Leo Price, 51, is on warfarin for AF. He has started using a home coagulometer but is worried because his numbers have been going up...

Leo shows Parveen his anticoagulant booklet and she sees that his INR readings, which normally are in the range 2.2-2.5, are hovering between 2.5-2.8. “Has anything changed recently, Leo?” asks Parveen. “Have you changed your diet or started taking any other medicines?” Leo looks sheepish and tells her he has started some thyroid tablets. “I haven’t mentioned it because you were closed so I got the prescription filled somewhere else and I felt guilty about it.”

What's happened?

Assuming Leo’s thyroid medication is for an underactive thyroid – as would be more likely at his age – his newly prescribed thyroid hormone is likely to be potentiating the anticoagulant effect of the warfarin.

While his INR is not straying too far from the target of 2.5, as recommended in patients with atrial fibrillation by the British Society of Haematology, the fact that there is an upwards trend in Leo’s measurements, coupled with the long half-life of levothyroxine, means that he may not be experiencing the full effect of the interaction at the moment, and his INR may increase further, potentially to a dangerous level.

Parveen should advise Leo to call or visit his anticoagulation clinic with his booklet and inform them about his new prescribed medication. They are likely to want to keep a closer eye on his INR than he is able to do himself through home monitoring, and may adjust his warfarin dosage as a result of their findings.

The bigger picture

While the interactions warfarin has with other drugs are well documented, less well known are the interactions that exist with food. Because warfarin works by interfering with the liver’s use of vitamin K, any changes in dietary intake of this nutrient can affect INR and may warrant a warfarin dose adjustment.

Of particular relevance are foodstuffs that are high in vitamin K, which include asparagus, leafy green vegetables, kiwi fruit, darker berries such as blackberries and blueberries, green beans, lettuce and pine nuts.

Two other interactions worthy of note include cranberry juice, which affects INR levels in several ways and therefore should be avoided by warfarin patients, and alcohol, which, in excess, affects liver functioning and therefore can be problematic in terms of warfarin efficacy.

Extend your learning

• The consequences of not maintaining control of INR can be serious. View the information here to understand how narrow the therapeutic window can be
• As mentioned in this scenario, warfarin is a drug that interacts with many other substances. An overview is provided by Anticoagulation Europe here (see ‘Some of the most common interactions with warfarin’).