Does Apixaban (Apixaban) affect the International Normalized Ratio (INR)?

Does Apixaban Affect the INR?

Yes, apixaban significantly elevates INR values, though INR should not be used to monitor apixaban's anticoagulant activity. 1

Mechanism and Extent of INR Elevation

Apixaban, as a direct factor Xa inhibitor, affects coagulation tests including prothrombin time (PT) and international normalized ratio (INR) in the following ways:

• Studies show 78.3% of patients on apixaban present with INR levels above the normal range 1
• The median INR in hospitalized patients on apixaban increases from 1.4 on day 1 of therapy to 1.7 by day 7 2
• The effect is dose-dependent, with a correlation between anti-Xa concentrations and INR values, though this correlation is less predictable than with rivaroxaban 3

Clinical Implications

INR Monitoring and Apixaban

• Unlike warfarin, apixaban does not require routine INR monitoring to guide dosing 4
• When switching from apixaban to warfarin, it's important to note that "apixaban affects INR, so that initial INR measurements during the transition to warfarin may not be useful for determining the appropriate dose of warfarin" 5
• The INR elevation caused by apixaban is not a reliable indicator of its anticoagulant effect 4

Extreme Cases

• Rare cases of extreme INR elevation have been reported, including a case of INR >27 in a patient with end-stage renal disease on hemodialysis taking apixaban 2.5 mg twice daily 6
• Such extreme elevations are uncommon but clinicians should be aware of this possibility, particularly in patients with renal impairment

Practical Considerations for Clinicians

Switching Between Anticoagulants

When transitioning between apixaban and warfarin:

1. From warfarin to apixaban:

   • Discontinue warfarin
   • Start apixaban when INR is below 2.0 5

2. From apixaban to warfarin:

   • One approach is to discontinue apixaban and begin both a parenteral anticoagulant and warfarin at the time the next dose of apixaban would have been taken
   • Discontinue the parenteral anticoagulant when INR reaches an acceptable range 5
   • Be aware that initial INR measurements will be affected by residual apixaban

Laboratory Testing

• Anti-Xa assay, not INR, is the appropriate test to measure apixaban's anticoagulant activity if needed 4
• In emergency situations where anti-Xa testing is unavailable, clinicians should be aware that elevated INR may be due to apixaban rather than indicating excessive vitamin K antagonist effect 1

Comparison to Other Anticoagulants

• Rivaroxaban elevates INR significantly more than apixaban (P<0.001) 1
• Unlike vitamin K antagonists (warfarin), where INR is maintained between 2.0-3.0 for therapeutic effect, the INR elevation with apixaban does not correlate with clinical efficacy 4

In summary, while apixaban does affect INR values, this effect should not be used to guide dosing decisions or assess anticoagulant efficacy. Clinicians should be aware of this effect when interpreting INR results in patients taking apixaban, particularly during transitions between different anticoagulants.