Doctors check for blood clots in the legs using a combination of a physical exam, a risk-scoring system, a blood test, and most often an ultrasound. The process is designed to be fast and stepwise, so many people can be cleared without ever needing imaging. If your doctor suspects a deep vein thrombosis (DVT), here’s what each step looks like and why it matters.
The Physical Exam
Your doctor will start by examining your legs, looking for swelling, tenderness, warmth, and changes in skin color. They’ll often measure the circumference of both calves and compare them, since a difference of more than 3 cm is a red flag. They’ll press along the path of the deep veins to check for localized tenderness, and they’ll look for pitting edema, where pressing on the swollen area leaves a temporary dent in the skin.
You may have heard of the Homan’s sign, where a doctor flexes your foot upward to see if it causes calf pain. This test has fallen out of favor because it misses too many clots and sometimes flags pain that has nothing to do with a blood clot. Modern diagnosis relies on more structured tools instead.
How Doctors Score Your Risk
Rather than relying on gut instinct, most doctors use a standardized checklist called the Wells score. It assigns points based on specific risk factors and symptoms, then places you into a risk category that determines what happens next.
You receive one point for each of the following:
- Active cancer
- Paralysis or recent leg immobilization (such as a cast)
- Recently bedridden for more than three days or major surgery within the past four weeks
- Tenderness along the deep veins
- Entire leg swollen
- Calf swelling more than 3 cm compared to the other leg
- Pitting edema in the symptomatic leg
- Visible surface veins that aren’t varicose veins
Two points are subtracted if another diagnosis, like a muscle strain or cellulitis, seems equally or more likely. A low score means a blood clot is unlikely, and your doctor may be able to rule it out with just a blood test. A higher score means imaging is needed.
The D-Dimer Blood Test
If your Wells score is low, the next step is usually a blood draw called a D-dimer test. When a blood clot forms and your body starts breaking it down, it releases tiny protein fragments called D-dimers into your bloodstream. A negative D-dimer result means those fragments aren’t elevated, which makes an active clot very unlikely. For people with a low-to-moderate risk score, a negative D-dimer is often enough to rule out DVT with no further testing needed.
The catch is that D-dimer levels rise for many reasons besides blood clots: recent surgery, infection, pregnancy, cancer, even normal aging. So a positive result doesn’t confirm a clot. It simply means imaging is the next step. This is why the test works best as a rule-out tool in lower-risk patients, not as a standalone diagnostic.
Ultrasound: The Primary Imaging Test
Duplex ultrasound is the go-to imaging test for suspected leg clots. It’s noninvasive, uses no radiation, and gives results in real time. The exam typically takes 30 to 90 minutes. You lie on a padded table while a technologist applies a water-soluble gel to your leg and moves a small handheld device called a transducer over the skin.
The key technique is called compression ultrasound. The technologist presses the transducer firmly against the vein at multiple points along the leg. A healthy, clot-free vein will flatten completely under this pressure, with the walls collapsing together. If a clot is present, the vein won’t compress fully. Loss of compressibility is the single most reliable indicator that a clot is inside the vein.
The exam also uses Doppler evaluation, which measures the speed and direction of blood flow. This helps detect areas where flow is sluggish or blocked, even if the clot isn’t directly visible. Together, the compression and Doppler components give a detailed picture of what’s happening inside both the deep and superficial veins. Ultrasound is highly accurate for clots in the upper leg and thigh veins (called proximal DVT), which are the most dangerous because they’re more likely to break off and travel to the lungs. It can be less reliable for smaller clots isolated in the calf, which is why doctors sometimes recommend a repeat ultrasound about a week later if calf DVT is suspected but the initial scan is inconclusive.
When Doctors Use CT or MRI Instead
Ultrasound handles the vast majority of cases, but certain situations call for different imaging. CT venography, which uses a contrast dye injected into a vein followed by a CT scan, is sometimes used when ultrasound isn’t available after hours or when a doctor needs to evaluate the pelvic veins or the large central vein in the abdomen (the inferior vena cava). Ultrasound doesn’t visualize these deeper areas as well.
MRI venography is another option, particularly useful for patients who have a cast or other physical barrier on the leg that prevents the compression technique ultrasound depends on. MRI also provides better imaging of pelvic and abdominal veins. Neither CT nor MRI is a routine first choice for a straightforward suspected leg clot, but they fill important gaps when ultrasound has limitations.
How the Steps Fit Together
The diagnostic process is designed like a funnel. Everyone starts with a physical exam and a Wells score. If the score is low, a D-dimer blood test can often end the workup right there. If the D-dimer comes back elevated, or if the clinical suspicion is moderate to high from the start, the patient goes straight to ultrasound. Advanced imaging with CT or MRI is reserved for cases where ultrasound can’t give a clear answer or can’t access the area in question.
This stepwise approach means that many people with leg pain or swelling can be reassured quickly without imaging. For those who do need an ultrasound, the test itself is painless, involves no needles, and provides answers the same day. The entire process from exam to diagnosis can often happen within a single clinic visit or emergency department stay.