My partner had a AVR in 2003 and has been on warfarin ever since with little to no problems. However since 2016 he has been very unstable from having elevated INR >8 to low INR 1.2. These occasions can last several weeks. He consistently takes his warfarin the same times each day, does not drink or smoke, we are careful on diet etc. His consultant can not seem to find the answer. Just wondering if anyone has had similar problems and if so, what the outcome was?