Question
Pictorially describe the extrinsic and intrinsic pathway of coagulation. Mention the tests used in their assessment with their normal ranges.
Answer
Extrinsic pathway Tissue factor (released from damaged tissue) → activates Factor VII → Factor VIIa-Tissue Factor complex → activates Factor X → Factor Xa (joins common pathway)
Intrinsic pathway Contact activation (exposed subendothelial collagen) → activates Factor XII → Factor XIIa → activates Factor XI → Factor XIa → activates Factor IX → Factor IXa (with Factor VIIIa as cofactor) → activates Factor X → Factor Xa (joins common pathway)
Common pathway Factor Xa (with Factor Va as cofactor) → converts Prothrombin (Factor II) to Thrombin (Factor IIa) → Thrombin converts Fibrinogen to Fibrin → Factor XIIIa cross-links fibrin → stable fibrin clot
Tests used and normal ranges
| Pathway assessed | Test | Normal range |
|---|---|---|
| Extrinsic + common | Prothrombin Time (PT) | 11–15 seconds |
| Extrinsic + common | INR (standardized PT) | 0.8–1.2 |
| Intrinsic + common | Activated Partial Thromboplastin Time (aPTT) | 25–35 seconds |
| Common pathway (fibrinogen to fibrin) | Thrombin Time (TT) | 14–16 seconds |
| Platelet-vessel wall interaction | Bleeding Time | 2–7 minutes |
| Fibrinogen level | Plasma fibrinogen assay | 200–400 mg/dL |