Normal PT and INR Reference Ranges
In adults without bleeding disorders or anticoagulation therapy, the normal prothrombin time (PT) is 11-13.5 seconds and the normal INR is 1.0-1.3. 1
Standard Reference Values
The established normal ranges for coagulation testing in healthy adults are:
- PT: 11-13.5 seconds 1
- INR: 1.0-1.3 (with values up to 1.4 considered within acceptable laboratory variation) 1
- PT ratio: <1.4 1
These values represent the baseline coagulation status in individuals not receiving anticoagulation therapy and without underlying coagulopathy. 1
Critical Context: INR Was Designed for Warfarin Monitoring
The INR was specifically designed and validated only for monitoring vitamin K antagonist (warfarin) therapy, not as a general screening test for bleeding risk or coagulopathy. 2, 1
- The INR standardization system emerged from decades of work to address variability in PT testing across different thromboplastin reagents used in laboratories. 2
- The mean normal prothrombin time (MNPT) is calculated as the geometric mean of PT values from at least 20 healthy individuals of both sexes. 2
- For patients on warfarin, the therapeutic INR target is typically 2.0-3.0 for most indications (atrial fibrillation, venous thromboembolism). 1, 3
Important Clinical Pitfalls
Do not use INR values to predict bleeding risk in patients not taking warfarin. 2, 1
- A systematic review found that pre-procedural INR had weak or no association with bleeding in 78 of 79 studies of non-warfarin patients. 2
- An INR of 1.4 in a patient not on anticoagulation does not represent clinically significant coagulopathy and does not predict adverse bleeding outcomes. 1
- Normal PT/INR values do not exclude all bleeding disorders, particularly platelet function disorders. 1
Plasma transfusion for mildly elevated INR values (below 1.5-2.0) in non-bleeding patients lacks biological plausibility and high-quality evidence of benefit. 2
- Plasma infusion does not meaningfully change INR values below 1.5 and only minimally impacts values below 2.0. 2
- The practice exposes patients to volumetric and immunologic risks without proven benefit. 2
Laboratory Variation Considerations
- PT results vary slightly between laboratories depending on the thromboplastin reagent and instrumentation used. 1
- The International Sensitivity Index (ISI) of the thromboplastin reagent is used to standardize PT results into INR values using the formula: INR = [PT patient/PT normal]^ISI. 2
- A PT ratio or INR >1.4 is generally considered abnormal for patients not on anticoagulation when invasive procedures are planned. 1