Cheap, Fast Test Measures Blood Clot Risk
Accurately rules out those who don't need risky and costly therapy
WEDNESDAY, Sept. 24, 2003 (HealthDayNews) -- A new kind of blood test gives doctors a quick, inexpensive way to identify emergency room patients who are at high risk of life-threatening blood clots that are generated in leg veins, researchers report.
The test, costing no more than a few hundred dollars, was nearly 100 percent accurate in separating low-risk patients from those with similar symptoms who needed an ultrasound diagnostic test that can cost $10,000 or more, says a report in the Sept. 25 issue of the New England Journal of Medicine.
Swift diagnosis of the condition, called deep-vein thrombosis, is essential because a clot that forms in a leg can travel to the lungs, killing or crippling the patient. A decision to use anti-clotting therapy -- which carries its own risk -- traditionally has been made on the basis of a doctor's clinical assessment, backed up by testing, with an ultrasound scan regarded as the gold standard.
The new test measures blood levels of d-dimers, blood clot fragments that can be a sensitive marker of danger. In a study of more than 1,000 patients with suspected deep-vein thrombosis, the test erred only twice in selecting patients who did not need the ultrasound exam, says the report by Dr. Philip S. Wells and colleagues at the University of Ottawa in Canada.
"These tests are not now widely used, but they are poised to really gain momentum," says Dr. Ben deBoisblanc, a professor of medicine and physiology at Louisiana State University, who has done his own studies in the field. "In the next year or two, you will see this become the most common procedure for excluding patients from ultrasound."
A number of d-dimer tests have been available for years, but those that give precise results have been too complicated for quick use, while easy-to-use tests have given imprecise results. The Wells study used two new-generation tests that can give precise results and are easily done in a clinical setting.
The selection process still starts with a doctor's clinical judgment, the journal report says. The doctor first assigns points on the basis of conditions known to be associated with a risk of deep-vein thrombosis -- swelling of the leg, prolonged immobility, previous deep-vein thrombosis, an active cancer and so on.
In the study, patients judged to be at high risk because of those criteria were divided into two groups. One group went straight to ultrasound imaging, while patients in the other group had ultrasound only if the d-dimer test showed danger.
D-dimer testing said that ultrasound exams were not needed for 39 percent of the patients given the test, so the number of ultrasound exams that ordinarily would have been done was nearly halved, the report says. And only two of the patients with negative d-dimer tests had clots, meaning that the test was 99.6 percent accurate in saying when ultrasound was unnecessary.
So the combination of clinical judgment and the d-dimer test "simplifies the diagnosis of deep-vein thrombosis in outpatients without compromising safety," the researchers conclude.
The stage is now set for widespread use of d-dimer testing for patients brought to a hospital with suspected deep-vein thrombosis, deBoisblanc says. "All we need to do is to get the message out to doctors about how to use these tests," he says.
A negative d-dimer test "goes a long way to telling whether a patient has a clot," he says. "In the right scenario, using this test means that you can stop right there. If the test is positive and you still don't know, you can do the additional tests."