My 60 yr old wife has her heart valve replaced with a St Jude bi-leaf mechanical valve a week ago. Currently she is on 3mg warfarin per day. Advice has been given on food and drugs management. She is to maintain her INR between 2 and 3. What we want to know is whether the INR varies significantly from day to day outside the range or it is fairly steady and not easily affected by small daily variations in diet. The other question is whether home machines for INR monitoring are reliable. Many thanks. TTH