Thrombin Time (TT) Test

     TT has an important role as a screening test because it measures the conversion of fibrinogen to fibrin by adding excess thrombin to undiluted plasma. Because the additional clotting factors previously measured in the PT and APTT have no effect on this test, TT is generally useful for evaluating other parameters affecting the formation of fibrin. There can be interference with the conversion of fibrinogen to fibrin for three major reasons: the presence of hypofibrinogenemia or dysfibrinogenemia, the presence of heparin, and the presence of fibrin degradation products (FDP). In rare cases, autoantibodies against thrombin (e.g., induced by topical thrombin application or the use of fibrin sealants) and myeloma proteins can also interfere with fibrin formation and result in an abnormal TT. The TT is useful in corroborating an abnormal FDP result and can verify that the citrated blood sample was drawn through an indwelling heparinized catheter that was not well flushed. An extremely prolonged TT usually indicates a heparin effect. If the sample is contaminated with heparin, it can be absorbed with Hepzyme. The testing can then be repeated, or the specimen can be redrawn.

     The general reference interval for the TT is 10–16 seconds. TT’s sensitivity can be increased by diluting the thrombin reagent to give a control of 16–18 seconds. The TT in preterm and term infants is longer than the adult reference interval even though the fibrinogen level is within the same normal reference interval, which can be explained by the presence of a distinct fetal fibrinogen molecule with altered function. The TT generally becomes normal within a few days after birth.

Thrombin time procedure

1. Pre-warm 0.1 mL patient or control plasma.
2. Add 0.2 mL pre-warmed thrombin reagent and start timing device.
3. Stop timing device upon formation of a clot.

• Note: Optimal reaction temperature is 37°C. 
• Reference interval: 10-16 seconds.

NOTE: Conditions associated with prolonged thrombin time

• Hypofibrinogenemia
• Dysfibrinogenemia
• Paraproteins (e.g., cryoglobulin)
• Presence of heparin
• Presence of fibrin degradation products
• Presence of plasmin