cook G, Holzenburg A. 3-Dimensional structure of membrane-bound coagulation factor VIII: modeling of the factor VIII heterodimer within a 3-dimensional density map derived by electron crystallography. Blood. Feb 15 2002;99(4):1215-23; Roberts HR, Hoffman M. Hemophilia A and B. In: Beutler E, Lichtman MA, Coller BS, et al, eds. Williams Hematology. 6th ed. NY: McGraw-Hill; 2001:1639-57; and Roberts HR. Thoughts on the mechanism of action of FVIIa. Presented at: Second Symposium on New Aspects of Haemophilia Treatment; 1991; Copenhagen, Denmark.
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Possible genetic outcomes in individuals carrying the hemophilic gene.
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Photograph of a teenage boy with bleeding into his right thigh as well as both knees and ankles.
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Photograph of the right knee in an older man with a chronically fused, extended knee following open drainage of knee bleeding that occurred many years previously.
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Photograph depicting severe bilateral hemophilic arthropathy and muscle wasting. The 3 punctures made into the left knee joint were performed in an attempt to aspirate recent aggravated bleeding.
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Radiograph depicting advanced hemophilic arthropathy of the knee joint. These images show chronic severe arthritis, fusion, loss of cartilage, and joint space deformities.
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Radiograph depicting advanced hemophilic arthropathy of the elbow. This image shows chronic severe arthritis, fusion, loss of cartilage, and joint space deformities.
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Photograph of a hemophilic knee at surgery, with synovial proliferation caused by repeated bleeding; synovectomy was required.
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Large amount of vascular synovium removed at surgery.
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Microscopic appearance of synovial proliferation and high vascularity. If stained with iron, diffuse deposits would be demonstrated; iron-laden macrophages are present.
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Large pseudocyst involving the left proximal femur.
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Transected pseudocyst (following disarticulation of the left lower extremity due to vascular compromise, nerve damage, loss of bone, and nonfunctional limb). This photo shows black-brown old blood, residual muscle, and bone.
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Dissection of a pseudocyst.
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Transected pseudocyst with chocolate brown-black old blood.
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Photograph of a patient who presented with a slowly expanding abdominal and flank mass, as well as increasing pain, inability to eat, weight loss, and weakness of his lower extremity.
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Plain radiograph of the pelvis showing a large lytic area.
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Intravenous pyelogram showing extreme displacement of the left kidney and ureter by a pseudocyst.
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Photograph depicting extensive spontaneous abdominal wall hematoma and thigh hemorrhage in an older, previously unaffected man with an acquired factor VIII inhibitor.
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Magnetic resonance image of an extensive spontaneous abdominal wall hematoma and thigh hemorrhage in an older, previously unaffected man with an acquired factor VIII inhibitor.
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Coagulation Cascade
