Michael Streiff, M.D., Medical Director of the Anticoagulation Management Service and Outpatient Clinic at Johns Hopkins, talks about the benefits of self testing for warfarin patients.
Millions of people take Coumadin (warfarin) to help prevent stroke and to treat deep venous thrombosis and pulmonary embolism. But warfarin is notorious for not playing well with others -- its effectiveness can be altered by anything from vitamin-K-rich foods to other common medications. A study published in the Journal of the American Medical Association found that one out of every seven drug-related emergency room visits was caused by either insulin or warfarin.
In October 2006, the FDA required drug manufacturers to include a "black-box” warning on Coumadin and warfarin labels. Black-box warnings are included when a drug carries major risks of serious side effects or death. The warfarin warning emphasizes the risk of potentially fatal hemorrhage.
Warfarin works by inhibiting the production of blood-clotting proteins. In the right amounts, it is a very effective anticoagulant. However, excessive doses of warfarin can reduce the levels of blood-clotting proteins to dangerously low levels, resulting in an increased risk of hemorrhage. Factors that increase your risk of warfarin-caused bleeding include being over 65, high blood pressure, cerebrovascular disease, severe heart disease, renal insufficiency, and cancer. Major bleeding is also more likely to occur soon after starting warfarin, but the cumulative risk also rises with continued use of warfarin. Not surprisingly, patients on more intensive warfarin therapy are also at higher risk for major bleeding.
To make sure that they are taking the correct dosage of warfarin, people who take warfarin must undergo frequent blood tests to measure their prothrombin time, or how long it takes for blood to clot. Prothrombin time results are expressed as International Normalized Ratio (INR) numbers. For most patients, the safe range is 2.0 - 3.0 -- higher INRs are associated with an increased risk of bleeding, while INRs below 2 provide less effective protection against clot formation. "Published studies clearly indicate that warfarin patients who undergo frequent INR testing spend a greater proportion of time within the therapeutic range than patients who have less frequent INR assessments,” says Michael Streiff, M.D., Assistant Professor of Medicine and Medical Director of the Anticoagulation Management Service and Outpatient Clinic at Johns Hopkins.
Typically, patients on a stable dose of warfarin have their INR measured at least once a month in an anticoagulation clinic at a hospital. If patients want to monitor their INR more closely, however, self-monitoring is an option that’s shown promising results over the past few years.
Dr. Streiff notes, "While some warfarin patients like the reassurance of a visit to their doctor’s office, motivated patients who stick to their self-monitoring program will do just as well as they did when they went to an anticoagulation clinic.”
There are several potential benefits of self-monitoring. Says Dr. Streiff, "It’s especially useful if the patient starts seeing signs that their INR might be high, such as bruising. Self-testing offers the patient a convenient way to check his or her level of anticoagulation promptly.” (Other signs of excessive bleeding include nosebleeds, bloody gums, weakness, and blood in urine or feces.) Self-monitoring also has the advantage of being portable -- patients can check their INR while out of town.
Clinical trials tend to support self testing.The Lancet published a review of 14 studies of patients who self-monitored their warfarin use. Overall, people who went to a physician or anticoagulation clinic to measure their INR had more than twice as many thromboembolic events (such as stroke) as warfarin users who checked their own INR.
At-home INR monitors are similar to those used by people with diabetes. A patient pricks his or her finger with a tiny needle and places the blood on a test strip. The test strip is read by a tabletop monitor that displays the INR. The patient calls the doctor’s office with the INR, and, if necessary, the doctor adjusts the warfarin doses. The patient may be advised to cut warfarin tablets in half, or the doctor may call the pharmacy with a new prescription.
Self-monitoring for warfarin is not for everyone. Patients need to have enough motor control to properly use the needle and test strip. The machines and test strips are also expensive, and insurance may not cover the costs. Medicare covers self-monitoring equipment only for people with mechanical heart valves.
ON THE HORIZON -- Genetic testing may someday help further reduce the risk of warfarin-related bleeding. Researchers have found that genetic differences influence the way the body metabolizes both warfarin and vitamin K. Dr. Streiff says, "While genetic tests are still in their research phase, some day soon they may help physicians tailor initial warfarin dosages to each patient, which could make the initial phase of warfarin use safer.”
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