Sometimes, blood clots can form in the deep veins of the body, typically in the legs. This condition is known as deep vein thrombosis (DVT) and it shouldn’t be taken lightly. If the blood clot breaks free from the vein in which it formed, it can travel to other parts of the body and cause life-threatening complications such as pulmonary embolism. Many of the risk factors of DVT are the same as the risk factors for varicose veins and spider veins, such as obesity, pregnancy, and injury to the veins. If you have suspected DVT, a vein specialist in Houston can perform medical tests to confirm the diagnosis.
Quick Answer: DVT is diagnosed by combining a clinical assessment of how likely a clot is, a D-dimer blood test and, most importantly, a venous duplex ultrasound of the leg. A normal D-dimer can rule out DVT in lower-risk patients, but a raised level only means imaging is needed. The ultrasound checks whether the deep veins compress normally and whether blood flows through them; a vein that will not compress contains clot. CT or MR venography is used when clots in the pelvis or abdomen are suspected.
Meet your vein specialist
Dr. Syed Raza, MD
Vascular & Interventional Radiology · Founder, Premier Vein & Vascular Center
Fellowship-trained interventional radiologist certified by the American Board of Radiology and the Royal College of Radiologists (UK). Former Chief of Interventional Radiology at UTMB Galveston and Memorial Hermann Memorial City, with 15+ years treating varicose veins, venous reflux and peripheral arterial disease.
Medical History
The vein specialist will begin your appointment by reviewing your medical history. You will be asked to disclose your pre-existing medical conditions, along with any prescribed drugs, over-the-counter medicines, or herbal supplements you may be taking.
Symptom Review
Next, the doctor will ask you about your symptoms. DVT doesn’t always cause noticeable symptoms, but when it does, patients may experience swelling and pain in the affected leg. Most often, the pain develops in the calf muscle. It typically feels like muscle cramping. The doctor will also perform a physical exam and may notice that one calf muscle looks larger than the other.
Blood Test
Blood tests are commonly used to diagnose DVT. In almost all cases, patients with DVT show elevated levels of a substance called D dimer. D dimer is a clot-dissolving substance that the body releases in response to the breaking up of a blood clot.
Ultrasound
Ultrasound tests use sound waves to develop real-time images of the internal structures of the body. You may already be familiar with the use of ultrasound in prenatal care, but it has other uses as well. Your doctor can place a special gel on the calf muscle or other area where a blood clot is thought to be. Then, a transducer is placed on the skin and moved around. The transducer transmits sound waves, which are used to create the images. Your vein specialist may need to perform multiple ultrasound tests over time to determine whether the blood clot is getting larger.
How DVT Is Diagnosed: The Big Picture
Diagnosing deep vein thrombosis is a step-by-step process rather than a single test. Symptoms of DVT overlap with many other leg problems, and some clots cause no symptoms at all, so doctors combine three pieces of information: how likely a clot is based on your history and exam, a blood test that helps rule a clot out, and imaging that shows the clot directly. Ultrasound is the cornerstone. In most cases, a properly performed venous duplex ultrasound confirms or excludes DVT the same day.
Speed matters. A clot in the thigh or pelvic veins can extend or break off and travel to the lungs, so suspected DVT should be assessed within hours to a day, not weeks.
Estimating the Likelihood of a Clot
Before ordering tests, clinicians usually estimate the pretest probability of DVT using a structured checklist, the most widely used being the Wells score. It assigns points to findings such as:
- Active cancer or recent cancer treatment
- Recent surgery, hospital stay or bed rest, or a leg in a cast
- Tenderness along the deep veins of the thigh or calf
- Swelling of the entire leg, or a calf that measures noticeably larger than the other
- Pitting swelling only in the symptomatic leg
- Visible superficial veins that are not varicose veins
- A previous DVT
- Whether another diagnosis seems at least as likely
The result places a patient in a lower- or higher-probability group, which determines what happens next. In a lower-probability patient, a normal D-dimer can safely rule out a clot in many cases. In a higher-probability patient, we go straight to ultrasound, because a normal D-dimer is not reassuring enough on its own.
What a D-Dimer Test Can and Cannot Tell You
D-dimer is a fragment released when the body breaks down a clot. The test is very sensitive, which makes a normal result useful for ruling DVT out in the right patient. It is not specific, though. D-dimer levels rise in many situations unrelated to DVT, including:
- Pregnancy and the weeks after childbirth
- Recent surgery or injury
- Infection and inflammation
- Cancer
- Older age, which is why many labs use an age-adjusted cutoff for patients over 50
So an elevated D-dimer does not mean you have a clot. It simply means a clot has not been ruled out and imaging is needed. People already taking blood thinners can also have a falsely reassuring result.
Venous Duplex Ultrasound: The Main Test for Diagnosing DVT
A venous duplex ultrasound combines a black-and-white picture of the veins with Doppler, which shows blood flow in color and sound. It is painless, uses no radiation or dye, and usually takes 20 to 45 minutes.
The key maneuver is compression. A healthy vein collapses completely when the technologist presses on it with the probe. A vein containing clot does not. The technologist works down the leg in small steps from the groin to the calf, pressing at each level, and uses Doppler to check that blood flows normally and increases when the calf is squeezed. The scan can also show:
- Whether a clot is fresh or older and partly scarred
- How far up the leg the clot extends, which affects treatment
- Whether a clot is in the deep veins or in a superficial vein, which is managed differently
- Other causes of swelling, such as a Baker’s cyst behind the knee, a muscle tear or fluid collection
Some centers perform a limited scan of the thigh and the back of the knee only. In that case, if the first scan is negative but suspicion remains, a repeat scan in about a week checks whether a small calf clot has extended. A whole-leg duplex that includes the calf veins avoids that wait in many cases. Accreditation of the vascular lab matters here, because the accuracy of ultrasound depends heavily on technique and experience.
Repeat imaging also has a role after diagnosis. A follow-up scan helps show whether a clot is shrinking on treatment and gives a new baseline, which makes it easier to tell an old clot from a new one if symptoms ever return.
When Other Imaging Is Needed
Ultrasound cannot always see veins deep in the pelvis or abdomen clearly. When a clot is suspected above the groin, for example with swelling of the entire leg up to the hip, other tests may be used:
- CT venography uses contrast dye and a CT scanner to show the pelvic and abdominal veins.
- MR venography gives similar detail without radiation and is useful in some patients, including selected pregnant patients.
- Catheter venography, in which dye is injected directly into a vein, is now reserved mostly for planning procedures such as clot removal or stenting, and for conditions like iliac vein compression (May-Thurner syndrome).
If there are any symptoms suggesting the clot has reached the lungs, such as breathlessness or chest pain, a CT pulmonary angiogram is the usual test for pulmonary embolism. Clots in the arm, often linked to IV lines, ports or vigorous repetitive arm use, are diagnosed with ultrasound of the arm and neck veins.
DVT vs. Conditions That Look Like It
Many painful, swollen legs turn out not to be DVT. A careful exam and ultrasound sort these out.
| Condition | Typical features | How it is confirmed |
|---|---|---|
| Deep vein thrombosis | One-sided calf or leg swelling, aching or cramping pain, warmth | Venous duplex ultrasound |
| Superficial thrombophlebitis | Tender, red, firm cord along a surface vein, often a varicose vein | Ultrasound, which also checks it is not near the deep veins |
| Ruptured Baker’s cyst | Sudden pain behind the knee and calf, sometimes bruising at the ankle | Ultrasound |
| Calf muscle strain | Pain after exertion or a specific movement, focal tenderness | Exam; ultrasound if unclear |
| Cellulitis | Spreading redness, heat, often fever, skin tenderness | Clinical exam; ultrasound to exclude a clot |
| Chronic venous insufficiency or lymphedema | Gradual swelling, often both legs, worse at day’s end | Duplex ultrasound for reflux; clinical assessment |
How We Evaluate Suspected DVT at Premier Vein
- Focused history. We ask when the swelling or pain began, about recent surgery, travel, injury, hormone use, pregnancy, cancer, previous clots and family history of clotting, plus any breathing symptoms.
- Examination. We compare both legs, measure calf size, check for tenderness along the deep veins, look at the skin and superficial veins, and examine pulses and the knee.
- Duplex ultrasound in our accredited lab. Our IAC-accredited vascular lab performs a complete venous duplex study, so the result is available promptly and interpreted by a physician experienced in vein imaging.
- A clear plan. If a clot is found, we explain its location and what it means, start or coordinate treatment, and arrange follow-up imaging. If there is no clot, we explain what is causing your symptoms and how to treat it.
What Happens After DVT Is Confirmed
Once the diagnosis is made, the next questions are where the clot is, why it formed and how to treat it. Most patients are treated with blood thinners (anticoagulants), often as tablets that can be started right away, usually for at least three months. Depending on the circumstances, we may look for a trigger such as recent surgery or immobility, or recommend further testing if a clot occurred without an obvious cause.
Large clots in the thigh or pelvic veins that cause severe swelling sometimes benefit from catheter-based clot removal to reduce the risk of long-term leg damage. Compression and walking help with symptoms, and follow-up ultrasound checks how the clot is resolving. You can read more on our deep vein thrombosis treatment page. Some patients go on to develop lasting swelling or skin changes from valve damage, known as post-thrombotic syndrome, which overlaps with chronic venous insufficiency and is something we monitor for.
Call 911 or go to the nearest emergency room if you have:
- Sudden shortness of breath or rapid breathing
- Chest pain that is sharp or worse with a deep breath
- Coughing up blood
- A racing heartbeat, lightheadedness or fainting
- A leg that is massively swollen, blue or pale and very painful
Getting a DVT Evaluation in Houston
If you have new swelling, pain or tightness in one leg, especially after surgery, a long trip or a period of reduced mobility, do not wait to see whether it settles. Contact your doctor the same day, and if you have any breathing symptoms, call 911. For leg symptoms without warning signs, a vein specialist in Houston with an accredited vascular lab can usually confirm or rule out a clot quickly.
Dr. Syed Raza is a fellowship-trained interventional radiologist with more than 15 years of experience diagnosing and treating venous disease, including DVT. Our office in Houston also serves patients from Katy and the surrounding area, same-week appointments are usually available, and most insurance plans, including Medicare, are accepted. You can contact us or call (832) 321-5355.
Talk With a Specialist at Premier Vein
Have questions about your symptoms or treatment options? Dr. Syed Raza and the team at Premier Vein & Vascular Center can evaluate your concerns and explain your options clearly. Same-week appointments are usually available, and most insurance plans are accepted.
Frequently Asked Questions
Can a DVT be missed on ultrasound?
Ultrasound is very accurate for clots in the thigh and behind the knee, but small clots in the calf veins or clots in the pelvis can be harder to see. If symptoms persist after a negative scan, a repeat ultrasound or a different imaging test may be recommended.
Can a blood test alone diagnose DVT?
No. A D-dimer blood test can help rule out DVT in people at lower risk when it is normal, but it cannot confirm a clot. A raised D-dimer has many other causes, so imaging, usually a duplex ultrasound, is needed to make the diagnosis.
How long does a DVT ultrasound take?
A venous duplex ultrasound of one leg usually takes about 20 to 45 minutes. It is painless, although pressing over a tender area can be briefly uncomfortable. Results are often available the same day.
Should I go to the ER for a suspected DVT?
If you have chest pain, shortness of breath, coughing up blood or fainting, call 911 or go to the emergency room because these can signal a pulmonary embolism. For leg swelling or pain without these symptoms, contact your doctor or a vein specialist the same day to arrange prompt ultrasound testing.
Can you have a DVT without swelling?
Yes. Some clots cause only mild calf aching or tightness, and some cause no symptoms at all, especially small clots or clots in patients who are bedbound. That is why doctors rely on risk factors and imaging rather than symptoms alone.
What is the difference between DVT and a superficial clot?
A DVT forms in the deep veins inside the muscles and carries a risk of traveling to the lungs. A superficial clot, or thrombophlebitis, forms in a vein just under the skin and causes a tender, red cord. Superficial clots are usually less dangerous, but an ultrasound is still needed to make sure they are not close to or extending into the deep veins.
How is DVT diagnosed during pregnancy?
Ultrasound is the first test for suspected DVT in pregnancy because it is safe for the baby and uses no radiation. D-dimer is less helpful because it rises naturally during pregnancy. If a pelvic clot is suspected and ultrasound is unclear, MR venography without contrast may be considered.
Trusted Health Resources
- Venous Thromboembolism, National Heart, Lung, and Blood Institute (NIH)
- About Venous Thromboembolism (Blood Clots), Centers for Disease Control and Prevention