Empiric warfarin dose adjustment

[lilkotori]

Is there any "guideline" to empiric warfarin dose adjustment when an interacting agent is added on board? I was told that antibiotics like metronidazole and Bactrim would require anywhere from 1/4 - 1/3 empiric warfarin dose reduction, whereas levofloxacin would require about 15% dose reduction. However, depending on the sources I look at, the range varies (eg, Micromedex: no warfarin dose adjustment necessary with levofloxacin). Are there other agents besides the aforementioned antibiotics and amiodarone that may require empiric warfarin dose adjustments? Any advice would be appreciated. Thanks!

---

Members don't see this ad.

 

[Dalteparin]

Are there other agents besides the aforementioned antibiotics and amiodarone that may require empiric warfarin dose adjustments? Any advice would be appreciated. Thanks!

You mean besides everything?

In all seriousness, dosing warfarin is different for each individual and I'm somewhat skeptical of dosing guidelines that claim to work for everybody who takes warfarin. When making these adjustments, it's a good idea to consider the following:

1. What is your patient's baseline sensitivity to warfarin? Do small changes in their dose result in big changes in their INR?
2. Duration of therapy - a one-time dose of metronidazole is a much smaller concern than 6-9 months of rifabutin therapy.
3. Patient-specific risk factors. For example, I like for little old ladies, people who work construction and other folks at risk of a fall/head injury to have an INR closer to 2. On the other hand, I've known some patients who could have an INR of 2 and still develop a DVT! 😱

Hope that helps!

 

[Pharmavixen]

I have a pt right now where I've been sucked into chasing the INR all over the map. He was on 6 mg/day of warfarin, when a silly dr started him on clarithromycin for no reason (pt has COPD, but no signs of exacerbation), and his INR went to 4.1. I recommended holding warfarin x 1 dose, d/cing the clarithromycin, and dropping the warfarin dose to 5mg. So the repeat INR a wk later was 1.8. Pt is newly transitioned from dalteparin to warfarin after a DVT; an appropriate warfarin dose hadn't yet been established; he's been on warfarin for < 1 month.

I think I jumped the gun, d/cing the clarithromycin AND dropping the warfarin dose. What are other folks' experiences? Does the rule, "Only change one thing @ a time" apply here?

A colleague who worked in an anticoagulation clinic had the same problem with pts who binge-drink on wkends, then come to get their INR checked on Monday.

 

[lilkotori]

Thanks for all your good advices! Is prophylactic warfarin dose adjustment then necessary for short term antibiotics in stable patient? I understand patient's risk of thrombosis vs bleeding must be considered, and that the range of warfarin dose adjustment (even prophylactic adjustment) will differ depending on this benefit vs risk ratio, but would one prophylactically reduce or increase warfarin doses (like with amiodarone) based on the fact that there exists a signiciant drug-drug interaction due to the temporary addition of an interacting agent (eg, short term metronidazole, levofloxacin, Bactrim)? Micromedex and literature search touched upon this a little, but very little recommendations in terms of management seems to be available. Thanks again for all your help!

 

[Rebel78]

Is there any "guideline" to empiric warfarin dose adjustment when an interacting agent is added on board?

Yes, Pradaxa 150mg bid, and let them go on with their life.

Just kidding...................sort of 😀