Thrombin time
Fibrin and control · detail · Final step only Reptilase
Exogenous thrombin is added to citrated plasma, bypassing the entire cascade so that only the final step — fibrinogen to fibrin — is timed. It is the test that separates a fibrinogen problem or a thrombin inhibitor from a factor deficiency higher up the cascade.
Traced from the start
- Tissue factor and the extrinsic triggerVitamin K cycle and gamma-carboxylation
- Contact activation system (factor XII, prekallikrein, high-molecular-weight kininogen)
- Subendothelial collagen and von Willebrand factor
- Tissue factor-factor VIIa complex (extrinsic tenase)Factor IXaFactor VIIIa and its von Willebrand factor carrierActivated platelet phospholipid surface
- Factor XaFactor VaProthrombin (factor II)
- Thrombin (factor IIa)
- Thrombin time
Detail
- Normal range
- About 14-19 seconds, reagent dependent
- Measures
- Fibrinogen concentration and function, plus the presence of any thrombin inhibitor
- Prolonged by
- Heparin, dabigatran, hypofibrinogenaemia, dysfibrinogenaemia, high fibrin degradation products in DIC, and paraproteins
- Reptilase time
- Uses a snake venom enzyme unaffected by heparin: prolonged in dysfibrinogenaemia but normal in heparin contamination, which is how the two are told apart
When it goes wrong
Isolated prolonged thrombin time in a patient with a central venous catheter
Heparin contamination of the sample; a normal reptilase time confirms it and the test should simply be repeated on a peripheral draw
Practise this structure
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