INR Response to Oral Vitamin K: Absorption Considerations
A drop in INR from 5.9 to 2.0 following 5mg oral vitamin K, with subsequent plateau, does not necessarily suggest an absorption issue—this pattern is consistent with normal vitamin K pharmacokinetics and the expected time course of warfarin reversal. 1, 2
Understanding the Expected Response Pattern
The scenario you describe is actually within the expected pharmacologic response to oral vitamin K:
- Oral vitamin K produces measurable INR reduction within 24 hours, with approximately 95% of patients showing INR decrease and 85% achieving INR <4.0 by 24 hours 1, 2
- The initial drop from 5.9 to 2.0 demonstrates adequate absorption and bioavailability of the oral vitamin K dose 1
- A plateau at INR 2.0 after initial correction is expected because vitamin K does not produce an immediate coagulant effect—it requires 1-2 hours minimum for measurable improvement, and the full effect unfolds over 24-48 hours 3
Why the INR May Not Drop Further
Several physiologic factors explain why the INR stabilizes rather than continuing to decline:
- Warfarin has a long half-life (36-42 hours) and continues to exert anticoagulant effects even after vitamin K administration, creating a dynamic equilibrium 1
- The 5mg oral dose is appropriate for INR 5-10 range and is designed to bring INR into a safe therapeutic range (typically 2.0-3.0), not to completely normalize it 1, 2
- Vitamin K works by enabling synthesis of new clotting factors, which takes time—the INR reflects the balance between residual warfarin effect and newly synthesized vitamin K-dependent factors 3, 4
When to Suspect True Absorption Issues
Absorption problems would manifest differently than your scenario:
- Failure of INR to decrease at all within 24 hours after oral vitamin K would suggest malabsorption 5
- INR remaining >4.0 at 24 hours despite appropriate oral vitamin K dosing raises concern for inadequate absorption 1, 5
- Clinical context matters: active vomiting, diarrhea, or known malabsorptive conditions (celiac disease, inflammatory bowel disease, short gut syndrome) increase likelihood of absorption failure 6
Appropriate Next Steps
For an INR that dropped from 5.9 to 2.0 but plateaued:
- Recheck INR within 24-48 hours to confirm stability and ensure it doesn't rebound as vitamin K effect wanes 2, 7
- Hold warfarin temporarily until INR trends demonstrate appropriate trajectory back toward therapeutic range 1, 2
- Resume warfarin at 10-20% reduced dose (80-90% of previous weekly total) once INR approaches therapeutic range to prevent recurrent supratherapeutic levels 7
- Investigate precipitating factors: recent dietary changes (decreased vitamin K intake), new medications, alcohol use, or intercurrent illness that may have caused the initial INR elevation 6
Common Pitfalls to Avoid
- Do not administer additional vitamin K when INR has already reached 2.0—this risks overcorrection, warfarin resistance for up to one week, and potential thrombotic complications 1, 8
- Do not assume absorption failure based solely on INR plateau at a therapeutic level—this is the intended effect 1, 2
- Do not restart warfarin at the previous dose—always reduce by at least 10-20% to prevent recurrence 7
- Avoid subcutaneous vitamin K if additional dosing were needed, as it has unpredictable and delayed absorption compared to oral administration 1, 5