Q: I had a heart valve replacement 12 years ago. I need to take warfarin to avoid blood clots on the valve. I have been reading about the newer blood thinners that don't require regular blood tests. I am tired of so many blood tests. Can I ask my doctor about making a switch?

A: 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 valves. But they also tend to cause small blood clots that can lead to strokes.

That's why people who get a mechanical heart valve take the anticoagulant ("blood thinner") warfarin (Coumadin). It does not actually make blood less thick. The drug interferes with the production of clotting factors made by the liver.

By decreasing production of these factors a little but not too much the risk of forming blood clots is reduced. The trade-off is the increased risk of bleeding. The key is finding the right balance of benefit and risk.

People on warfarin 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. The usual INR range for people with a mechanical heart valve is 2.5 to 4.0

If the INR is too high, it means the bleeding risk is now excessive and the dose of warfarin is reduced. If the INR is the warfarin is too low, the risk of blood clotting on the valve has increased. So, the person needs a higher warfarin dose. Of course, this assumes the person has been taking warfarin exactly as instructed.

Changes in diet or medicine can affect the INR. So any significant change can alter the INR, sometimes a lot. And 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.

Within the next few years, I do think a new anticoagulant that needs less monitoring than warfarin will be approved to prevent clots on mechanical heart valves.

(Howard LeWine, M.D., is a practicing internist at Brigham and Women's Hospital in Boston and Chief Medical Editor of Internet Publishing at Harvard Health Publications, Harvard Medical School.)

(For additional consumer health information, please visit http://www.health.harvard.edu.)