A patient on therapeutic warfarin will most likely have which combination of lab results?

• A. Normal PT/INR, increased aPTT, prolonged bleeding time, low platelet count
• B. Increased PT/INR, increased aPTT, normal bleeding time, normal platelet count
• C. Normal PT/INR, normal aPTT, normal bleeding time, normal platelet count
• D. Increased PT/INR, normal aPTT, prolonged bleeding time, low platelet count

Answer: (B)

Warfarin interferes with vitamin K–dependent clotting factors (II, VII, IX, X) and some anticoagulant proteins, which mainly slows the extrinsic pathway. This shows up as an increased PT/INR on the lab tests. Because these factors contribute to both the extrinsic and common pathways, aPTT can also be prolonged, though the most consistently affected test is the PT/INR. Platelet number and function aren’t directly altered by warfarin, so bleeding time remains normal and platelet count is typically normal.

So the pattern you’d expect with therapeutic warfarin is an increased PT/INR and often an increased aPTT, with normal bleeding time and normal platelet count. The other patterns imply issues with platelets or different pathways (or no anticoagulation effect), which doesn’t fit how warfarin works.