An 80-year-old man with a recent myocardial infarction has PT 52 sec, INR 5.5, and aPTT 50 sec. This pattern most likely reflects which therapy?

• A. Nontherapeutic dose of low-molecular-weight heparinoid
• B. Supratherapeutic dose of coumadin
• C. Subtherapeutic dose of coumadin
• D. Fibrinolytic agent such as tissue plasminogen activator

Answer: (B)

Warfarin therapy primarily lengthens the extrinsic pathway, which is measured by PT and INR. An INR of 5.5 shows a markedly elevated level, indicating excessive anticoagulation. While warfarin also lowers vitamin K–dependent coagulation factors II, IX, and X and can prolong the aPTT, the PT/INR is the most sensitive test for warfarin effect, and a supratherapeutic dose can produce a high INR with a parallel but less dramatic rise in aPTT. The combination of a very high INR with a prolonged PT matches overdosing on coumadin, rather than heparin-based therapy (which would elevate aPTT more than INR) or a fibrinolytic agent (which would not specifically cause such a high INR).