Your INR (International Normalized Ratio) is a measurement of how quickly you clot (stop bleeding). It is an important measurement if you are using coumadin (warfarin) a drug that "thins your blood", reducing how quickly you clot. Patients are given coumadin because they have a clotting problem; for example, they may have suffered from pulmonary embolisms or strokes. A pulmonary embolism occurs when a clot blocks off blood to your lungs and a stroke is a similar problem, but it affects your brain.
If you are on coumadin, you will undergo a bleeding test to determine your INR. Ideally, your INR should be higher than normal (normal is around 1), but not TOO high. If your INR is too high, your blood is too thin and you are at risk of bleeding out.
For example, if a healthy person gets hit hard enough in the arm, they will get a bruise. A bruise is a sign that the person was bleeding underneath his skin, but the blood clotted and he has began healing. Now, if your INR is too high, you may take significantly longer to clot because your blood is very thin (clotting factors are missing).
The reason I mention this is because a gentleman was telling me about a time when his INR was high and although he was told he needed to come in to see his family doctor, he didn't. I don't know if the doctor didn't clearly convey the danger of the situation, or if he just ignored his physician's warning, but he didn't bother to make an appointment. Later that week, he went to his chiropractor who did some work on his back. He ended up having a sore back the next day; he thought he broke his back because the pain felt the same as when he broke his back years ago. Turns out that the pressure the chiropractor put on his back caused him to bleed internally because his blood was so ridiculously thin. The blood didn't have anywhere to go because his skin was intact, so it pooled in his lower back. Blood pooling into a restricted space increased the pressure in his back and caused the pain.
This man was very fortunate that the blood pulled in a relatively safe way. If he bled into his chest cavity, for example, this could stop your heart from beating and be fatal.