Assessing your risk

Deep vein thrombosis may first be suspected after a medical history and physical exam. The information gathered from these initial tests will help your doctor determine whether your risk level for having deep vein thrombosis is low, medium, or high. Your risk level will help your doctor decide the appropriate testing for deep vein thrombosis.

Ultrasound test

Ultrasound is the main test used to help diagnose deep vein thrombosis. It creates a picture of the flow of blood through the veins. You might need more than one ultrasound, usually done a few days to a week apart. This is called serial testing. The testing sequence for deep vein thrombosis is based on your risk level and the results from your initial ultrasound.

Additional tests

Additional tests may be used when ultrasound results are unclear. These tests may help diagnose or exclude a blood clot in the leg but are not frequently needed. Additional tests may include:

• D-dimer test.
• Magnetic resonance imaging (MRI).
• Computed tomography (CT) scan.
• Venogram.

If you are treated with anticoagulant medicines, you may need periodic blood tests to monitor the effects of the anticoagulant on the blood. Blood tests include:

• Activated partial thromboplastin time (APTT) to monitor treatment with standard or unfractionated heparin.
• Prothrombin time (PT), also referred to as INR, to monitor treatment with warfarin (Coumadin).

If you are suspected of having pulmonary embolism, you may have a lung scan, a spiral CT scan, or a pulmonary angiogram. For more information, see the topic Pulmonary Embolism.

Early Detection

Special blood tests may help identify inherited blood-clotting abnormalities that can increase your risk of forming blood clots. However, screening for these factors is not routinely done and is somewhat controversial.

In general, screening is sensible if you have or have had one or more of the following:⁶

• A blood clot in a vein that has no clear cause
• A blood clot at age 45 or younger
• A first-degree family member (mother, father, brother, or sister) who has had a blood clot in a vein before age 45 or has had problems with blood clotting
• A blood clot in a vein at an unusual location, such as the gastrointestinal region, the brain, or the arms

Some doctors believe that checking for clotting problems with a blood test can help prevent deep vein thrombosis in people who have an increased risk and are in a high-risk situation (such as upcoming surgery). If a blood test finds clotting problems, then preventive measures can be taken.