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Things I Do But Should I: Warfarin

Vanessa Cardy, MD and Adrien Selim, MD
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In this chapter of things I do but should I: A patient with low INR, to bridge or not to bridge with heparin; How often should patients with stable INR be monitored; what’s the proper  warfarin starting dose; when starting warfarin, do we need to bridge with heparin until the INR is therapeutic?

 

Pearls:

  • You can safely extend INR intervals up to 12 weeks for patients who have been on a stable warfarin dose.

  • Using a decision algorithm or nomogram to adjust INRs increases patient’s time within therapeutic window.

  • The standard initiation dose of warfarin is 5mg but you can consider 10mg in a younger patient, and lower doses in elderly patients.

  • Bridging with LMWH when initiating warfarin for nonvalvular afib is probably not necessary for patients with no prior history of CVA/TIA.

 
  • Review pointers from September 2015 PC RAP based on American College of Chest Physicians:

    • INR < 4.5: no vitamin K for for one out of range INR below 5, continue dose and retest in 1-2 weeks. No benefit in adjustments in this group.

    • INR > 4.5-10: no vitamin K, stop warfarin and let INR trickle down. Though bleeding risk does increase after INR 4.5, no change in bleeding or thromboembolism risk in this group with use of vitamin K.

  • How often do you check an INR with someone on stable dose?

    • A few randomized control trials looked at people on stable dose of therapeutic INRs over three months: no difference in terms of INR control, thrombotic or bleeding outcomes when checking INR every 4 vs. 6 to 8 to 12 weeks.

    • Recommendation: check every 12 weeks (Grade 2B - weak recommendation based on moderate quality data)

    • Antithrombotic Therapy and Prevention of Thrombosis (9th Edition), Published: February 2012. Chest American College of Chest Physicians.

  • How do you adjust warfarin doses?

    • Recommendation: Chest guidelines recommend paper-based algorithm or computer program (Grade 2C)

    • One observational study compared time in therapeutic range for those using algorithm vs. those not using and found  a 6-14% increase in those using the algorithm

    • Nomogram for warfarin INR goal 2-3 from 2013 Canadian Family Physician:

      • < 1.5: give one extra dose, increase weekly dose by 10-20%

      • > 1.5-2: increase weekly dose by 5-10%

      • > 3-3.5: decrease weekly dose by 5-10%

      • > 3.6-4.9: hold one extra dose, decrease weekly dose by 10-20%

      • > 5-9: hold two doses, decrease weekly dose by 10-20%

    • Dumont Z et. al. Warfarin: its highs and lows.
      Can Fam Physician. 2013 Aug;59(8):856-60. PMID: 23946031

    • Antithrombotic Therapy and Prevention of Thrombosis (9th Edition), Published: February 2012. Chest American College of Chest Physicians.

  • Bridging subtherapeutic INRs with warfarin?

    • Review article concluded risk of thromboembolic event for patient with single out of range INR is low and bridging with LMWH does not add benefit. Includes patients with afib, VTE and mechanical valve disease. High-risk patients (recent thromboembolic event) were underrepresented.

    • Recommendation: No evidence to support bridging with heparin for single subtherapeutic INR (Grade 2C)

    • Clark NP. Frequency of monitoring, non-adherence, and other topics dear to an
      anticoagulation clinic provider. J Thromb Thrombolysis. 2013 Apr;35(3):320-4. PubMed PMID: 23494486

    • Antithrombotic Therapy and Prevention of Thrombosis (9th Edition), Published: February 2012. Chest American College of Chest Physicians.

  • What starting dose of warfarin?

    • Recommendation: Younger, healthier patients who have low risk of bleeding, you can start with 10mg for two days. Those with higher bleeding risk or with comorbidities, start with 2-3mg. (Grade 2C)

    • Antithrombotic Therapy and Prevention of Thrombosis (9th Edition), Published: February 2012. Chest American College of Chest Physicians.

  • Do you use heparin at the time of initiation of warfarin in patients with afib?

    • One observational study looked at those who were bridged and not bridged , found no difference in stroke between the two groups but the bridging group had higher risk of bleeding.

Recommendation: With nonvalvular afib and no prior CVA or TIA, you don’t need to bridge. With history of CVA or TIA, judgement call to bridge either way.

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