Firstly, 3.5 is not exceptional. What is the therapeutic range they have set? Mine is 2.5 to 3.5, so 3.5 for me would be fine, and a bit over would not worry me either. When was your last test, and what was the result then? Speed of change is relevant.
Secondly, how did they check your INR? My Coaguchek XS is usually 0.1 or 0.2 higher results than when I test with a blood draw at a clinic (though blood draws are quite rare now - the CoaguCheck machines are perfectly accurate enough.
Thirdly I am always wary of big changes in dose. So for me I would have to be A LOT over my maximum to do skip a whole dose, and there is a danger of yo-yoing with big adjustments, making it take ages to get back on track. . Remembering that Warfarin takes about 3 days to take effect.
Finally, has anything else changed? Are you now on antibiotics, or do you have a cold? Both of these push my INR up.
So, questions summarised:
1. What is your therapeutic range?
2. when was your last test, and what was the result?
3. Do you have a cold?
4. Have you recently started antibiotics (or any other new medication)?
5. Is the weather there gorgeous today or not? (Hint: it's gorgeous compared with London right now ...)