Expected Prothrombin Time (PT) on Coumadin
For patients on Coumadin (warfarin), the PT is not the primary monitoring parameter—instead, the INR (International Normalized Ratio) should be used, with a therapeutic target of 2.0-3.0 for most indications, which typically corresponds to a PT of approximately 1.5-2.5 times the control value, though this varies significantly by thromboplastin reagent used. 1, 2
Why INR, Not PT, Is the Standard
The PT alone is unreliable for monitoring warfarin because different thromboplastin reagents produce vastly different PT values for the same level of anticoagulation 1, 3, 4. The INR was specifically developed by the World Health Organization to standardize PT results across laboratories and eliminate this variability 3, 4.
- The INR mathematically corrects the PT using the International Sensitivity Index (ISI) of the specific thromboplastin reagent, making results comparable regardless of which laboratory performs the test 1, 4
- Baseline PT for patients not on warfarin is typically 11-13.5 seconds, with an INR of approximately 1.0 2, 5
- Therapeutic anticoagulation produces PT values roughly 1.5-2.5 times the control, but this ratio is reagent-dependent and should not be used for clinical decision-making 1
Target INR Ranges by Indication
Standard therapeutic range (INR 2.0-3.0): 1, 2
- Atrial fibrillation with stroke risk factors
- Venous thromboembolism (DVT/PE)
- Rheumatic heart disease
- Bioprosthetic heart valves (first 3 months)
Higher intensity range (INR 2.5-3.5): 1, 2, 5
- Mechanical prosthetic heart valves
- Recurrent systemic embolism despite adequate anticoagulation
- Patients at extremely high risk of stroke
Monitoring Schedule
- 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
- Once stable, INR can be checked as infrequently as every 4 weeks
- Resume frequent monitoring during medication changes (especially antibiotics), dietary changes, illness, or any bleeding 1, 2
Critical Pitfalls to Avoid
Do not rely on PT values alone for warfarin monitoring—always use INR 1, 3, 4. Different thromboplastin reagents with varying ISI values (ranging from 1.3 to 2.1) produce dramatically different PT results for the same degree of anticoagulation 1, 6.
Lupus anticoagulants interfere with INR accuracy: 7
- These antibodies prolong the baseline PT and can cause INR values that overestimate the true level of anticoagulation
- Consider chromogenic factor X assay for more accurate monitoring in these patients
Laboratory-specific factors affecting PT/INR: 1, 4
- Use thromboplastin reagents with ISI ≤1.7 for optimal accuracy
- Automated clot detectors can influence INR measurements
- Citrate concentration in collection tubes affects results (use 3.2% citrate)
- Underfilling blood collection tubes spuriously prolongs PT
Concurrent medications significantly affect warfarin response: 1
- Amiodarone reduces warfarin clearance with peak interaction effects at 7 weeks—requires close INR monitoring at least weekly during first 6 weeks
- Many antibiotics, antifungals, and other drugs alter warfarin metabolism
Clinical Significance of Out-of-Range Values
Subtherapeutic anticoagulation (INR <2.0): 2, 5
- Significantly increases risk of thromboembolism
- Provides inadequate protection against stroke and clot formation
Supratherapeutic anticoagulation: 2, 5
- INR 4.0-5.0: Reduce or omit next dose, resume at lower dose when INR approaches therapeutic range
- INR 5.0-9.0 without bleeding: Omit 1-2 doses; consider oral vitamin K 1-2.5 mg if bleeding risk factors present
- INR >9.0 without bleeding: Give oral vitamin K 3-5 mg with expected reduction in 24-48 hours
- Serious bleeding or life-threatening overdose: Vitamin K 10 mg IV over 30 minutes plus fresh frozen plasma or prothrombin complex concentrate
The risk of bleeding increases exponentially when INR exceeds 4.0 and becomes sharply elevated at INR ≥5.0 2, 5.