Diagnosis of a blood clot begins with a medical history and physical examination. If a clot is suspected, a number of tests can be used to confirm the diagnosis, including:

• Doppler ultrasound. Imaging technology that uses sound and/or motion waves to produce visual images of the shape and outline of various blood vessels and organs in the body (e.g., the heart), as well as any obstructions to blood flow.
  
  
• CAT scan (computed axial tomography). A test that uses a series of x-rays taken from various angles to produce a computerized three-dimensional image of a body structure. A special dye (contrast medium) is sometimes injected prior to imaging.
  
  
• MRI (magnetic resonance imaging). A noninvasive or minimally invasive test that uses magnetic fields to produce high-resolution cross-sectional or three-dimensional visual images of the targeted area. Often, a magnetic resonance angiogram (MRA) is also done after the injection of a dye to view blood flow through the coronary arteries or carotid arteries.

Physicians may also order a venogram to assess for a blood clot in a vein. This diagnostic test involves the use of a special dye (contrast medium) that is injected into the vein of the affected area prior to an x-ray. The resulting film reveals the shape, size and composition of the vein, as well as any obstructions to blood flow. If this test is performed on an artery, it is known as an angiogram (e.g., a pulmonary angiogram). With the advent of high-resolution, noninvasive imaging, however, angiograms are usually not needed.