Doctors check for blood clots using a combination of physical exams, blood tests, and imaging, with the specific approach depending on where they suspect the clot is located. The process typically starts with a risk assessment and a simple blood draw, then moves to ultrasound or CT imaging if needed. Most evaluations can be completed within a few hours.
The Physical Exam
The first thing a doctor does is look at your symptoms and examine the affected area. For a suspected clot in the leg (deep vein thrombosis, or DVT), they’ll check for unilateral swelling, warmth, redness, tenderness along the deep veins, and pitting edema. They may also feel for a firm, rope-like cord under the skin, which can indicate a clotted vein. These signs are useful starting points, but none of them confirm a clot on their own. Conditions like muscle injuries, infections, and even a ruptured cyst behind the knee can look nearly identical.
You may have heard of the Homan’s sign, where a doctor pushes your foot upward to see if it causes calf pain. This test is largely historical at this point. It only triggers pain in about one-third of people who actually have a DVT, and it produces false positives in roughly 20% of people without one. Most clinicians no longer rely on it.
Scoring Your Risk Level
Before ordering any tests, doctors use a standardized checklist called the Wells score to estimate how likely a clot is. For DVT, the checklist assigns one point each for factors like active cancer, recent surgery or prolonged bed rest, leg swelling, calf swelling more than 3 cm larger than the other leg, pitting edema, and tenderness along the deep veins. If another diagnosis seems equally or more likely (such as a pulled muscle), two points are subtracted. The total determines whether you’re categorized as low, moderate, or high probability, and that classification guides what happens next.
A separate version of the Wells score exists for pulmonary embolism (a clot in the lungs). The principle is the same: the score tells your doctor whether they can safely rule out a clot with a blood test alone or whether imaging is needed right away.
The D-Dimer Blood Test
If your risk score is low or moderate, the next step is usually a D-dimer blood test. D-dimer is a protein fragment released when a blood clot breaks down. A normal or low D-dimer level is strong evidence that you don’t have an active clot, and in many cases, it’s enough to end the workup entirely without imaging.
The catch is that D-dimer levels rise with age, inflammation, infection, pregnancy, recent surgery, and other conditions. This makes the test less useful for ruling out clots in older adults, because their baseline levels are naturally higher. To address this, doctors often use an age-adjusted threshold for patients over 50: your age multiplied by 10 (in micrograms per liter). A 70-year-old, for example, would use a cutoff of 700 instead of the standard 500. This adjustment helps avoid unnecessary imaging while still catching real clots.
An elevated D-dimer doesn’t confirm a clot. It simply means imaging is the next step.
Ultrasound for Leg Clots
Duplex ultrasound is the primary imaging test for DVT. It’s noninvasive, involves no radiation, and can be done at the bedside or in a vascular lab. The technician applies a probe to your leg and presses down on the veins at roughly 2-centimeter intervals, from the groin all the way to the ankle. A healthy vein collapses flat under this pressure. A vein containing a clot doesn’t compress fully.
Beyond compression, the ultrasound uses Doppler technology to visualize blood flow. The technician checks whether flow patterns in both legs are symmetrical, since a clot disrupts normal flow on the affected side. In an acute clot, the vein typically appears larger than normal and the clot itself looks soft and deformable. Sometimes a portion of the clot is seen floating loosely at its edge, which can influence treatment decisions.
The entire exam usually takes 20 to 45 minutes. If the initial scan is negative but suspicion remains high, your doctor may recommend a repeat ultrasound in about a week, since small clots in the calf can sometimes be missed on the first pass.
CT Scans for Lung Clots
When a pulmonary embolism is suspected, doctors turn to CT pulmonary angiography, or CTPA. This is a specialized CT scan timed so that contrast dye highlights the arteries in your lungs at just the right moment. The radiologist looks for “filling defects,” which are dark spots inside the bright, contrast-filled arteries where a clot is blocking flow. CTPA is highly sensitive and highly specific, making it the first-line imaging tool for PE in most patients.
The scan itself takes only a few minutes. You’ll lie on the CT table, receive an IV injection of contrast dye, and hold your breath briefly while the scanner captures images. Results are often available within the hour, which matters because PE can be life-threatening and speed is critical.
In some cases, an echocardiogram (an ultrasound of the heart) may be done alongside or before CTPA, particularly if you’re hemodynamically unstable. It can reveal signs of strain on the right side of the heart caused by a large clot. However, even when the echocardiogram suggests PE, a CTPA is still performed to confirm the diagnosis.
When CT Isn’t an Option
CTPA requires iodine-based contrast dye, which can harm the kidneys. For patients with acute kidney injury or chronic kidney disease (particularly those with very low kidney function), a ventilation-perfusion scan, or V/Q scan, is the preferred alternative. This nuclear medicine test uses a small amount of radioactive tracer to compare airflow and blood flow in the lungs. Areas where air reaches the lung tissue but blood does not suggest a clot is blocking the artery.
Pregnant patients are another group where the testing approach changes. Guidelines from radiology and obstetrics organizations recommend starting with a Doppler ultrasound of the legs. If that’s negative, a chest X-ray followed by a low-dose perfusion-only scan (using a reduced amount of radioactive tracer) is the next step. This approach minimizes radiation exposure. If the chest X-ray shows a significant abnormality, however, the perfusion scan becomes unreliable and CTPA may still be necessary.
MRI and Other Specialized Tests
MRI can detect both arterial and venous clots without radiation or iodine contrast. A technique called magnetic resonance direct thrombus imaging can distinguish fresh clots from older, chronic ones based on differences in how the clot appears on the scan. This is useful when doctors need to determine whether a clot found on ultrasound is new (and needs aggressive treatment) or old (and may already be stable).
MRI isn’t used as a first-line test for most clot evaluations because it’s slower, more expensive, and less widely available on an emergency basis. But it fills an important gap for patients who can’t receive CT contrast and aren’t good candidates for a V/Q scan.
How the Steps Fit Together
The diagnostic process is designed as a funnel. Your doctor starts broad, with a clinical score and a blood test, and narrows down only as needed. For many low-risk patients, a normal D-dimer ends the evaluation entirely. For moderate- and high-risk patients, imaging follows quickly. The specific imaging test depends on where the clot is suspected (legs versus lungs), whether you’re pregnant, and how well your kidneys are functioning.
Most of these tests are available in emergency departments and can be completed the same day. If you’re sent home after a negative D-dimer, your doctor may still ask you to return if symptoms worsen or new symptoms appear, such as sudden shortness of breath, chest pain, or rapid swelling in one leg.