Normal INR in a Patient on ASA 75 mg OD
The expected International Normalized Ratio (INR) in a patient taking Aspirin (Acetylsalicylic Acid) 75 mg once daily should be within the normal range of 0.8-1.2, as aspirin at this dose does not affect the INR measurement.
Understanding INR and Aspirin
- INR is a standardized measurement used to monitor the effectiveness of vitamin K antagonists like warfarin, not antiplatelet medications like aspirin 1
- Aspirin at low doses (75-100 mg daily) functions as an antiplatelet agent by inhibiting thromboxane A2, which affects platelet aggregation through a different mechanism than the coagulation cascade measured by INR 1
- Unlike warfarin and other vitamin K antagonists that affect the extrinsic coagulation pathway (measured by INR), aspirin does not impact the production of clotting factors II, VII, IX, and X 1
Clinical Implications
- Patients on aspirin 75 mg daily without concomitant anticoagulant therapy should maintain a normal INR (0.8-1.2) 1
- If an elevated INR is observed in a patient taking only aspirin, other causes should be investigated, such as liver disease, vitamin K deficiency, or undisclosed use of anticoagulants 2
- Aspirin affects platelet function and primary hemostasis rather than the secondary coagulation cascade measured by INR 1
Important Distinctions
- When aspirin is used in combination with warfarin (for specific indications like mechanical heart valves), the target INR is determined by the warfarin therapy, not the aspirin 1
- For patients with mechanical heart valves on both warfarin and aspirin, the recommended INR target is 2.0-3.0 or 2.5-3.5 depending on valve type and risk factors, with aspirin dose typically 75-100 mg daily 1
- For patients with atrial fibrillation on warfarin who cannot tolerate aspirin and clopidogrel, the target INR is 2.0-3.0 1
Common Pitfalls to Avoid
- Do not confuse the bleeding risk of aspirin with an effect on INR measurement 3, 4
- Avoid assuming that a patient with an elevated INR is necessarily non-compliant with aspirin therapy; the elevated INR must be due to other factors 2
- Do not use INR to monitor the antiplatelet effect of aspirin; specific platelet function tests would be needed for this purpose 1, 5
- Remember that aspirin can increase bleeding risk through its antiplatelet effect even though it does not affect the INR 3, 4
Special Considerations
- In patients requiring both antiplatelet and anticoagulant therapy (e.g., recent stent plus atrial fibrillation), the INR target is determined by the anticoagulant indication, not by the aspirin 1
- When transitioning between therapies or adding aspirin to existing anticoagulation, remember that aspirin itself will not alter the INR but may increase bleeding risk 4