Understanding the key laboratory tests used to assess hemostasis and coagulation pathways is essential in diagnosing bleeding and clotting disorders. Each test evaluates different factors involved in the intrinsic, extrinsic, and common pathways of the coagulation cascade.
Major Coagulation Tests
| Test | Pathway Tested | Factors Evaluated | Clinical Use / Interpretation |
|---|---|---|---|
| Prothrombin Time (PT) | Extrinsic & Common | VII, X, V, Prothrombin, Fibrinogen | Prolonged in vitamin K deficiency, warfarin therapy, or liver disease |
| International Normalized Ratio (INR) | Standardized form of PT | Same as PT | Used to monitor warfarin therapy globally |
| Partial Thromboplastin Time (PTT) | Intrinsic & Common | XII, XI, IX, VIII, X, V, Prothrombin, Fibrinogen | Prolonged in hemophilia A/B, heparin therapy, or DIC |
| Thrombin Time (TT) | Final step of coagulation | Fibrinogen → Fibrin | Prolonged with low fibrinogen or heparin excess |
| Fibrin Degradation Products (FDPs) | Fibrinolytic system | Fibrin breakdown fragments | Elevated in DIC, severe trauma, or thrombolysis |
Key Points to Remember
- PT/INR → Assesses extrinsic pathway (factor VII).
- PTT → Assesses intrinsic pathway (factors VIII, IX, XI, XII).
- TT → Directly tests conversion of fibrinogen to fibrin.
- FDP → Indicates fibrin breakdown, elevated in DIC.
- Normal ranges vary by lab but must always be interpreted in the clinical context (e.g., anticoagulant therapy, liver function).
Learning Objective
By the end of this topic, medical students should be able to:
Interpret key coagulation laboratory tests (PT, INR, PTT, TT, FDP) and identify their diagnostic significance in bleeding and thrombotic disorders.
