Doctors use several strategies to diagnose DVT, including a scan that uses sound waves to look at clotting in the veins (ultrasonography) and blood tests (d-dimer tests) that help measure whether a clot has formed and is breaking down. Doctors may repeat ultrasonography within 1 week if the initial scan is negative. To decide when to do these tests, doctors often assess a patient's medical background, symptoms, and physical examination. They sometimes sum the presence or absence of 9 items (the Wells rule) to help sort patients into low, moderate, or high probability of having DVT. For example, in the original study that developed the Wells' rule, only 3% of patients who were classified as low risk by the rule had DVT. Few additional studies document how well the Wells rule places people into a low-risk group.