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One-Stage Factor Assay: Extrinsic Coagulation System

When prothrombin time (PT) and activated partial thromboplastin time (APTT) are prolonged, a one-stage assay is used to detect a deficiency of factor II, factor V, or factor X. If PT is abnormal but APTT is normal, factor VII may be deficient.

Purpose

• To identify a specific factor deficiency in persons with prolonged PT or APTT . 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.
• When the patient is factor deficient and receiving blood component therapy, tell him that he may need a series of tests.
• Withhold oral anticoagulants before the test. If they must be continued, note this on the laboratory slip.
• 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.

Precautions

• If the patient has a suspected coagulation defect, 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.
• 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. The reference ranges for the factors are as follows:

• factor II: 80% to 120% of normal
• factor V: 50% to 150% of normal
• factor VII: 65% to 140% of normal
• factor X: 45% to 155% 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. Deficiency of factor II, factor VII, or factor X may indicate hepatic disease or vitamin K deficiency. Deficiency of factor X may also indicate disseminated intra vascular coagulation (DIC). Factor V deficiency suggests severe hepatic disease, DIC, or fibrinogenolysis. Deficiencies of all four factors may be congenital; absence off factor II is lethal.

Interfering factors

• Failure to mix the sample and the anticoagulant adequately, or to send the sample to the laboratory immediately or place it on ice.
• Hemolysis due to rough handling of the sample.
• Oral anticoagulants (possible increases due to inhibition of vitamin K­ dependent synthesis and activation of tilling factors II, VII, and X, which form in the liver).

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