Thrombin Time (TT)

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Subject: Thrombin Time (TT)

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Definition

  • TT measures the time of conversion of fibrinogen into fibrin once thrombin (used as a reagent) is added.

  • Normal range: 14–21 seconds (may vary depending on reagent and equipment used).

Use

Interpretation

Causes of Increased (Prolonged) TT

  • Very low fibrinogen (<80 mg/dL)

  • Dysfibrinogenemia

  • Interference with polymerization of fibrinogen

    • Fibrin degradation products, such as DIC and pathologic or therapeutic fibrinolysis. TT is not recommended in the diagnosis of pathologic fibrinolysis or DIC because of very low specificity and low sensitivity

    • High concentrations of monoclonal immunoglobulins

    • Uremia

  • Heparin therapy

Limitations

  • Preanalytic conditions may interfere with this test.

    • Improper filling of blood collecting tube or use of wrong tubes (containing different anticoagulant than recommended or no anticoagulant)

    • Clots in specimen

    • Hemolysis

    • Heparin contamination of blood, such as drawings from IV lines with heparin flushes (when heparin contamination is suspected, a reptilase time can be performed instead [see above])

  • Hyperlipidemia may artificially prolong the thrombin time obtained by optical equipment (most modern machines). In such cases, the assay can be done on equipment that uses mechanical clotting.

  • Results are unreliable in patients with high fibrinogen (>500 mg/dL).

  • Patients previously exposed to bovine thrombin to arrest bleeding may develop thrombin antibodies.

  • Use of various radiocontrast agents may affect test results.

 
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