Thrombin Time (TT)
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Thrombin is added to patient’s plasma and time required for clot formation is noted.
(1) Water bath at 37°C
(2) Test tubes (75 × 12 mm)
Venous blood is collected from antecubital fossa with a plastic, siliconized glass, or polypropylene syringe and a large bore needle (20 or 21 G in adults, 22 or 23 G in infants). Blood should never be collected from indwelling intravenous lines, as these often contain heparin. Glass syringe should not be used for collection since it activates coagulation. The blood is drawn gently but quickly after a single, smooth venepuncture. The needle is detached from the syringe, and the sample is passed gently into the plastic container. After capping the container, the blood and the anticoagulant are mixed immediately by gentle inversion 5 times. The anticoagulant used for coagulation studies is trisodium citrate (3.2%), with anticoagulant to blood proportion being 1:9. Most coagulation studies require platelet poor plasma (PPP). To obtain PPP, blood sample is centrifuged at 3000-4000 revolutions per minute for 15-30 minutes. Coagulation studies are carried out within 2 hours of collection of sample.
Take 0.1 ml of buffered saline in a test tube and add 0.1 ml of plasma. Note clotting time after addition of 0.1 ml of thrombin solution.
± 3 seconds of control.
(1) Disorders of fibrinogen: Prolongation of TT occurs in afibrinogenemia (virtual absence of fibrinogen), hypofibrinogenemia (fibrinogen less than 100 mgs/dl), and dysfibrinogenemia (dysfunctional fibrinogen).
(2) Heparin therapy: Heparin inhibits action of thrombin.
(3) Presence of fibrin degradation products (FDPs): These interfere with fibrin monomer polymeri-zation. TT is repeated using a mixture of normal plasma and patient’s plasma. If TT remains prolonged, then FDPs are present (provided patient is not receiving heparin).