Medicine & USMLE

Extrinsic Pathway of Coagulation

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Coagulation
  1. Common Pathway of Coagulation
  2. Extrinsic Pathway of Coagulation
  3. Intrinsic Pathway of Coagulation
  4. Hemophilia A
  5. Hemophilia B
  6. Hemophilia C
  7. Factor V Leiden
  8. Antithrombin-3 Deficiency

Summary

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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
    • 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)