Serious illness or death can occur if blood clots (thrombi) form in the deep veins of the legs (a situation known as deep venous thrombosis or DVT) and then become dislodged and flow with the bloodstream to the lungs. Thrombi that reach the lungs are called pulmonary emboli. Deep venous thrombosis and pulmonary emboli are treated with blood thinners (anticoagulants). Unfortunately, anticoagulants themselves can cause side effects (such as bleeding) that may be fatal, so they must be used carefully and only in confirmed cases of DVT. Confirmation can be obtained by using one of several available ultrasonography methods that use sound waves to detect the presence of thrombi. The most commonly used method is called simplified compression ultrasonography. While this test reliably identifies (or rules out) thrombi above the knee, it cannot detect thrombi in the calf veins (below the knee). A significant clinical problem is that once thrombi form in the calf veins, they sometimes extend upward above the knee before dislodging and causing pulmonary emboli. For this reason, patients with negative results on simplified compression ultrasonography must be retested a week later to detect thrombi that were missed on the first test. Not only is repetition of the test expensive and inconvenient, it is frequently unnecessary because 80% of retested patients have no evidence of thrombi. Another test, called comprehensive duplex ultrasonography, is more time-consuming and difficult to perform but can effectively detect thrombi in the calf vein and the upper leg and may not require retesting when the initial results are negative.