Does Clexane (enoxaparin), a low molecular weight heparin, affect the International Normalized Ratio (INR)?

Does Clexane Affect INR?

Clexane (enoxaparin) does not significantly affect INR in most clinical situations, as low molecular weight heparins primarily inhibit factor Xa rather than thrombin and do not require INR monitoring. 1

Mechanism and Monitoring

• LMWHs like enoxaparin have greater inhibitory activity against factor Xa than thrombin, which is the primary target measured by INR testing 1
• LMWHs can be administered without coagulation monitoring because they exhibit predictable pharmacokinetic and pharmacodynamic properties, unlike unfractionated heparin 1
• The International Society on Thrombosis and Haemostasis recommends that LMWH generally does not require routine anti-Xa monitoring, except in specific populations including obesity, renal impairment, pregnancy, or extremes of weight 2

Important Clinical Caveat: Point-of-Care Testing

When patients receive both warfarin and LMWH concurrently, point-of-care INR devices may produce falsely elevated INR readings, but standard laboratory INR measurements remain accurate 3:

• In patients receiving warfarin plus LMWH, point-of-care INR devices showed a mean divergence of 24.19% compared to laboratory values, versus only 7.21% in warfarin-only patients 3
• This discrepancy resulted in a 25% rate of discordant dosing decisions in the warfarin plus LMWH group compared to 8% in the warfarin-only group 3
• Patients receiving concurrent LMWH and warfarin should have INRs checked by standard reference laboratory methods, not point-of-care devices 3

Reagent-Specific Considerations

• Some thromboplastin reagents used for INR testing are more sensitive to heparin than others, potentially causing falsely elevated INR values 4
• The degree of INR prolongation depends on the specific reagent formulation, with some reagents showing sensitivity to even low levels of therapeutic heparin while others remain resistant up to 0.9 U/mL 4
• This reagent sensitivity can result in INR values falsely elevated by 2-55%, potentially leading to premature cessation of heparin therapy 4

Bridging Therapy Context

• When bridging from warfarin to LMWH, enoxaparin is typically started after the INR falls to 1.5 or below 5
• LMWH should overlap with warfarin initiation for a minimum of 5 days or until the INR exceeds 2.0 for at least 24 hours 6
• During bridging therapy, standard laboratory INR monitoring remains the appropriate method for warfarin dose adjustment 1

Practical Algorithm

For patients on LMWH alone:

• No INR monitoring is required 1
• Use anti-Xa levels only if the patient has obesity, renal impairment, pregnancy, or extreme body weight 2

For patients on both LMWH and warfarin:

• Use standard laboratory INR testing, not point-of-care devices 3
• Be aware that some laboratory reagents may still show heparin sensitivity 4
• If INR appears unexpectedly elevated, consider testing after heparin removal with a heparin adsorbent system to verify true warfarin effect 4