Question:
Which of the following statements is TRUE about Warfarin?
Which of the following statements is TRUE about Warfarin?
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Warfarin's anticoagulant effect is delayed and it is monitored by a specific coagulation test involving the extrinsic pathway.
Updated On: Jun 23, 2026
- It inhibits Vitamin K-dependent clotting factors by blocking Vitamin K epoxide reductase
- It acts immediately on administration and reaches full effect within 2 hours
- It is monitored by measuring aPTT
- It crosses the placenta and is safe in pregnancy
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The Correct Option is A
Solution and Explanation
Step 1: Mechanism of Warfarin.
Warfarin inhibits Vitamin K epoxide reductase (VKORC1), the enzyme responsible for recycling oxidized Vitamin K epoxide back to the active reduced form (Vitamin K hydroquinone). This prevents the gamma-carboxylation of glutamic acid residues on Vitamin K-dependent clotting factors (II, VII, IX, X) and anticoagulant proteins (Protein C, Protein S). These factors become inactive without gamma-carboxylation.
Step 2: Onset of action.
Warfarin does NOT act immediately. Its anticoagulant effect is delayed because it acts by preventing synthesis of new clotting factors -- the existing circulating factors must be depleted. The full effect takes 3-5 days. The earliest effect (at 24-36 hours) reflects depletion of Factor VII (shortest half-life, 4-6 hours), which causes initial PT prolongation but a paradoxical transient pro-thrombotic state due to early depletion of Protein C and S (also Vit K-dependent).
Step 3: Monitoring.
Warfarin is monitored by PT/INR (Prothrombin Time/International Normalized Ratio), NOT aPTT. aPTT monitors heparin (unfractionated).
Step 4: Pregnancy safety.
Warfarin CROSSES the placenta and is TERATOGENIC (Warfarin embryopathy: nasal hypoplasia, stippled epiphyses, CNS defects). It is contraindicated in the first trimester and near delivery. Heparin (which does NOT cross the placenta) is used instead.
Answer: It inhibits Vitamin K-dependent clotting factors by blocking Vitamin K epoxide reductase
Warfarin inhibits Vitamin K epoxide reductase (VKORC1), the enzyme responsible for recycling oxidized Vitamin K epoxide back to the active reduced form (Vitamin K hydroquinone). This prevents the gamma-carboxylation of glutamic acid residues on Vitamin K-dependent clotting factors (II, VII, IX, X) and anticoagulant proteins (Protein C, Protein S). These factors become inactive without gamma-carboxylation.
Step 2: Onset of action.
Warfarin does NOT act immediately. Its anticoagulant effect is delayed because it acts by preventing synthesis of new clotting factors -- the existing circulating factors must be depleted. The full effect takes 3-5 days. The earliest effect (at 24-36 hours) reflects depletion of Factor VII (shortest half-life, 4-6 hours), which causes initial PT prolongation but a paradoxical transient pro-thrombotic state due to early depletion of Protein C and S (also Vit K-dependent).
Step 3: Monitoring.
Warfarin is monitored by PT/INR (Prothrombin Time/International Normalized Ratio), NOT aPTT. aPTT monitors heparin (unfractionated).
Step 4: Pregnancy safety.
Warfarin CROSSES the placenta and is TERATOGENIC (Warfarin embryopathy: nasal hypoplasia, stippled epiphyses, CNS defects). It is contraindicated in the first trimester and near delivery. Heparin (which does NOT cross the placenta) is used instead.
Answer: It inhibits Vitamin K-dependent clotting factors by blocking Vitamin K epoxide reductase
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