What are the target International Normalized Ratio (INR) and partial thromboplastin time (PTT) for patients on warfarin (coumarin) therapy?

Target INR and PTT for Warfarin Therapy

The target INR for warfarin therapy is 2.0-3.0 for most indications, with PTT not being used to monitor warfarin (PTT monitors heparin, not warfarin). 1, 2

Critical Clarification: PTT Does Not Monitor Warfarin

• PTT (partial thromboplastin time) is not used to monitor warfarin therapy - it monitors unfractionated heparin therapy 3
• Warfarin is monitored exclusively using the INR (International Normalized Ratio), which standardizes prothrombin time (PT) measurements 3, 1
• Warfarin may increase aPTT values incidentally, but this is not clinically relevant for monitoring purposes 2

Standard Target INR Ranges by Indication

For most clinical indications, the target INR is 2.0-3.0, aiming for a midpoint of 2.5: 3, 1, 2

• Atrial fibrillation (non-valvular): INR 2.0-3.0 3, 2
• Venous thromboembolism (DVT/PE): INR 2.0-3.0 2, 4
• Tissue heart valves: INR 2.0-3.0 3, 2
• Bileaflet mechanical valve in aortic position: INR 2.0-3.0 2, 4
• Post-myocardial infarction (moderate-intensity): INR 2.0-3.0 2

Higher intensity anticoagulation (INR 2.5-3.5) is required for: 3, 1

• Mechanical prosthetic heart valves (most types): INR 2.5-3.5 3, 1
• Recurrent thromboembolism despite warfarin: INR 2.5-3.5 3
• High-risk post-MI patients (alternative regimen): INR 3.0-4.0 2

Why INR 2.0-3.0 is the Standard

This range maximizes efficacy while minimizing bleeding risk based on decades of clinical trial data: 3

• Thromboembolic risk increases significantly when INR falls below 2.0 3, 1
• Bleeding risk increases exponentially when INR exceeds 3.0, becoming clinically unacceptable above 5.0 3, 1, 4
• Intracranial hemorrhage risk rises sharply when INR exceeds 3.5 3
• The optimal balance between stroke prevention and bleeding occurs at INR 2.5 (range 2.0-3.0) 3, 2

Common Pitfall: Lower INR Targets Lack Evidence

Targeting INR ranges below 2.0 (such as 1.5-2.0 or 1.6-2.6) is not supported by robust evidence and may provide inadequate protection: 3, 1

• Some Asian countries have proposed lower targets (INR 1.6-2.6), but only one small trial of 115 patients supports this approach 3
• The conventional evidence-based INR target of 2.0-3.0 should be employed globally 3
• Lower intensity anticoagulation (INR 1.3-1.6) can suppress prothrombin activation but has not been validated for clinical efficacy in preventing thromboembolism 5

Monitoring Frequency and Quality Control

Initial monitoring should be daily until INR stabilizes, then progressively less frequent: 1, 2

• Check INR daily until therapeutic range is reached and sustained for 2 consecutive days 1
• Then 2-3 times weekly for 1-2 weeks 1
• Then weekly for 1 month 1
• Once stable, intervals can extend to every 4 weeks 1, 2

Time in therapeutic range (TTR) is critical for outcomes: 3

• Patients should maintain INR within target range at least 65% of the time 3
• In usual care, patients are only in therapeutic range 33-64% of the time 2
• Anticoagulation clinics achieve 56-93% time in therapeutic range 2
• Studies show only 60.9% of INR measurements fall within target range even at tertiary hospitals 6

Management of Elevated INR Without Bleeding

For INR 4.0-5.0: 1

• Reduce or omit next warfarin dose 1
• Resume at lower dose when INR approaches therapeutic range 1

For INR 5.0-9.0: 1

• Omit 1-2 doses of warfarin 1
• Consider oral vitamin K 1-2.5 mg if patient has bleeding risk factors 1

For INR >9.0: 1

• Give oral vitamin K 3-5 mg 1
• Expect INR reduction within 24-48 hours 1

For serious bleeding or life-threatening overdose: 1

• Administer vitamin K 10 mg by slow IV infusion over 30 minutes 1
• Plus fresh frozen plasma or prothrombin complex concentrate 1

Special Populations Requiring Dose Adjustment

Lower maintenance doses are typically needed for: 2

• Elderly patients (>65 years) due to increased sensitivity 1, 2
• Asian patients (may require mean daily dose of 3.3 mg) 2
• Patients with certain CYP2C9 and VKORC1 genetic variations 2
• Debilitated patients 2