Doctors use several strategies to diagnose DVT. These include a scan that uses sound waves to look at clots in the veins of the upper leg (ultrasonography), blood tests that help measure whether a clot has formed and is breaking down (d-dimer tests), and an x-ray taken after dye is injected into a foot vein (venography). Doctors may repeat an ultrasonography after 1 week if the initial scan is negative to see whether any small clot in the lower leg grew into the upper leg during the week. They may also do additional tests, such as venography, in patients with positive d-dimer test results. While all of these strategies seem to be reasonably accurate and safe, we do not know whether some are better than others.