A deep vein thrombosis (DVT) is a blood clot that forms in one of the deep veins in your body, usually in one of your legs. A blood clot that forms in an artery or vein, and restricts blood flow is called a thrombus.

It’s made up of proteins and platelets. Platelets are one of the three major types of blood cells. They help your blood clot and control bleeding.

Several different things can cause a DVT.

Among them are:

  • surgery
  • injury to the vein
  • inflammation of the walls of the vein

While there are several symptoms of a DVT, imaging is usually necessary to diagnose the condition and locate the clot. An ultrasound is one commonly used test to confirm whether a blood clot — or some other health problems — is causing your symptoms.

DVT symptoms include:

  • pain and swelling in the lower leg
  • redness or discoloration of the skin near the clot
  • warmth around the affected area

Based on your symptoms and your medical history, your healthcare provider may recommend an ultrasound or another test to help diagnose a suspected DVT. This decision is often made in an emergency room when a person arrives with DVT symptoms.

An ultrasound uses sound waves to create moving images of blood flowing through your veins and arteries. Unlike an X-ray, for example, an ultrasound does not use any radiation.

During an ultrasound procedure, you’ll wear a hospital gown and be covered by a sheet. Only the leg being evaluated will be exposed. The head of the bed should be at a 30- to 45-degree angle to help force more blood flow down to your legs.

Depending on where the clot is likely to be, you’ll rest your leg at an angle that’s comfortable for you, but also allows the ultrasound technician or the doctor to move the ultrasound “wand” or probe easily on the surface of your leg.

A common location for DVT is behind the knee, but a clot can form in any of the deep blood vessels.

Ultrasound gel will be rubbed across a wide area of your leg. The gel is safe and painless.

It forms a bond between the skin and the probe, making it easier for the sound waves to reach the blood vessels under the skin. Any space between the probe and the skin would cause the images to be lost.

The probe is moved slowly and gently across your leg, allowing sound waves to penetrate the skin to the blood vessels and tissue underneath. The waves form images that appear on a computer screen nearby. When a DVT is identified, a still picture of it can be made.

The person doing the ultrasound may want to get a few angles of the DVT to better understand its size and location. The procedure should take less than 30 minutes.

After the test, the ultrasound gel will be cleaned off of your leg. Your healthcare provider will then decide whether any treatment is necessary that day. If the DVT doesn’t appear to be a threat, you may have a few more ultrasounds to see if the thrombus is growing or moving.

A thrombus that moves toward the heart can be a serious health risk. A blood clot that moves to the heart and then into the lungs is called a pulmonary embolism (PE). It can be life threatening.

According to the National Blood Clot Alliance, an ultrasound finds about 95 percent of DVTs in the large veins above the knee. Usually, no other test is required if a clot is identified through ultrasound.

Ultrasound identifies only about 60 to 70 percent of DVTs in calf veins. These clots are less likely to become PEs than those that form above the knee.

An ultrasound is used more than other types of imaging tests when a DVT is suspected. If an ultrasound can’t provide a definitive diagnosis, other screenings may be necessary.

These screenings include:

  • Pulmonary ventilation/perfusion (VQ) scan: A VQ scan is a series of two lung scans. These scans measure where the blood flows in your lungs and how well it’s flowing.
  • Computed tomography (CT) scan: A CT scan test uses special X-ray and computer equipment to create three-dimensional scans of the blood vessels.
  • Magnetic resonance imaging (MRI): An MRI uses a large magnetic field and radio frequencies to show detailed pictures of blood vessels and soft tissue.
  • Venography: Venography is a test that uses a special dye that’s injected into a large vein. An X-ray then highlights the flow of the dyed blood through the vein suspected of having a clot.

A blood test may also be ordered. If you have a DVT, you may have an elevated level of a substance in your blood known as “D-dimer.”

If you’ve been diagnosed with a DVT, the goal of treatment is to keep the thrombus from getting bigger or moving to the lungs. A smaller clot that doesn’t appear to be at risk of becoming a PE may be treated with anticoagulant drugs. These include warfarin (Coumadin), apixaban (Eliquis), and others.

Anticoagulants, also known as “blood thinners,” won’t break up an existing clot. Instead, they may help keep an existing clot from growing larger. Stronger anticoagulants, such as heparin, may be injected into your bloodstream.

Anticoagulants may be required for a few months. They may be required indefinitely if you have other medical conditions, such as atrial fibrillation, that put you at a higher risk for future blood clots.

Different drugs, called thrombolytics or “clot busters,” may be given in more serious cases.

These drugs, which are given intravenously or with a catheter inserted directly into a blood vessel, help break up existing clots. Thrombolytics are very strong and can cause bleeding problems in some people.

A special umbrella-shaped filter may be inserted into the large vein, called the vena cava, in your abdomen. It can help block a thrombus from reaching your lungs.

If no blood clot is found, your healthcare provider will look for other possible causes of pain and swelling in your legs.

The problem could be a muscle strain, for example. Or, it could be heart problem, such as heart failure or cardiomyopathy, which could be any of several diseases of the heart tissue. These heart problems can cause fluid to build up in your legs.