June 24, 2011
Dear Mayo Clinic:
I've had two blood clots in two years, but in the past nine months have had no problems. How long do I need to be on warfarin? I'm worried about another blood clot but don't like taking medication.
Blood clots in veins are common and have a tendency to recur, particularly once you stop taking blood-thinning (anticoagulant) medication. Determining how long you should remain on an anticoagulant, such as warfarin (Coumadin), can be complicated. The risk of developing another blood clot if warfarin is stopped has to be balanced against the risk of bleeding complications if the medication is continued. The decision also depends on what caused the clot, as well as your preferences about taking medication.
Warfarin is a drug that decreases the blood's ability to clot and helps prevent blood clots from forming in blood vessels. Although it won't dissolve blood clots that have already formed, warfarin can keep existing clots from becoming larger.
The first issue to consider is the presence of risk factors which may contribute to the development of your blood clots. Risk factors can be divided into either acquired or inherited categories. Acquired risk factors may be temporary, such as surgery or injury. If so, treatment with warfarin for three to six months is usually enough. However, some acquired risk factors may be permanent, such as incurable cancer. In that situation, lifelong blood-thinner therapy is typically recommended.
A number of inherited conditions increase the tendency for forming blood clots. They are divided into mild and aggressive categories. For mild conditions, treatment with warfarin is typically limited to three to six months. For more aggressive inherited clotting conditions — such as deficiencies of certain proteins or some genetic mutations — a longer duration of warfarin is recommended.
In all of these situations, treatment recommendations are fairly straightforward. Yet, some people with blood clots do not have an acquired or an inherited reason for forming clots. Instead, the clots seem to form for no particular reason. Because the clotting has no good explanation, predicting if these people will have another blood clot is difficult. In this situation, the risk of recurrent blood clots should be weighed against the risk of bleeding on blood-thinner therapy when deciding how long to continue warfarin treatment.
A typical approach is to complete three to six months of treatment. At that point, you and your health care provider should discuss how well you're tolerating the medication. Issues to consider include whether you've had bleeding complications and whether the dosing of warfarin has been easy or if multiple dosing changes have been required.
Your preferences should also be taken into account. People typically fall into one of three categories. Some want to stay on warfarin because they fear another clot. Others want to stop warfarin due to the inconvenience of taking and monitoring the medication. And some are torn between the two extremes and don't know exactly what to do.
For the third group, additional testing can help determine their risk of further blood clots. A blood test called fibrin D-dimer measures the level of ongoing clotting in the blood. If D-dimer is elevated one month after stopping warfarin, the annual risk of recurrence is about 10 percent. If D-dimer is normal, the risk of recurrence is about 3 percent.
Ultrasound imaging, which uses sound waves to create pictures of the inside of your body including the veins, can also be useful. If an ultrasound shows that the previous blood clot is gone, the risk of recurrence is quite low. If the clot is still present, the risk of recurrence is higher.
Before you decide, talk to your doctor about these topics to help determine your risk of developing blood clots in the future and the role that ongoing anticoagulant medication can play in reducing that risk.
— Robert McBane, M.D., Cardiovascular Diseases, Mayo Clinic, Rochester, Minn.