Normally, blood clots do not form inside deep veins of the legs. When they do (a condition known as deep venous thrombosis or DVT), they can lead to tissue damage or even fatal complications. Blood thinners, which are used to treat DVT, can prevent most of these problems or help resolve them if they already exist. Because blood thinners can cause serious bleeding, it is important to use them only when necessary. Diagnostic accuracy of DVT is therefore very important. When DVT is suspected, a patient's history, symptoms, and physical examination may help assess whether the probability of DVT is low, moderate, or high. However, clinical history and physical findings are not reliable enough to make a firm diagnosis. Since the body begins breaking down blood clots as soon as they are formed, a simple blood test—the D-dimer test—can determine whether breakdown products from blood clots are present. However, increased amounts of D-dimer can also be found in some people without blood clots. The test is therefore most useful in ruling out DVT when high levels of D-dimer are absent. Also, there are several D-dimer tests. Some take a long time to complete, and some don't perform as well as others.