X-Ray, Ultrasound, CAT Scan, MRI and D dimer Testing

The diagnosis of a deep venous thrombosis (DVT) depends in part on the size, location, and symptoms of the blood clot. Medical imaging techniques such as ultrasound, MRI and CAT (CT) scans may locate blood clots. Blood tests for circulating D dimer and blood pressure tests such as plethysmography can also help make a diagnosis. Despite these tests, however, the only definitive diagnosis for DVT remains a venography, or x-ray of the veins.

A DVT may not produce any noticeable symptoms. If symptoms are present, they usually include local swelling, pain, and tenderness. Further, the skin around the blood clot site may be red, or hot to the touch. These symptoms can also indicate a number of other conditions, including cellulitis and chronic venous insufficiency.

DVT symptoms may be vague, may indicate other diseases, or may not appear at all. Because of this, a physical examination is insufficient to make a diagnosis. Medical imaging techniques are essential to confirm DVT.

Venography continues to be the definitive medical imaging tool for DVT diagnosis. With venography, dye is injected into the blood vessels to help visualize the circulation. The dye is seen on an x-ray, displaying the passage of blood through the body. A DVT appears where there is blockage of blood flow.

Venography is an effective means of diagnosing DVT, but the procedure has some drawbacks. Venography cannot be used repeatedly, and some studies indicate that the x-ray dye may actually promote blood clot formation. As medical imaging techniques become more sophisticated, alternative imaging tools are becoming available.

After venography, an ultrasound is perhaps the most effective medical imaging tool used in DVT diagnosis. Ultrasound uses high frequency sound waves to generate an image of the blood vessels. Two forms of ultrasound are used in DVT diagnosis.

Compressive Ultrasound: The ultrasound probe gently presses against the blood vein to check for rigid, inflexible areas.

Duplex Ultrasound: Duplex ultrasound monitors the same symptoms as compressive ultrasound and also provides information on blood flow. Duplex ultrasound works best for thigh blood clots: the procedure is less sensitive to DVTs in the pelvis and calves.

In addition to ultrasound, magnetic resonance imaging (MRI) and CAT scanning may be used to detect a DVT. Magnetic imaging uses radio waves to generate images of the internal organs, while a CAT scan is a special x-ray that takes multiple x-rays of "slices" of the body. Neither MRI nor CAT scans are commonly used to diagnose DVT.

Blood tests for D dimer may be used during DVT diagnosis. D dimer is a by-product of fibrin development. Fibrin is an essential component of blood coagulation and its presence indicates blood clotting activity. However, D dimer testing is not infallible: pregnancy, surgery, and cancer can also increase D dimer levels and yield false positive results for a DVT. On the other hand, a negative result for D dimer is usually accurate, and indicates that DVT is unlikely.

Plethysmography compares blood pressure in the arms and legs. A blockage can be detected when there is a significant difference in blood pressure. This method is usually used to test for blockages in the leg. While abnormal results don't prove the presence of DVT, normal results indicate that blood clots are unlikely.

 

 

Beers, M. H. & Berkow, R. (ed). Venous thrombosis [electronic version]. The Merck Manual of Diagnosis and Therapy, 17th Edition. Merck Research Laboratories, NJ, 1999.
www.merck.com/mrkshared/mmanual/section16/chapter212/
212g.j sp.

National Heart, Lung, and Blood Institute. (nd). What is deep vein thrombosis? Retrieved January 16, 2004, from
dci.nhlbi.nih.gov/Diseases/Dvt/DVT_WhatIs.html.

 

National Library of Medicine. (updated 2004). Venous insufficiency. Retrieved January 16, 2004, from
www.nlm.nih.gov/medlineplus/ency/article/000203.htm.

 

National Library of Medicine. (updated 2004). Deep venous thrombosis. Retrieved January 16, 2004, from
www.nlm.nih.gov/medlineplus/ency/article/000156.htm.