The Editorial Executive Committee welcomes letters, which should be less than 250 words. Before a decision to publish is made, letters which refer to a published article may be sent to the author for a response. Any letter may be sent to an expert for comment. When letters are published, they are usually accompanied in the same issue by their responses or comments. The Committee screens out discourteous, inaccurate or libellous statements. The letters are sub-edited before publication. Authors are required to declare any conflicts of interest. The Committee's decision on publication is final.

Letter to the Editor

I was interested to read your article ‘How to manage warfarin therapy’ (Aust Prescr 2015;38:44‑8). In the article and subsequent online quiz, it mentions beetroot as being one of the foods that can affect INR, which I found rather unusual. After having worked as a senior pharmacist on a cardiothoracic ward for a number of years, I have counselled countless patients on warfarin and factors that can influence INR and I have never heard of beetroot being one of them. After doing some of my own research, I came across the vitamin K contents of beetroot, which was listed to be approximately 0.3 micrograms per 100 g in comparison with spinach 540 micrograms per 100 g.

Consequently, I believe that consuming beetroot while taking warfarin would have an insignificant effect on INR compared to other foods. I also noted in the quiz that vitamin C was listed as not affecting INR and, although there is limited evidence, there are a number of case reports of vitamin C at high doses affecting INR. Vitamin C is also listed in the Western Australian Department of Health’s Living with Warfarin: Information for Patients,1 so I believe that it is worth mentioning as something that could possibly affect INR.

Louise Vanpraag
Senior pharmacist
Freemantle Hospital

Philip A Tideman, Rosy Tirimacco, Andrew St John and Gregory W Roberts, authors of the article, comment:

Louise Vanpraag rightly points out that the beetroot bulb is a negligible source of vitamin K. It was our oversight in not explicitly naming the beetroot leaves as the rich source of vitamin K rather than the bulb.

While there have been two separate case reports of a possible interaction between high doses of vitamin C and warfarin causing an elevated INR, three separate crossover trials using daily vitamin C doses of 1–10 g for periods of one week to six months have failed to reveal an interaction.