Subcutaneous Heparin in Patients Taking Coumadin
Subcutaneous heparin should not be given to patients taking Coumadin (warfarin) unless there is a specific bridging indication, as concurrent use increases bleeding risk without additional therapeutic benefit. 1
Rationale for Avoiding Concurrent Use
Heparin and warfarin are both anticoagulants that work through different mechanisms:
- Warfarin (Coumadin) - Inhibits vitamin K-dependent clotting factors (II, VII, IX, X)
- Heparin - Activates antithrombin III to inhibit thrombin and factor Xa
When used together without proper management, these medications can lead to excessive anticoagulation and increased bleeding risk.
Appropriate Scenarios for Combined Use
There are specific clinical scenarios where heparin and warfarin may be used together:
1. Bridging Therapy
- When initiating warfarin therapy: Heparin provides immediate anticoagulation while waiting for warfarin to reach therapeutic effect (4-5 days)
- During this overlap period, INR should be monitored daily
- Heparin should be discontinued once the INR reaches therapeutic range for 2 consecutive days 2
2. Temporary Interruption of Warfarin
- For patients requiring surgery or procedures where warfarin must be temporarily discontinued
- Not applicable to routine anticoagulation management
FDA-Approved Guidance
The FDA label for Coumadin specifically addresses this situation:
"During initial therapy with Coumadin, the interference with heparin anticoagulation is of minimal clinical significance. To ensure therapeutic anticoagulation, continue full dose heparin therapy and overlap Coumadin therapy with heparin for 4 to 5 days and until Coumadin has produced the desired therapeutic response as determined by INR, at which point heparin may be discontinued." 2
Monitoring Requirements During Transition
If heparin and warfarin must be used together during bridging:
INR monitoring should be performed at least:
- 5 hours after the last IV bolus dose of heparin
- 4 hours after cessation of continuous IV heparin infusion
- 24 hours after the last subcutaneous heparin injection 2
Be aware that warfarin may increase aPTT test results even without heparin
A severe elevation in aPTT (>50 seconds) with INR in therapeutic range indicates increased bleeding risk 3
Contraindications
The FDA label for heparin specifically states that low-dose prophylactic heparin should exclude "patients receiving oral anticoagulants" 1. This is a direct contraindication to the routine practice of giving subcutaneous heparin to patients already on warfarin.
Clinical Implications
- Increased Bleeding Risk: Combining these medications without proper management significantly increases hemorrhagic complications
- No Added Benefit: For a patient adequately anticoagulated on warfarin, adding heparin provides no additional therapeutic benefit
- Monitoring Challenges: Concurrent use makes it difficult to interpret coagulation tests and properly adjust dosing
Conclusion
The practice of giving subcutaneous heparin to patients already taking warfarin is not advisable outside of specific bridging scenarios. When bridging is necessary, careful monitoring and adherence to established protocols are essential to minimize bleeding risk.