What are the expected International Normalized Ratio (INR) and Prothrombin Time (PT) levels for a patient with multiple strokes on warfarin (coumarin)?

Target INR and PT Levels for Patients with Multiple Strokes on Warfarin

For patients with multiple strokes on warfarin, the target INR should be 2.5 with a range of 2.0-3.0. 1

INR Targets Based on Stroke Etiology

Atrial Fibrillation

• For patients with ischemic stroke or TIA with persistent or paroxysmal (intermittent) atrial fibrillation, anticoagulation with adjusted-dose warfarin with a target INR of 2.5 (range 2.0-3.0) is recommended. 1
• The safety and effectiveness of warfarin therapy depends critically on maintaining the INR within this therapeutic range. 1
• Patients should aim for a time in therapeutic range (TTR) >65% to ensure optimal protection against recurrent stroke. 1

Mechanical Heart Valves

• For patients with ischemic stroke who have mechanical prosthetic heart valves, a higher target INR of 3.0 (range 2.5-3.5) is recommended. 1
• For patients with mechanical prosthetic heart valves who have recurrent ischemic stroke despite adequate anticoagulation, adding aspirin 75-100 mg/day to warfarin therapy is reasonable if the patient is not at high bleeding risk. 1

Rheumatic Heart Disease

• For patients with rheumatic mitral valve disease, whether or not AF is present, long-term warfarin therapy with a target INR of 2.5 (range 2.0-3.0) is recommended. 1, 2
• Antiplatelet agents should not be routinely added to warfarin in patients with rheumatic heart disease to avoid additional bleeding risk. 1

Cardiomyopathy

• For patients with ischemic stroke who have dilated cardiomyopathy, warfarin with an INR of 2.0-3.0 may be considered for prevention of recurrent events. 1

Monitoring and Management

INR Monitoring Frequency

• INR should be determined at least weekly during initiation of anticoagulant therapy. 1
• Once anticoagulation is stable (INR consistently in therapeutic range), monitoring can be reduced to at least monthly. 1
• More frequent monitoring is needed when starting or stopping other medications that may interact with warfarin. 2

PT Correlation with INR

• PT is the laboratory test used to calculate INR, but the raw PT value itself varies between laboratories. 3
• INR standardizes PT results across different laboratories and is the preferred measurement for monitoring warfarin therapy. 3
• A target INR of 2.0-3.0 typically corresponds to a PT that is approximately 1.3-1.8 times the laboratory control value, but this can vary based on the laboratory's reagents. 3

Special Considerations

Elderly Patients

• Patients 60 years or older exhibit greater than expected PT/INR response to warfarin and may require lower doses to achieve therapeutic INR. 3
• For elderly patients (>75 years) with AF, careful monitoring is especially important as they are at greatest risk of intracranial bleeding during warfarin therapy. 4

Factors Affecting INR Stability

• Only about 75% of patients achieve initial INR stabilization (defined as three consecutive INR values between 2.0-3.0). 5
• Even after initial stabilization, approximately 30% of subsequent INR values fall outside the therapeutic range. 5
• Age ≥75 years, hypertension, and prior stroke are positively associated with achieving INR stabilization, while heart failure is negatively associated. 5

Risks of Non-Therapeutic INR

Subtherapeutic INR (< 2.0)

• Significantly increases the risk of thromboembolism and recurrent ischemic stroke. 1, 4
• For patients with subtherapeutic INR who develop acute ischemic stroke, intravenous thrombolysis may be considered if INR is ≤1.7. 6

Supratherapeutic INR (> 3.0)

• INR levels >3.0 are associated with a greater incidence of major bleeding. 1, 4
• The risk of bleeding increases exponentially with INR and becomes clinically unacceptable once the INR exceeds 5.0. 4
• Intracranial hemorrhage risk rises significantly when the INR exceeds 3.5. 1

Common Pitfalls to Avoid

• Targeting lower INR ranges (e.g., 1.5-2.0) is not supported by strong evidence and may lead to inadequate protection against thromboembolism. 2
• Inconsistent vitamin K intake through diet can cause fluctuations in INR values, making it important to maintain consistent dietary habits. 2
• Drug interactions can significantly alter warfarin metabolism and effect, requiring more frequent monitoring when starting or stopping other medications. 2, 3
• Failing to adjust warfarin dosing based on patient characteristics such as age, weight, and comorbidities can lead to poor INR control. 7