Are INR and PT the Same?
No, INR and PT are not the same—PT (prothrombin time) is the raw clotting time measured in seconds, while INR (International Normalized Ratio) is a mathematically standardized value derived from the PT to account for variability between different laboratory reagents and methods. 1
Key Differences
Prothrombin Time (PT)
- PT is measured in seconds and represents the actual clotting time of plasma after adding thromboplastin (tissue factor) and calcium 1
- PT values vary significantly depending on the specific thromboplastin reagent used and the measurement technique/instrument 1
- PT results are not interchangeable between laboratories using different reagents, making direct comparison unreliable 1
International Normalized Ratio (INR)
- INR is a calculated, standardized value derived from the PT using the formula: INR = (PT/MNPT)^ISI, where MNPT is the mean normal prothrombin time and ISI is the international sensitivity index 1
- INR was developed by the WHO in the early 1980s specifically to standardize PT results across different laboratories and reagents for monitoring vitamin K antagonist (warfarin) therapy 1, 2
- INR provides equivalence between various reagents and techniques, allowing consistent anticoagulation monitoring regardless of which laboratory performs the test 1
Critical Clinical Distinction
The most important clinical point: INR is NOT the same as PT ratio (patient PT/control PT), which is a common source of confusion 1. The PT ratio is simply the patient's PT divided by a normal control PT, without the ISI correction factor that makes the INR standardized 1.
Why This Matters Clinically
- Subtle PT changes may not be detected when reported as INR, particularly in early warfarin therapy or mild coagulopathy 1
- In one COVID-19 study, PT was modestly prolonged in non-survivors (15.5 vs 13.6 seconds), but these subtle changes would likely not be picked up if reported as INR 1
- During warfarin initiation (first 72-96 hours), the INR is less reliable because it predominantly reflects Factor VII depletion (6-hour half-life) while Factor II/prothrombin (60-72 hour half-life) remains near-normal, creating a falsely reassuring INR 3, 4
Practical Implications
For Warfarin Monitoring
- Always overlap heparin with warfarin for at least 4-5 days until the INR is therapeutic, regardless of early INR elevation, because true anticoagulation requires Factor II depletion 3, 4
- Never discontinue heparin based solely on an elevated INR in the first 72-96 hours of warfarin therapy 3
- Once steady-state is achieved (after 4-6 days), the INR becomes a reliable indicator as all vitamin K-dependent factors reach equilibrium 3
For Non-Warfarin Patients
- INR was designed specifically for warfarin monitoring based on patients on stable anticoagulation for at least 6 weeks 1, 3
- There is no high-quality evidence supporting INR targets or plasma transfusion for mildly elevated INR (1.0-2.0 range) in patients not on vitamin K antagonists 1
Common Pitfalls to Avoid
- Do not use "PT ratio" and "INR" interchangeably—they are mathematically different and clinically distinct 1
- Do not assume all laboratories report the same value—even with INR standardization, variability exists between laboratories using the same reagents and instruments 5, 6
- Do not rely on early INR values during warfarin initiation as the sole indicator of adequate anticoagulation 3, 4
- Do not apply INR targets from warfarin studies to non-warfarin patients—the INR standardization was never validated for this purpose 1