What are the implications of an INR (International Normalized Ratio) of 2.5, prothrombin time of 25.5 seconds, and aPTT (activated Partial Thromboplastin Time) of 37 seconds in a patient taking 2.5mg of warfarin (coumarin), a vitamin K antagonist?

INR 2.5 on Warfarin 2.5mg: Therapeutic and Appropriate

Your patient's INR of 2.5 is within the standard therapeutic range of 2.0-3.0 for warfarin therapy and requires no dose adjustment. 1, 2

Current Anticoagulation Status

• The INR of 2.5 represents optimal anticoagulation for most indications including atrial fibrillation, venous thromboembolism, and rheumatic heart disease, with the target being 2.5 (range 2.0-3.0). 1, 3
• The prothrombin time of 25.5 seconds is elevated as expected with therapeutic warfarin, as the PT directly correlates with INR values. 4, 2
• The aPTT of 37 seconds (reference 24-33) is mildly elevated, which is a known effect of warfarin even without heparin therapy, and does not indicate a problem or need for intervention. 2, 5

Clinical Significance of These Values

• **Subtherapeutic anticoagulation (INR <2.0) significantly increases thromboembolism risk**, while INR values >3.0 are associated with greater incidence of major bleeding, especially intracranial hemorrhage when INR rises above 3.5. 1, 6
• At INR 2.5, the patient is optimally protected against thromboembolism while minimizing bleeding risk. 1
• The risk of bleeding increases exponentially when INR exceeds 4.0, but at 2.5, bleeding risk remains at baseline therapeutic levels. 1, 6

Warfarin Effect on aPTT

• Warfarin can increase the aPTT even in the absence of heparin, and this is expected. 2, 5
• A retrospective study demonstrated statistically significant elevation of aPTT (mean 55.10 seconds vs baseline 30.79 seconds) when PT was therapeutic on warfarin, with good linear correlation (r=0.821) between PT and aPTT elevation. 5
• The aPTT elevation to 37 seconds is consistent with therapeutic warfarin effect and does not require additional investigation unless there are clinical signs of bleeding. 2, 5

Management Recommendations

• Continue warfarin 2.5mg daily without dose adjustment. 1, 2
• Recheck INR in 1-4 weeks depending on stability, as intervals between INR determinations should be based on the patient's reliability and response once therapeutic range is achieved. 2
• Maintain time in therapeutic range (TTR) >65% to ensure optimal protection, as patients managed by anticoagulation clinics achieve TTR of 56-93% compared to only 33-64% in usual care. 2
• More frequent monitoring is required during fluctuations in diet, weight changes, medication changes, illness, or any indication of bleeding. 1

Important Caveats

• Drug interactions can significantly alter warfarin metabolism—obtain additional INR determinations when other medications (including over-the-counter drugs and botanicals) are initiated, discontinued, or taken irregularly. 2
• Inconsistent vitamin K intake through diet can cause INR fluctuations; counsel the patient to maintain consistent dietary habits. 1, 3
• Elderly patients (>65 years) may require lower maintenance doses due to increased sensitivity and have higher bleeding risk at any given INR level. 1, 6, 2
• Asian patients may require lower warfarin doses, with Chinese outpatients requiring mean daily doses of 3.3±1.4 mg to achieve INR 2.0-2.5. 2