Warfarin Monitoring: INR (International Normalized Ratio)
The International Normalized Ratio (INR) is the laboratory test used to monitor warfarin therapy. 1, 2
Why INR Instead of PT
The INR standardizes prothrombin time (PT) results across different laboratories and thromboplastin reagents, eliminating the significant variability that made raw PT values unreliable for warfarin monitoring. 3 While the PT measures the same clotting pathway, different thromboplastin reagents produce vastly different PT values for identical levels of anticoagulation, making the INR essential for consistent dosing decisions. 2, 4
Target INR Ranges
For most indications (atrial fibrillation, venous thromboembolism, bioprosthetic valves), maintain INR between 2.0-3.0. 1, 2
- For mechanical prosthetic heart valves and patients at extremely high embolic risk, target INR 2.5-3.5. 1, 2
- The standard target of 2.5 ± 0.5 applies to most patients, while higher-risk patients with certain mechanical heart valves require 3.0 ± 0.5. 3
Monitoring Schedule Algorithm
- Check INR daily until therapeutic range is reached and sustained for 2 consecutive days
- Then check 2-3 times weekly for 1-2 weeks
- Then weekly for 1 month
During maintenance phase: 1, 2
- Check INR every 1-2 months once stable
- Can extend to every 4-6 weeks in patients with consistently stable values 1, 5
Resume frequent monitoring immediately when: 2
- Any medication changes occur
- Dietary changes
- Intercurrent illness
- Any bleeding episodes
- Weight changes 1
Critical Technical Considerations
Use thromboplastin reagents with International Sensitivity Index (ISI) less than 1.5, preferably less than 1.2. 1 This improves precision of INR measurements and reduces variability. 3
Ensure proper blood collection technique: 3, 1
- Use 3.2% citrate collection tubes (not 3.8%, which spuriously elevates INR)
- Avoid underfilling tubes, as excess citrate prolongs PT artificially
Common Pitfall: INR Reliability During Initiation
The INR is less reliable during the first few days of warfarin therapy because it was validated on patients receiving stable anticoagulation for at least 6 weeks. 3 However, the INR remains more reliable than unconverted PT ratios even during initiation, so continue using INR from day one. 3
When INR May Be Misleading
The INR is designed and validated exclusively for monitoring vitamin K antagonist therapy. 3 Using INR to assess bleeding risk in patients not on warfarin (such as those with liver disease, acute illness, or pre-procedural screening) lacks validity and should be avoided. 3 The INR does not predict procedural bleeding or general coagulopathy outside the context of warfarin therapy. 3
Additional Monitoring Beyond INR
The FDA label emphasizes monitoring for: 6
- Drug-drug interactions (especially enzyme inducers/inhibitors)
- Dietary changes affecting vitamin K intake
- Changes in medications or botanicals
- Signs of bleeding (major or minor)
- Patient compliance 1