A patient started on warfarin develops a PT elevation within 24 to 36 hours, well before the aPTT rises into the therapeutic range. Which factor accounts for this earliest laboratory change?
- A Factor II, because it has the longest half-life
- B Factor IX, because it is consumed fastest
- C Factor VII, because it has the shortest half-life among vitamin K dependent factors ✓
- D Factor X, because it sits at the convergence of both pathways
Correct answer: C. Factor VII, because it has the shortest half-life among vitamin K dependent factors
Explanation
Warfarin blocks vitamin K epoxide reductase, preventing gamma-carboxylation of factors II, VII, IX, and X and of proteins C and S. Factor VII has the shortest plasma half-life, roughly 4 to 6 hours, so its level falls first and the extrinsic pathway test, the PT, becomes prolonged earliest. Factors II and X have half-lives measured in days, so full anticoagulation takes several days. This is why bridging heparin is used when rapid anticoagulation is needed.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
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