From the Guidelines
Target INR Range for Warfarin Anticoagulation
The target international normalized ratio (INR) for sufficient anticoagulation with warfarin should be between 2.0 to 3.0 1.
High-Risk Conditions
For certain high-risk conditions, such as mechanical heart valves, a higher target INR range of 2.5 to 3.5 may be indicated 1.
Monitoring INR Levels
The INR should be monitored regularly, ideally every 1 to 4 weeks, to ensure therapeutic levels are maintained and to minimize the risk of bleeding complications 1.
Dose Administration
Warfarin is typically administered orally once daily, with a dose of 2.5 to 10 mg per day 1.
Special Considerations
In patients with a history of ischemic stroke or TIA and an aortic mechanical prosthesis, it is recommended to maintain the INR at a higher target of 3.0 (range, 2.5–3.5) or to add aspirin 75 to 100 mg daily 1.
Bleeding Risk
Bleeding that occurs at an INR of 3.0 is frequently associated with trauma or an underlying lesion in the gastrointestinal or urinary tract 1.
Initiation of Warfarin
It is prudent to initiate warfarin at lower doses in the elderly, as patients 75 years of age require 1 mg/d less than younger individuals to maintain comparable prolongation of the INR 1.
Long-term Management
Long-term management is challenging for patients who have experienced bleeding during warfarin anticoagulation yet require thromboembolic prophylaxis, and a target INR of 2.0 to 2.5 seems sensible for patients with mechanical prosthetic heart valves and persistent risk of bleeding during anticoagulation in the therapeutic range 1.
From the FDA Drug Label
The dose of warfarin should be adjusted to maintain a target INR of 2.5 (INR range, 2.0 to 3.0) for all treatment durations. For patients with a St. Jude Medical bileaflet valve in the aortic position, a target INR of 2.5 (range, 2.0 to 3.0) is recommended. For patients with tilting disk valves and bileaflet mechanical valves in the mitral position, the 7th ACCP recommends a target INR of 3.0 (range, 2.5 to 3.5). For patients with caged ball or caged disk valves, a target INR of 3.0 (range, 2.5 to 3.5) in combination with aspirin, 75 to 100 mg/day is recommended. For patients with bioprosthetic valves, warfarin therapy with a target INR of 2.5 (range, 2.0 to 3.0) is recommended for valves in the mitral position and is suggested for valves in the aortic position for the first 3 months after valve insertion. A moderate dose regimen (INR 2.0 to 3.0) is recommended for patients with valvular disease associated with atrial fibrillation, patients with mitral stenosis, and patients with recurrent systemic embolism of unknown etiology.
The target INR for anticoagulation with Warfarin varies depending on the indication:
- Venous Thromboembolism: 2.0-3.0
- Atrial Fibrillation: 2.0-3.0
- Mechanical Heart Valves:
- Aortic position: 2.0-3.0 (St. Jude Medical bileaflet valve), 2.5-3.5 (tilting disk valves and bileaflet mechanical valves)
- Mitral position: 2.5-3.5 (tilting disk valves and bileaflet mechanical valves), 2.5-3.5 (caged ball or caged disk valves)
- Bioprosthetic Heart Valves: 2.0-3.0
- Recurrent Systemic Embolism and Other Indications: 2.0-3.0 2
From the Research
International Normalized Ratio (INR) for Warfarin Therapy
The International Normalized Ratio (INR) is a test used to measure the time it takes for blood to clot and is primarily used to monitor patients on warfarin therapy. The ideal INR range for warfarin therapy varies depending on the patient's condition and the reason for anticoagulation.
- For patients with atrial fibrillation, the recommended INR range is 2.5 +/- 0.5 indefinitely 3.
- For patients with mechanical valves, the recommended INR range is 3.0 +/- 0.5 indefinitely 3.
- For patients with venous thromboembolism, the recommended INR range is 2.5 +/- 0.5, with the length of therapy determined by associated conditions 3.
- A study on age and body weight adjusted warfarin initiation program for ischemic stroke patients found that the median warfarin maintenance dose was 4 mg per day, with an INR range of 2-3 4.
Factors Affecting INR
Several factors can affect INR levels, including:
- Factor IX levels: patients with lower factor IX levels may be at a higher risk of bleeding even with a therapeutic INR 5.
- Age and body weight: these factors can affect the maintenance dose of warfarin and the resulting INR levels 4.
Risks of Subtherapeutic Anticoagulation
Subtherapeutic anticoagulation, defined as an INR value below the therapeutic range, can increase the risk of thromboembolism. However, a study found that patients with stable INRs who experience a significant subtherapeutic INR value have a low risk of thromboembolism in the ensuing 90 days 6.
INR Testing Intervals
The frequency of INR testing can vary depending on the patient's condition and the stability of their INR levels. A study found that extended INR testing intervals (> 5 weeks) can be safely implemented in stable warfarin patients, with no significant difference in bleeding events or emergency department visits compared to patients with non-extended INR testing intervals 7.