I never, in 8.5 yrs, had my INR go above 4, and when it was, the doc helped me with my dosage to lower it. Even going to 4 was a one time deal, UNTIL I got bacterial endocarditis. So, to make a long story short, when your INR goes too high like mine did, to 9.3, you are hospitalized and given vit K shots and also some FFP (fresh frozen plasma). Also, I was taken off coumadin totally, and not even given heparin or lovenox, for a few days, until I was stablized.
Really, the best advice is not to worry, because your INR usually stays within range or just a bit above or below. Going bonkers like mine did was a rarity, caused by a bad bacteria. Everyone with a valve replacement should know the symptoms of endocarditis, because many hospital ER's and urgent care docs don't suspect this first. They think you have the flu.
One more point, some say an INR of 5 isn't dangerous. This may be true, but mine was at 6.6 when I had blood in my urine. I was already getting internal bleeding. So, I would be concerned if mine ever was at 5. Also, to say you'd have to be at 10 to have a real problem is nuts. The night after my 6, I had a bad headache which turned out to be a brain bleed. So my INR was between 6 and 9 when this happened. It was found to be 9.3 two days after it was in the 6 range.