Deep venous thrombosis (DVT) is a condition in which blood clots (thrombi) form in the deep veins of the legs. The condition is dangerous because pieces of the clots can break off and travel through the bloodstream to the lungs. This problem, called pulmonary embolism (PE), can be fatal. Doctors typically treat patients who have DVT or PE with blood-thinning drugs called anticoagulants. Unfortunately, these drugs can cause side effects (such as bleeding), so doctors must use them only when they are sure of the diagnosis. If a patient is suspected to have 1 of these conditions, experts recommend that his or her doctor first evaluate the probability of the disease according to clinical findings (the clinical probability) and perform a series of tests that may include ultrasonography to look for a clot in the leg veins, a blood test called a d-dimer study to look for breakdown products from blood clots, and special lung scans (spiral computed tomography and ventilation–perfusion scans). However, there is limited information about how doctors apply these testing strategies and about the relationship between the testing strategies and patient outcomes.