Normally, blood clots do not form inside deep veins. When they do (a condition known as deep venous thrombosis or deep-vein thrombosis), they can lead to tissue damage or even fatal complications. Blood thinners can prevent most of those problems, but because these drugs can cause serious bleeding, it is important to use them only when necessary. Diagnostic accuracy is therefore very important. When deep venous thrombosis is suspected, a patient's medical background, symptoms, and results on physical examination help in estimating whether the probability of deep venous thrombosis is low, moderate, or high (the “prior probability” of disease being present). By themselves, however, these 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 if breakdown products from blood clots are present. Unfortunately, d-dimer can also be found in some people without blood clots. The test is therefore most useful in ruling out deep venous thrombosis when d-dimer is absent.