The blood test result is referred to as an INR (International Normalised Ratio). This is a measure of the time it takes for your blood to clot. The higher the INR, the thinner the blood and the less warfarin you need. The lower the INR the more likely it is that you will develop a clot, so you may need more warfarin. Your warfarin levels need to be in the right range for your individual set of circumstances.