Thrombin time is the clotting time of plasma on addition of thrombin. It measures the amount and quality of fibrinogen. Thrombin time is prolonged when the fibrinogen levels fall below 70-100mg/dL. The levels of other coagulations factors do not affect thrombin time. Thrombin time is also prolonged by the presence of fibrin/fibrinogen degradation products. It can also be prolonged with a very high fibrinogen levels. Dilution of test plasma normalizes the thrombin time in such patients.
The tests is performed on platelet poor plasma. Thrombin solution is added to PPP and the time for the clot to form is measured. The nature of the clot is also observed. The clot may be transparent or opaque. The test is performed with commercially available bovine thrombin. The concentration of thrombin is adjested to get a thrombin time of 15 on normal plasma. If a more concentrated thrombin is used mild defects in fibrinogen may not be detected. The test is performed with paired test as well as control samples. The thrombin time is the mean of the two readings. Normal thrombin time is two ±2 seconds from the control.
Causes of Prolongation
- Therapy with unfractionated heparin. Low molecular weight heparin does not alter the thrombin time appreciably.
- Hypofibrinogenaemia and occasionally hyperfibrinogenaemia may prolong thrombin time. The mechanism of prolongation by hyperfibrinogenaemia is unclear but hyperfibrinogenaemia may interfere with fibrin polymer formation.
- Paraproteins may interfere with polymerization of fibrin and prolong thrombin time.
Short Thrombin Time
The thrombin time is shortened when the coagulation is activated.
Nature of the Clot
Transparent fluffy clots indacetes an anomaly in fibrin polymerization. This seen with dysfibrinogenaemias either congenital on seen in liver disease.
Reptilase Clotting Time
Reptilase clotting time is a variation of thrombin time. Clotting is induced by reptilase, an enzyme prepared from snake venom. This test is unaffected by heparin. In samples contaminated with heparin the thrombin time is prolonged where as the reptilase time is normal. On the other hand dysfbrinogenaemia affects the reptilase clotting time more than thrombin time. In patinets with hypofibrinogenaemia the prolongation is equal.