What are the normal values for Prothrombin Time (PT) International Normalized Ratio (INR)?

Normal Values for Prothrombin Time (PT) International Normalized Ratio (INR)

The normal INR range for individuals not on anticoagulation therapy is 0.9-1.1, with an INR above 1.5 considered discoagulopathy that significantly increases bleeding risk. 1

Understanding INR

The International Normalized Ratio (INR) is a standardized measurement specifically designed and validated to evaluate coagulation status and bleeding risk in patients receiving vitamin K antagonists, such as warfarin. It was developed to standardize prothrombin time (PT) results across different laboratories and reagents.

What INR Measures

• INR measures the depression of vitamin K-dependent clotting factors (II, VII, X)
• PT is measured in seconds and depends on the thromboplastin reagent and measurement technique
• The PT in seconds is transformed to INR using a mathematical formula to achieve equivalence between various reagents and techniques 2

INR Calculation

The INR is calculated using the following formula:

• INR = (PT/MNPT)^ISI
  • Where MNPT is the Mean Normal Prothrombin Time
  • ISI is the International Sensitivity Index 3

Clinical Interpretation of INR Values

For Patients NOT on Anticoagulation

• Normal range: 0.9-1.1 1
• INR below 1.0: No intervention required before procedures 1
• INR above 1.5: Considered discoagulopathy with significantly increased bleeding risk 1

For Patients ON Anticoagulation (Target Ranges)

Different conditions require different target INR ranges:

1. Venous thromboembolism (VTE):

   • Target range: 2.0-3.0 (target INR of 2.5) 1

2. Atrial Fibrillation:

   • Target range: 2.0-3.0 2

3. Mechanical Heart Valves:

   • St. Jude Medical bileaflet valve in aortic position: 2.0-3.0 (target 2.5)
   • Tilting disk valves and bileaflet valves in mitral position: 2.5-3.5 (target 3.0)
   • Caged ball or caged disk valves: 2.5-3.5 (target 3.0) plus aspirin 75-100 mg/day 2

4. Recurrent thromboembolism while on therapeutic warfarin:

   • Target range: 2.5-3.5 1

Important Considerations

• INR values above 4.0 generally provide no additional therapeutic benefit and increase bleeding risk 2
• The risk of bleeding increases exponentially with INR and becomes clinically unacceptable once the INR exceeds 5.0 4
• INR does not adequately predict bleeding risk in patients with liver disease or acute illness who are not on anticoagulant treatment 1
• For elderly patients (>75 years) with atrial fibrillation, a lower target INR of 2.0-2.5 may be considered to reduce the risk of intracranial bleeding 4

Clinical Application

• Regular monitoring of INR is essential for patients on vitamin K antagonists
• More frequent INR monitoring is recommended during fluctuations in diet and weight, changes in medications, during illness, or with any indication of bleeding 1
• Studies have shown that patients monitored using INR values (rather than PT in seconds) have more stable anticoagulation with less variation in values 5

The standardization of INR across different laboratories and reagents has significantly improved the safety and efficacy of anticoagulation therapy, allowing for more consistent monitoring and dosing adjustments regardless of the laboratory performing the test.