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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.
- 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.
- 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.
- 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.
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.
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.
- 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).