Introduction: Who Should Undergo Diagnostics

Venous thromboembolism is not always easy to spot on your own. Some people with blood clots experience obvious symptoms, while others have no warning signs at all until serious complications develop. If you have recently undergone major surgery, suffered a significant injury, or spent extended time unable to move—such as during bed rest or a long flight—you should be especially alert to the possibility of VTE.^[1] These situations create conditions where blood flow slows down, making it easier for clots to form in damaged or stagnant veins.

You should seek medical evaluation promptly if you notice sudden swelling, pain, tenderness, warmth, or redness in one leg or arm. These are common signs that a deep vein thrombosis, or DVT, may have developed in the deeper veins of your limbs.^[3] The symptoms usually affect just one side of the body, and the pain often gets worse when you walk or stand. Sometimes the skin over the affected area may appear darker or discolored, though this can be harder to see if you have brown or black skin.^[7]

Even more urgent is the need to recognize symptoms of pulmonary embolism, which happens when a clot breaks loose and travels to the lungs. Sudden shortness of breath, rapid breathing, chest pain that worsens when you take a deep breath or cough, a fast or irregular heartbeat, coughing up blood, or feeling lightheaded and faint are all warning signs that require immediate emergency care.^[5] Pulmonary embolism can be life-threatening, and every minute counts in getting proper treatment. If you experience these symptoms, you should call emergency services right away rather than waiting to see if they improve on their own.^[3]

Certain groups of people face higher risk and should be particularly vigilant about seeking diagnostics. If you are over 60 years old, overweight, a smoker, or have a personal or family history of blood clots, your chances of developing VTE are increased.^[7] People with cancer, heart failure, varicose veins, or those taking hormone-based medications such as birth control pills also need to be more aware of the possibility. Women who are pregnant or have recently given birth—especially in the first six weeks after delivery—should watch for symptoms as well.^[5]

Anyone who has been hospitalized recently, especially following surgery, or who has been confined to bed for days or weeks should understand that this lack of movement dramatically increases the risk of blood clots.^[3] Even long journeys by plane, car, or train lasting more than three hours can contribute to clot formation. If you are in any of these situations and develop symptoms, do not hesitate to seek medical attention. Early diagnosis can prevent dangerous complications and save lives.

Classic Diagnostic Methods

When you arrive at the hospital or doctor’s office with suspected VTE, the healthcare provider will begin by taking a detailed medical history and asking questions about your symptoms. They will want to know when the symptoms started, whether you have had recent surgery or injuries, if you have been traveling, and whether you have any risk factors such as cancer or previous blood clots.^[1] This conversation helps the provider assess how likely it is that you have VTE and guides which tests to order next.

The physical examination is another important first step. Your provider will carefully look at your legs or arms for swelling, check for areas of tenderness or warmth, and observe any changes in skin color. They will measure the size of your limbs to compare one side with the other, as a clot often causes noticeable swelling on just one side.^[12] For suspected pulmonary embolism, the provider will listen to your heart and lungs and check your vital signs, including your heart rate, breathing rate, blood pressure, and oxygen levels.

One of the first blood tests your provider may order is called a D-dimer test. This test measures a specific type of protein that appears in your blood when clots are breaking down. Almost everyone with a severe DVT has elevated D-dimer levels.^[12] A normal D-dimer result can help rule out VTE, meaning it is unlikely you have a blood clot. However, a high D-dimer does not confirm VTE on its own, because many other conditions—such as infections, inflammation, pregnancy, recent surgery, or cancer—can also raise D-dimer levels.^[12] For this reason, additional imaging tests are almost always needed to confirm the diagnosis.

The standard and most common test to diagnose deep vein thrombosis is a duplex ultrasound. This is a painless, non-invasive test that uses sound waves to create pictures of the blood flowing through your veins.^[12] During the test, a technician will gently move a small handheld device called a transducer over the skin of your leg or arm. The ultrasound can show whether a clot is present and where it is located. Sometimes you may need to have more than one ultrasound over several days to check if a clot is growing or if a new one has formed.^[7] If a doctor suspects DVT based on your symptoms and risk factors, you should typically be referred to the hospital within 24 hours to have this ultrasound scan performed.^[7]

For diagnosing pulmonary embolism, the most commonly used imaging test is a computed tomography angiogram, often called a CT scan or CTPA. This test involves injecting a special dye into your veins through an intravenous line, then using a CT scanner to take detailed cross-sectional images of your chest.^[4] The dye makes the blood vessels in your lungs show up clearly on the images, allowing doctors to see if any clots are blocking the arteries. The entire test usually takes only a few minutes, though you may need to wait for the dye to circulate through your body first.

In some cases, especially if a CT scan cannot be done or if the results are unclear, doctors may use a ventilation-perfusion scan, also called a V/Q scan. This is a type of nuclear medicine test that examines both air flow and blood flow in your lungs.^[5] First, you breathe in a small amount of radioactive gas while a camera takes pictures of how air moves through your lungs. Then, a different radioactive substance is injected into your vein, and more pictures show how blood flows through your lungs. If there is a mismatch—meaning air is reaching an area but blood is not—this suggests a clot may be blocking blood flow to that part of the lung.

Less commonly, a test called venography might be performed. During venography, a dye is injected into a large vein in your foot or ankle, and X-rays are taken to create a picture of the veins in your legs.^[12] This test is invasive and not used very often anymore because ultrasound is usually sufficient and carries less risk. Venography may be considered when ultrasound results are inconclusive and the suspicion for DVT remains high.

For pulmonary embolism, additional tests may help assess how the clot is affecting your body. A chest X-ray is often performed, though it cannot diagnose pulmonary embolism directly. Instead, it helps rule out other conditions that might cause similar symptoms, such as pneumonia or a collapsed lung.^[4] An echocardiogram, which is an ultrasound of the heart, may be done to check if the clot is putting strain on the right side of your heart or if there are clots visible in the heart chambers.^[5]

Pulse oximetry is a simple test that measures how much oxygen is in your blood. A small clip-like device is placed on your fingertip, and it shines light through your skin to estimate oxygen levels.^[4] Low oxygen levels can suggest that a pulmonary embolism is preventing your lungs from working properly. An electrocardiogram, or EKG, records the electrical activity of your heart and can show signs of strain or irregular heart rhythms that sometimes occur with pulmonary embolism.

In rare situations where other tests have not provided clear answers, or when a patient cannot undergo CT scanning, a pulmonary angiogram might be necessary. This is an invasive procedure where a thin tube called a catheter is inserted into a blood vessel in your groin or arm and guided up to the arteries in your lungs.^[4] Dye is then injected directly into these vessels while X-ray images are taken. Although this test gives very detailed information, it is used only when absolutely needed because it carries more risks than non-invasive tests.

Diagnostics for Clinical Trial Qualification

When patients are being considered for enrollment in clinical trials studying new treatments for venous thromboembolism, specific diagnostic tests and criteria are often required to ensure that participants truly have the condition being studied and that it is safe for them to participate. These requirements help researchers gather reliable data and protect patient safety throughout the trial.

Confirming the diagnosis with objective testing is essential before a patient can join a VTE clinical trial. Most trials require that deep vein thrombosis be documented by duplex ultrasound or, less commonly, by venography.^[14] For pulmonary embolism, confirmation typically comes from a CT angiogram or a ventilation-perfusion scan showing clear evidence of blood clots in the lung arteries. Clinical trials generally do not accept patients based only on symptoms or physical examination findings—they need hard evidence from imaging or laboratory tests.

The timing of the diagnosis matters as well. Many clinical trials studying acute VTE treatment will only accept patients who have been diagnosed very recently, often within the past few days or weeks. This ensures that researchers are testing treatments on fresh clots rather than old, partially resolved ones. Trials may exclude patients whose clots have been present for longer periods or who have already received extensive treatment, as these factors could affect how well a new therapy appears to work.

Blood tests beyond the D-dimer are often part of clinical trial screening. Kidney function tests are particularly important because many blood-thinning medications used to treat VTE are processed by the kidneys, and dosing may need to be adjusted or the medication avoided entirely in patients with severe kidney problems.^[14] Liver function tests may also be required, as some anticoagulant drugs can affect the liver or need dose adjustments in people with liver disease.

Patients entering trials may need a complete blood count to check their red blood cells, white blood cells, and platelets. Very low platelet counts can increase bleeding risk, which might make it unsafe for someone to receive blood-thinning medications in a trial. Similarly, severe anemia might be a reason to exclude someone, as blood thinners carry a risk of causing bleeding that could worsen anemia.

Pregnancy testing is standard for women of childbearing age who are considering participation in VTE trials, because many anticoagulant medications can harm a developing fetus. Women who are pregnant or planning to become pregnant are typically excluded from trials testing new blood thinners, though separate studies may exist specifically for pregnant women with VTE.

Some clinical trials require assessment of bleeding risk before enrollment. This might include a detailed medical history asking about any previous episodes of abnormal bleeding, ulcers, recent surgeries, or conditions that could make bleeding more likely. Occasionally, special blood tests that measure how well your blood clots—such as prothrombin time or activated partial thromboplastin time—may be performed to establish a baseline before starting any experimental treatment.

Imaging to assess the extent or severity of the clot is another common requirement. For DVT trials, researchers may want to know exactly where the clot is located and how large it is, which can be determined by careful ultrasound examination. For pulmonary embolism studies, the number and size of clots in the lungs, and whether they are causing strain on the heart, may influence eligibility. Some trials focus specifically on patients with massive or high-risk pulmonary embolism, while others may include only those with less severe disease.

Echocardiography might be required in trials enrolling patients with pulmonary embolism to evaluate whether the right side of the heart is under strain from the clot. Certain trials testing new treatments specifically target patients whose hearts show signs of strain, as these individuals are at higher risk for serious complications and may benefit most from aggressive therapies.

Clinical trials may also require documentation that standard diagnostic procedures have ruled out other conditions that could mimic VTE. For example, trials might exclude patients whose leg swelling is due to infection, injury, or chronic venous problems rather than an acute blood clot. Similarly, for pulmonary embolism trials, researchers may want to ensure that shortness of breath is not due to pneumonia, heart failure, or lung disease rather than a clot.

Genetic testing is not typically required for entry into most VTE treatment trials, but some research studies specifically focus on people with inherited clotting disorders. In those cases, blood tests to identify mutations in genes that affect clotting—such as Factor V Leiden or prothrombin gene mutations—may be part of the screening process.

The overall goal of these diagnostic requirements is to create a well-defined group of participants with confirmed VTE whose outcomes can be meaningfully compared. Standardized testing ensures that everyone in the trial has the same condition diagnosed using the same methods, which makes the results more reliable and applicable to real-world clinical practice.