INR Calculator
Calculate International Normalized Ratio from prothrombin time for anticoagulation monitoring
Calculate INR
Patient's actual PT measurement
Laboratory control sample PT
Reagent sensitivity factor (0.9-1.7)
Results
INR Interpretation
Enter patient PT, control PT, and ISI values to calculate INR
Clinical Significance: Complete all required values for assessment
Risk Assessment: Unknown
Therapeutic Ranges
• < 1.1: Normal (no anticoagulation)
• 1.1 - 2.0: Elevated, not therapeutic
• 2.0 - 3.0: Therapeutic for most conditions
• 2.5 - 3.5: Mechanical heart valves
• > 4.0: Excessive, bleeding risk
⚠️ Medical Disclaimer: This calculator provides INR calculations for educational purposes only. INR values must be interpreted in clinical context with patient history and current medications. Always consult healthcare professionals for anticoagulation management decisions. Never adjust anticoagulant doses based solely on calculator results.
Common Indications for INR Monitoring
• Atrial fibrillation: Target INR 2.0-3.0
• Deep vein thrombosis (DVT): Target INR 2.0-3.0
• Pulmonary embolism: Target INR 2.0-3.0
• Mechanical heart valves: Target INR 2.5-3.5
• Stroke prevention: Target INR 2.0-3.0
Example INR Calculations
Normal Patient
Scenario: Healthy individual, no anticoagulation
Patient PT: 13 seconds
Control PT: 12 seconds
ISI: 1.0
INR Calculation: (13 ÷ 12)^1.0 = 1.08
Interpretation: Normal INR (< 1.1)
Warfarin Patient
Scenario: Atrial fibrillation on warfarin therapy
Patient PT: 26 seconds
Control PT: 12 seconds
ISI: 1.2
INR Calculation: (26 ÷ 12)^1.2 = 2.4
Interpretation: Therapeutic range (2.0-3.0)
Over-anticoagulated Patient
Scenario: Patient with elevated bleeding risk
Patient PT: 45 seconds
Control PT: 12 seconds
ISI: 1.1
INR Calculation: (45 ÷ 12)^1.1 = 4.2
Interpretation: Excessive anticoagulation (> 4.0)
Quick Reference
INR Formula
Normal Values
• PT: 11-13 seconds
• INR: 0.8-1.1
• ISI: 0.9-1.7
Target Ranges
• Most conditions: 2.0-3.0
• High-risk conditions: 2.5-3.5
• Mechanical valves: 2.5-4.0
Key Points
INR standardizes PT measurements across laboratories
Higher INR indicates increased bleeding time
Essential for warfarin dose monitoring
INR >4.0 requires immediate attention
ISI varies by reagent manufacturer
Understanding INR and Prothrombin Time
What is INR?
The International Normalized Ratio (INR) is a standardized measurement of blood clotting time. It unifies prothrombin time (PT) results across different laboratories and reagents, allowing consistent interpretation worldwide.
Clinical Applications
INR is primarily used to monitor anticoagulant therapy, assess bleeding risk, evaluate liver function, and screen for coagulation disorders. It measures the extrinsic and common pathways of blood coagulation.
INR Calculation Components
Patient PT
Actual clotting time measured in patient sample
Control PT
Reference clotting time from normal plasma pool
ISI (International Sensitivity Index)
Reagent-specific calibration factor
Factors Affecting INR
Factors Increasing INR
- • Anticoagulant medications (warfarin)
- • Liver disease or dysfunction
- • Vitamin K deficiency
- • Certain antibiotics
- • Disseminated intravascular coagulation (DIC)
- • Factor deficiencies (I, II, V, VII, X)
Factors Decreasing INR
- • Vitamin K supplementation
- • Thrombotic disorders
- • Pregnancy (early stages)
- • Certain medications (oral contraceptives)
- • Sample collection errors
- • Pre-analytical factors