Normal Values for PT/INR and APTT
The normal reference range for Prothrombin Time (PT) is typically 11-13.5 seconds, for International Normalized Ratio (INR) is 0.8-1.2, and for Activated Partial Thromboplastin Time (APTT) is 25-35 seconds, though exact values may vary slightly between laboratories. 1
Normal Reference Ranges
- PT typically ranges from 11-13.5 seconds in healthy individuals not on anticoagulation therapy 1
- INR in healthy individuals not on anticoagulant therapy normally ranges from 0.8-1.2 2
- APTT typically ranges from 25-35 seconds in individuals with normal coagulation function 1, 3
Clinical Significance of These Values
- INR was specifically designed and validated to standardize PT results for monitoring vitamin K antagonist therapy (like warfarin), not as a general predictor of bleeding risk in other contexts 4
- For patients on warfarin therapy, the therapeutic INR range is typically 2.0-3.0 for most indications 5, 6
- Higher INR targets (2.5-3.5) may be required for specific conditions such as mechanical heart valves 7
- The risk of bleeding increases exponentially when INR exceeds 5.0 7
Factors Affecting PT/INR and APTT Values
- PT/INR can be prolonged due to vitamin K deficiency, liver disease, or anticoagulant therapy 1
- APTT can be prolonged due to lupus anticoagulant, hemophilia, or heparin therapy 1
- Both PT/INR and APTT can be prolonged in conditions affecting multiple coagulation factors, such as liver disease or disseminated intravascular coagulation 1
Interpretation in Different Clinical Contexts
- For patients not on anticoagulation therapy, a PT ratio or APTT ratio >1.4 times the normal control is generally considered abnormal 5
- For patients on vitamin K antagonist therapy, INR values between 2.0-3.0 are typically targeted for most indications 5
- In certain high-risk conditions (such as mechanical heart valves), a higher INR target of 2.5-3.5 may be recommended 7
Important Considerations
- Storage conditions affect stability of PT/INR and APTT test results - samples are stable for up to 36 hours when frozen, but deteriorate within 12 hours at room temperature 3
- The INR system was specifically designed to standardize PT results for patients on vitamin K antagonist therapy and may not accurately reflect bleeding risk in other clinical scenarios 4
- Different laboratories may have slightly different reference ranges based on their specific reagents and testing methods 2, 8