How should the acenocoumarol dose be adjusted for a patient with a subtherapeutic INR (International Normalized Ratio) of 0.8, targeting an INR of 3, previously stabilized on 4 mg daily with an INR of 2.5?

Acenocoumarol Dose Adjustment for Subtherapeutic INR

For a patient with INR 0.8 (target 3.0) previously stable at INR 2.5 on 4 mg daily acenocoumarol, increase the weekly dose by approximately 10-20% and recheck INR within 1-2 weeks without bridging anticoagulation. 1

Rationale for Dose Adjustment

Your patient's INR has dropped from 2.5 to 0.8, representing a deviation of more than 0.5 below the therapeutic range. While the American College of Chest Physicians guidelines suggest continuing the same dose for single out-of-range INRs within 0.5 of target 1, this patient falls outside that recommendation with an INR 1.7 units below target.

Specific Dosing Strategy

• Current dose: 4 mg daily = 28 mg/week
• Recommended adjustment: Increase to approximately 30-32 mg/week (approximately 4.3-4.6 mg daily) 1
• Recheck INR: Within 1-2 weeks 1

The dose increase should be modest because:

• The patient was previously stable, suggesting this may be an isolated deviation 1
• Acenocoumarol has a shorter half-life than warfarin, making it more prone to INR fluctuations 2
• Overly aggressive dose increases risk overshooting into supratherapeutic range 1

No Bridging Anticoagulation Required

Do not routinely administer bridging therapy with heparin for this single subtherapeutic INR. 1 Evidence from 2,597 patients showed no significant difference in thromboembolic events between patients with single subtherapeutic INRs and those maintaining therapeutic levels 1. The incidence of thromboembolism was only 0.3% even without bridging 1.

Critical Considerations Before Adjusting

Investigate Potential Causes of INR Drop

Before increasing the dose, evaluate for:

• Medication interactions: Recent addition of enzyme-inducing drugs or discontinuation of drugs that potentiate anticoagulation 1
• Dietary changes: Increased vitamin K intake from green leafy vegetables 1
• Compliance issues: Missed doses are the most common cause of subtherapeutic INR 1
• Malabsorption: New gastrointestinal symptoms 1

If any reversible cause is identified, address it first before permanently increasing the maintenance dose 1.

Monitoring Strategy

• First INR recheck: 1-2 weeks after dose adjustment 1
• If INR remains subtherapeutic: Consider further 10-15% dose increase 1
• If INR overshoots: Return to previous dose or reduce slightly 1
• Once stable: Return to routine monitoring intervals 1

Special Considerations for Acenocoumarol

Acenocoumarol differs from warfarin in important ways:

• Shorter half-life: Results in more variable INR control 2
• Higher instability risk: Two-fold higher risk of unstable anticoagulation compared to warfarin 2
• Conversion factor: If considering switch to warfarin for persistent instability, use factor of 1.8 (warfarin dose = acenocoumarol dose × 1.8) 2

Patients with unstable acenocoumarol control who switch to warfarin show improvement in time within therapeutic range from 40% to 60% 2.

Common Pitfalls to Avoid

• Over-correction: Avoid increasing dose by more than 20% for a single low INR, as this frequently causes supratherapeutic overshoot 1
• Premature bridging: Unnecessary heparin bridging increases bleeding risk without proven benefit for single subtherapeutic INRs 1
• Inadequate follow-up: Failing to recheck INR within 1-2 weeks risks prolonged subtherapeutic anticoagulation 1
• Ignoring vitamin K intake: Dietary changes are a frequent but overlooked cause of INR variability 1