One-Stage Factor Assay: Intrinsic Coagulation System
When prothrombin time (PT) is normal but activated partial thromboplastin time (APTT) is abnormal, a one-stage assay is used to identify a deficiency in the intrinsic coagulation system: factor VIII, factor IX, factor XI, or factor XII.
Procedure and posttest care
Diluted samples of the patient's plasma are added to a substrate plasma deficient in a single factor. The activity of this mixture is compared with normal activity. Reference ranges for the factors are as follows:
If the clotting time for the substrate mixture is prolonged compared to normal, the patient may be deficient in the factor being tested. Factor VIII deficiency may indicate hemophilia A, von Willebrand's disease, or a factor VIII inhibitor. An acquired deficiency of factor VIII may result from disseminated intravascular coagulation or fibrinolysis. Factor VIII antigen and ristocetin cofactor tests distinguish between hemophilia A (and its carrier state) and von Willebrand's disease.
Factor IX deficiency may suggest hemophilia B, or it may be acquired as a result of hepatic disease, a factor IX inhibitor, vitamin K deficiency, or coumarin therapy. Factor VIII and IX inhibitors occur after blood transfusions in patients deficient in either factor and are antibodies specific to each factor.
Factor XI deficiency may appear after the stress of trauma or surgery, or transiently in neonates. Factor XII deficiency may be inherited or acquired (as in nephrosis) and may also appear transiently in neonates.
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