In vitamin K deficiency, which coagulation test is typically prolonged first?

• A. aPTT
• B. PT
• C. TT
• D. Platelet count

Answer: (B)

Vitamin K is required to activate factors II, VII, IX, and X. Among these, factor VII has the shortest half-life, so its activity drops first when vitamin K is deficient. The prothrombin time evaluates the extrinsic pathway (primarily factor VII) and the common pathway, so a deficiency in VII makes the PT become prolonged earliest. The aPTT measures the intrinsic pathway and tends to remain normal until more II and X are depleted, while the thrombin time reflects fibrinogen conversion to fibrin and is not an early indicator of vitamin K deficiency. Platelet count is not affected by vitamin K status. So the earliest test abnormality in vitamin K deficiency is the prolongation of the PT.