Extrinsic Pathway of Coagulation
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Coagulation
- Common Pathway of Coagulation
- Extrinsic Pathway of Coagulation
- Intrinsic Pathway of Coagulation
- Hemophilia A
- Hemophilia B
- Hemophilia C
- Factor V Leiden
- Antithrombin-3 Deficiency
Summary
Coming soon...
Key Points
- Extrinsic Pathway of Coagulation
- Also known as the tissue factor-mediated pathway
- Pathway
- Activation by cleavage of Factor VII → VIIa
- Requires tissue factor, calcium (Ca2+), phospholipid
- Vitamin K-dependent clotting factor
- Inhibited by Warfarin
- Factor VIIa + Tissue factor (as a cofactor) helps cleave Factor X → Xa
- Factor Xa marks the end of the extrinsic pathway and the beginning of the common pathway
- Activation by cleavage of Factor VII → VIIa
- Monitoring
- PT (prothrombin time)
- PT measures time to clotting (extrinsic and common pathways) after tissue factor is added
- PT is the best measure of warfarin effectiveness or vitamin K deficiency since factor VII has the shortest half-life among all vitamin K-dependent factors
- Many commercial PT tests also include heparin inhibitors like polybrene (reinforcing its utility for warfarin monitoring over heparin)
- Mnemonic: “play tennis outside” = PT measures extrinsic
- INR (international normalized ratio)
- INR is a normalized ratio of a patient’s PT over the international average (~1 is normal or average)
- PT (prothrombin time)