Heart valves keep blood flowing forward within the heart. If one of them stops working, it may need to be repaired or replaced. One replacement option is a mechanical valve. These last longer than biological (tissue covered) valves. But they also tend to cause small blood clots that can lead to strokes.
Thats why people who get a mechanical heart valve take the blood thinner warfarin (Coumadin). They need periodic blood tests to make sure that the amount of blood thinning is just right. Usually blood tests are done monthly.
The blood test is called an INR. It measures the time it takes for blood to clot. If the INR is too high (meaning blood is too thin), the warfarin dose is reduced. Too low, the dose is increased.
Changes in diet or medicine can affect the INR. For some people, INR varies considerably even without any changes in their daily routine. This means more frequent blood tests.
Unfortunately, there is no good alternative to warfarin right now. It was hoped that dabigatran (Pradaxa) would work as well as warfarin. But the clinical trial comparing the two drugs showed that the risk of blood clots on the valve and strokes was higher with dabigatran.