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Plasma Thrombin Time
Plasma thrombin time, or thrombin clotting time, measures how quickly a clot forms when a standard amount of bovine thrombin is added to a platelet poor plasma sample from the patient and to a normal plasma control sample After thrombin is added, the clotting time for each sample is compared and recorded. This test allows a quick but imprecise estimation of plasma fibrin ogen levels.
- To detect fibrinogen deficiency or defect
- To aid diagnosis of disseminated intravascular coagulation (DIC) and hepatic disease.
- To monitor the effectiveness of treatment with heparin or thrombolytic agents.
- Explain to the patient that this test is used to determine if blood clots normally.
- If possible, withhold heparin therapy before the test. If heparin must be continued, note this on the laboratory slip.
- Tell the patient 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.
- 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.
- If t he tube isn't filled to the correct volume, an excess of citrate appears in the sample. Completely fill the collection tube, and invert it gently several times to mix the sample and the anticoagulant thoroughly.
- To prevent hemolysis, avoid excessive probing during venipuncture and rough handling of the sample.
- Immediately send the sample on ice to the laboratory.
Normal thrombin times range from 7 to 12 seconds. Test results are usually reported with a normal control value.
A prolonged thrombin time may indicate heparin therapy, hepatic disease, DIC, hypofibrinogenemia, or dysfibrinogenemia. Patients with prolonged thrombin times may require measurement of fibrinogen levels; in suspected DIC, the test for fibrin split products is also necessary.
- Failure to use the proper anticoagulant, 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 or to excessive probing at the venipuncture site.
- Fibrinogen level < 100 mg/d1 (prolonged thrombin time)
- Fibrinogen inhibitors, such as streptokinase, urokinase, and tissue plasminogen activators (increase).
- Heparin (possible increase).