Richard N. Fogoros, MD, is a retired professor of medicine and board-certified internal medicine physician and cardiologist.

Douglas A. Nelson, MD, is a board-certified oncologist and hematologist who previously served for 13 years as a physician in the US Air Force.

If a healthcare provider suspects a patient has deep vein thrombosis (DVT), a condition defined 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 (PE).

Anyone who experiences symptoms of DVT should see a practitioner, 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.

Your healthcare provider 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 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 levels are elevated, it suggests that you may have a DVT, but there are other reasons for an elevated D-dimer; additional tests are required to confirm the presence of a DVT or PE.

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.

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

This is typically the preferred option for diagnosis. There are different types of venous ultrasonography:

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).

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 healthcare providers 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 practitioners 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 practitioners generally prefer to use the duplex ultrasonography method. However, some healthcare providers 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.

Some healthcare providers 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 another for blood clots. The MR machine uses these to create an image that allows medical professionals to discern between the two.

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 healthcare provider suspects you have a pulmonary embolism (PE), they may opt for a computed tomographic pulmonary angiography (CTPA)—a 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 test.

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

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.

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

A D-dimer is often used to rule out a clot. If it's abnormal, it indicates some form of activation of the clotting system, which may indicate a DVT. A D-dimer cannot be used to diagnose a blood clot, it can only suggest a blood clot in which case further tests are needed.

Imaging studies used to diagnose DVT include duplex ultrasound, venogram, and MRI.

Yes, DVT often goes unnoticed and the clot dissolves on its own. However, if it causes symptoms and is discovered, it should be treated to prevent serious complications such as a pulmonary embolism.

Other conditions that present similarly to DVT include venous insufficiency, superficial thrombophlebitis, a muscle strain, tear, or trauma, a Baker’s cyst, cellulitis, and lymphedema. 

Texas Heart Institute. Pulmonary embolism.

Centers for Disease Control and Prevention. Diagnosis and treatment of venous thromboembolism.

U.S. National Library of Medicine: MedlinePlus. D-dimer test.

American Academy of Orthopedic Surgeons: OrthoInfo. Deep vein thrombosis.

Institute for Quality and Efficiency in Health Care (IQWiG). Deep vein thrombosis (DVT): Overview.

Crisp JG, Lovato LM, Jang TB. Compression ultrasonography of the lower extremity with portable vascular ultrasonography can accurately detect deep venous thrombosis in the emergency department. Ann Emerg Med. 2010 Dec. 56(6):601-10. doi:10.1016/j.annemergmed.2010.07.010

Stanford University Healthcare. Duplex ultrasound for CVI.

The National Heart, Lung, and Blood Institute. Pulmonary embolism.

By Richard N. Fogoros, MD Richard N. Fogoros, MD, is a retired professor of medicine and board-certified in internal medicine, clinical cardiology, and clinical electrophysiology.

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