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Home :: Intrinsic coagulation system

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.

Purpose

  • To identify a specific factor deficiency.
  • To study patients with congenital or acquired coagulation defects
  • To monitor the effects of blood component therapy in factor-deficient patients

Patient preparation

  • Explain to the patient that this test is used to assess the function of the blood coagulation mechanism.
  • Tell him that a blood sample will be taken. Explain who will perform the venipuncture and when.
  • Reassure him that drawing a blood sample will take less than 3 minutes.
  • Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.
  • Withhold oral anticoagulants before the test. If such medications must be continued, note this on the laboratory slip.
  • When the patient is factor deficient and receiving blood component therapy, tell him that a series of tests may be needed to monitor therapeutic progress.
  • Inform the patient that food or fluids need not be restricted before the test.

Procedure and posttest care

  • Perform a venipuncture, and collect the sample in a 7-ml blue-top tube.
  • If a hematoma develops at the venipuncture site, apply warm soaks.
  • A patient with a bleeding disorder may require a pressure bandage to stop bleeding at the venipuncture site.
  • Resume administration of medications discontinued before the test.
Precautions
  • If a coagulation defect is suspected, avoid excessive probing during venipuncture, don't leave the tourniquet on too long (it will cause bruising), and apply pressure to the puncture site for 5 minutes, or until the bleeding stops.
  • Completely fill the collection tube, and invert it gently several times to mix the sample and the anticoagulant thoroughly.
  • Handle the sample gently to prevent hemolysis, and send it to the laboratory immediately or place it on ice.

Reference values

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:

  • factor VIII: 45% to 185% of normal
  • factor IX: 60% to 140% of normal
  • factor XI: 65% to 135% of normal
  • factor XII: 50% to 150% of normal
Abnormal findings

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.

Interfering factors

  • Failure to adequately mix the sample and the anticoagulant or to send the sample to the laboratory immediately.
  • Hemolysis due to rough handling of the sample.
  • Oral anticoagulants (decrease in factor IX).
  • Pregnancy (increase in factor VIII).

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