How Deep Vein Thrombosis Is Diagnosed

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If a doctor suspects a patient has deep vein thrombosis (DVT), a condition : by a blood clot forming in one of the deep veins, they will attempt to make a definitive diagnosis as quickly as possible. There is a potential for such a blood clot to loosen and travel to the lungs, which can cause a potentially life-threatening pulmonary embolism. Anyone who experiences symptoms of DVT should see a doctor, who will likely perform an ultrasound if they suspect the condition. Other tests, such as a venogram, impedance plethysmography, CT scan, or a D-dimer test, may also be used to identify DVT and/or its cause.

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Labs and Tests

Your doctor may order blood tests to determine if you have inherited a blood disorder associated with DVT and PE. The blood tests are also used to measure carbon dioxide and oxygen levels. A blood clot in the lungs can lower oxygen levels in the blood. 

A D-dimer test is usually used to rule out DVT in people with a low or intermediate risk for the condition. The test indicates whether you have elevated levels of D-dimer, a protein fragment that's left over from a clot once it's formed. If your D-dimer test is not elevated, chances are you do not have DVT.

While the D-dimer generally has reliable results, it cannot identify where the blood clot is. The other drawbacks of the D-dimer test are that it may not be as reliable for finding clots in pregnant women, people who take blood thinners, and those with a history of DVT.

Deep Vein Thrombosis Doctor Discussion Guide

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Imaging

While it's true signs and symptoms of DVT can mimic those of other conditions, if DVT is a possibility, a doctor will undoubtedly opt for imaging tests to get to the bottom of things.

Ultrasound

This is typically the preferred option for diagnosis. A venous ultrasound is usually done on people who have a history of DVT and are taking blood thinners and those who have a positive D-dimer test.

There are different types of venous ultrasonography:

  • Compression ultrasound (B-mode imaging): Similar to the duplex ultrasonography, compression ultrasound is a variation of the commonly-used medical ultrasound technique (also known as an “echo” test). A probe placed on the skin of the chest uses sound waves to construct an image of the tissue that lies beneath. The technician performing the ultrasound can then attempt to compress the vein by pushing on it with the ultrasound probe in the femoral vein (in the groin area) or the popliteal vein (behind the knee). Veins are typically highly compressible, which means they can be collapsed temporarily by applying pressure to them. But if DVT is present, a blood clot makes it difficult to compress the vein. A non-compressible vein is almost always an indication a DVT is present. The ultrasound technique can also be used to visualize the clot itself and to assess whether there's an obstruction of blood flow through the vein.
  • Duplex ultrasound (B-mode imaging and Doppler waveform analysis): Duplex ultrasonography uses high-frequency sound waves to visualize the flow of blood in the veins. It can detect blood clots in the deep veins and is one of the quickest, most painless, reliable, and noninvasive ways to diagnose DVT. The duplex ultrasonography also includes a color-flow Doppler analysis.
  • Color Doppler imaging: This produces a 2-D image of the blood vessels. With a Doppler analysis, a doctor can see the structure of the vessels, where the clot is located, and the blood flow. The Doppler ultrasound can also estimate how quickly blood is flowing and reveal where it slows down and stops. As the transducer is moved, it creates an image of the area. 

The reliability of these tests varies. For example, compression ultrasounds are best for detecting DVT in proximal deep veins, like femoral and popliteal veins (thighs), but duplex ultrasound and color Doppler imaging are best for DVT of the calf and iliac veins (pelvis).

Venogram

In the past, making a firm diagnosis of DVT required performing a venogram. With a venogram, a contrast iodine-based dye is injected into a large vein in the foot or ankle, so doctors can see the veins in the legs and hips. X-ray images are made of the dye flowing through the veins toward the heart. This allows for doctors and medical professionals to see major obstructions to the leg vein.

This invasive test can be painful and entails certain risks, such as infection, so doctors generally prefer to use the duplex ultrasonography method. However, some doctors will use a venogram for people who have had a history of DVT. Because blood vessels and veins in these individuals are likely damaged from previous clots, a duplex ultrasonography won't be able to detect a new clot like a venogram can.

Today, many doctors use magnetic resonance (MR) venography instead of the X-ray version because it's less invasive. The MR machine uses radio frequency waves to line up hydrogen atoms within tissues. When the pulse stops, the hydrogen atoms return to their normal state, giving off one type of signal for tissues in the body and other for blood clots. The MR machine uses these to create an image that allows doctors to discern between the two.

MRI and CT Scans

Magnetic resonance imaging (MRI) and computed tomography (CT) scans can create images of the organs and tissues in the body, as well as veins and clots. While useful, they are generally used in conjunction with other tests to diagnose DVT.

If your doctor suspects you have a pulmonary embolism (PE), they may opt for a computed tomographic pulmonary angiography (CTPA)—a standard chest X-ray test in which a contrast dye is injected into the arm. The dye travels through the blood vessels leading to the lungs to create clear images of the blood flow to the lungs in the images produced. 

Lung Ventilation-Perfusion Scans; Pulmonary Angiography

If a CPTA isn't available, you might get a lung ventilation-perfusion scan or a pulmonary angiography.

With the lung ventilation-perfusion scan, a radioactive substance shows the blood flow and oxygenation of the lungs. If you have a blood clot, the scan might show normal amounts of oxygen but slowed blood flow in parts of the lungs that have clotted vessels.

With a pulmonary angiography, a catheter from the groin injects a contrast dye into the blood vessels, which allows doctors to take X-ray images and follow the path of the dye to check for blockages. 

Impedance Plethysmography

Impedance plethysmography is another non-invasive test for diagnosing DVT. While this test is reliable, many hospitals do not have the equipment or the expertise readily available to perform this test efficiently.

In impedance plethysmography, a cuff (similar to a blood pressure cuff) is placed around the thigh and inflated in order to compress the leg veins. The volume of the calf is then measured (by means of electrodes that are placed there). When the cuff deflates, it allows the blood that had been "trapped" in the calf to flow out through the veins. The calf volume measurement is then repeated.

If DVT is present, the difference in volume (with the cuff inflated versus deflated) will be less than normal, indicating that the veins are partially obstructed by a blood clot.

Differential Diagnoses

Test results and a physical exam can help rule out (or in) other possible causes of your symptomsl. A few that will be considered:

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  1. Texas Heart Institute. Pulmonary embolism.

  2. Centers for Disease Control and Prevention. Diagnosis and treatment of venous thromboembolism. Updated March 14, 2019.

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