A patient is taking 10 mg per day of Coumadin (warfarin). The results of which of the following lab tests will be most impacted?

A patient is taking 10 mg per day of Coumadin (warfarin). The results of which of the following lab tests will be most impacted?

• A. Protein C
• B. Factor V Leiden
• C. Antithrombin
• D. Factor VIII

Answer: (A)

Warfarin blocks vitamin K–dependent gamma-carboxylation, reducing the synthesis of proteins II, VII, IX, X and the anticoagulant proteins C and S. Of these, Protein C has the shortest half-life, so its activity drops earliest and most markedly after starting warfarin. That makes Protein C activity the lab result most affected by a standard warfarin dose. The rapid fall in Protein C relative to procoagulant factors can even create a transient hypercoagulable state. Factor V Leiden is a genetic variant and isn’t changed by warfarin therapy, so it isn’t reflected by drug effect. Antithrombin and Factor VIII levels are not directly driven down by warfarin in routine dosing.

Warfarin blocks vitamin K–dependent gamma-carboxylation, reducing the synthesis of proteins II, VII, IX, X and the anticoagulant proteins C and S. Of these, Protein C has the shortest half-life, so its activity drops earliest and most markedly after starting warfarin. That makes Protein C activity the lab result most affected by a standard warfarin dose. The rapid fall in Protein C relative to procoagulant factors can even create a transient hypercoagulable state.

Factor V Leiden is a genetic variant and isn’t changed by warfarin therapy, so it isn’t reflected by drug effect. Antithrombin and Factor VIII levels are not directly driven down by warfarin in routine dosing.