Deep venous thrombosis (DVT) occurs when blood clots form in the large veins of the legs. Patients with DVT require treatment with blood thinners (anticoagulants) to help dissolve the clots. Before starting treatment with anticoagulants, doctors must be certain that the diagnosis of DVT is correct because these drugs, although appropriate for treatment of DVT, can also cause serious bleeding. Researchers have proven that several tests are reliable for diagnosing DVT when the condition occurs for the first time. However, after the condition has occurred once, the tests can be much less reliable for diagnosing a recurrence. Some of the changes caused by the first episode do not resolve promptly and may falsely indicate that a new clot has formed. As many as two thirds of patients with a suspected recurrence of DVT actually do not have new clots. To avoid treating patients unnecessarily, doctors often perform many confirmatory tests. Recently, researchers have observed that a test known as d dimer will produce positive results in patients with active clots but not in patients who do not have DVT.