Probable Elevation in International Normalized Ratio from Cranberry Juice

Abstract

Objective:

To report a case in which cranberry juice interacted with warfarin, resulting in an elevated international normalized ratio (INR).

Case Summary:

A 77-year-old male had maintained stable INR values for 6 months while on a warfarin maintenance dose of 24 mg/wk. On a routine follow-up examination, the INR was 5.9; one month prior to presentation, the INR had been 2.4. The patient reported several recent nosebleeds and a red mark on his left eye. He stated he had been adherent to therapy and denied any changes in medications or recent illnesses such as fever, flu, or diarrhea. His physical activity level and diet had been consistent, with no alcohol intake. The only change identified was the addition of daily cranberry juice (1–3 cups/day) for the last month. Cranberry juice was stopped, and the dose of warfarin was decreased to 22 mg/wk. Subsequently, the INR stabilized at the low end of the therapeutic range.

Discussion:

Whether cranberry juice interacts with warfarin is unclear. Three previously published case reports suggested a possible interaction. In at least 2 of these cases, confounding factors and incomplete information make assessing the influence of cranberry juice difficult. The interaction in our patient was scored as probable by the Naranjo probability scale, based on the temporal relationship, return to the therapeutic INR range after discontinuing cranberry juice, and elimination of alternative causes for the INR elevation.

Conclusions:

This case suggests that cranberry juice may interact with warfarin, leading to an elevated INR.