Doctors check for blood clots using a combination of risk scoring, blood tests, and imaging, with the specific steps depending on where the clot is suspected and how likely it seems based on your symptoms. The process is designed to rule clots out quickly when the risk is low and confirm them fast when the risk is high.
The First Step: Scoring Your Risk
Before ordering any tests, your doctor will assess how likely it is that you actually have a clot. This isn’t a guess. They use a standardized scoring system called the Wells Score, which assigns points based on specific risk factors: visible leg swelling and pain (3 points), a heart rate over 100 (1.5 points), recent surgery or immobilization within the past four weeks (1.5 points), a history of previous clots (1.5 points), and active cancer (1 point). The total score places you into a low, moderate, or high probability category.
This score matters because it determines what happens next. If your score is low, a simple blood test may be enough to rule out a clot entirely. If your score is high, you skip straight to imaging, because no blood test result would change the need to look directly at your veins or lungs.
The D-Dimer Blood Test
For people with low or moderate risk scores, the first test is usually a D-dimer blood draw. D-dimer is a protein fragment your body produces when it breaks down a blood clot. If your D-dimer level is normal (below 500 micrograms per liter for people under 50), you almost certainly don’t have a dangerous clot. The test catches over 97% of true clots, making it extremely reliable for ruling them out.
The catch is that D-dimer levels rise for many reasons besides clots: infection, inflammation, recent surgery, pregnancy, and simply getting older. In people over 50, the standard cutoff produces a lot of false alarms. To address this, many hospitals now use an age-adjusted cutoff. Instead of the flat 500 threshold, the cutoff becomes your age multiplied by 10. So for a 70-year-old, a D-dimer below 700 would be considered negative. This adjustment reduces unnecessary imaging without missing real clots.
A high D-dimer doesn’t confirm a clot. It just means one can’t be ruled out by blood work alone, and imaging is the next step.
Ultrasound for Leg Clots
When doctors suspect a deep vein thrombosis (a clot in the leg), the standard imaging test is a compression ultrasound. It’s noninvasive, painless, and doesn’t involve radiation. A technician presses an ultrasound probe against your leg at several points along the vein. A healthy vein compresses flat under pressure, like squeezing an empty tube. A vein with a clot inside it won’t compress fully, and that resistance is the diagnostic sign.
The test also shows blood flow patterns in real time, which helps identify partial blockages. The whole process typically takes 15 to 30 minutes. If the ultrasound is positive, treatment usually starts immediately. If it’s negative but clinical suspicion remains high, your doctor may repeat it in a week or move to other imaging.
CT Scan for Lung Clots
If a clot in the leg breaks loose and travels to the lungs, it becomes a pulmonary embolism, which is a medical emergency. The go-to test for this is a CT pulmonary angiography, or CTPA. A contrast dye containing iodine is injected into a vein in your arm, and the CT scanner takes rapid images timed to capture the moment that dye fills your pulmonary arteries. The scan itself takes only seconds.
On the images, a clot shows up as a dark spot where the bright contrast dye should be flowing freely. Radiologists look for arteries that are completely blocked, partially blocked with dye flowing around a central obstruction, or clots pressed against the vessel wall. The scan can also distinguish between a fresh clot and an older, chronic one based on the shape and angle of the blockage relative to the artery wall.
When CT Isn’t an Option
Some people can’t safely receive the iodine-based contrast dye used in CT scans. If your kidneys aren’t functioning well (specifically, a filtration rate below 30), the contrast poses a serious risk of further kidney damage. People with severe contrast allergies also need an alternative, as do younger women of reproductive age, for whom minimizing radiation exposure to the chest is a priority.
In these cases, doctors turn to a ventilation-perfusion scan, commonly called a V/Q scan. This two-part test has you inhale a small amount of radioactive tracer while a camera captures how air moves through your lungs. Then a second tracer is injected into your bloodstream to map blood flow. If an area of the lung is receiving air but not blood, that mismatch suggests a clot is blocking the artery feeding that region. For younger women with a low clinical suspicion of a clot, a V/Q scan can rule out pulmonary embolism just as reliably as a CT scan, with less radiation to breast tissue.
Less Common Tests
Two other imaging options exist but are used far less frequently. Venography involves injecting contrast dye directly into a vein in the foot, then taking X-rays as the dye travels up through the leg veins. It provides detailed images but is invasive and rarely necessary now that ultrasound technology has improved. MRI can be useful for detecting clots in unusual locations, particularly in the abdominal veins, where ultrasound has limited visibility. It’s not a routine first-line test for most suspected clots.
What the Process Looks Like in Practice
If you arrive at an emergency room or urgent care with a swollen, painful leg, here’s the typical sequence: your doctor evaluates your symptoms and risk factors using the Wells Score. If your risk is low, you get a D-dimer blood draw. A normal result means you go home. An elevated result sends you to ultrasound. If the ultrasound confirms a clot, treatment starts that day, usually with blood-thinning medication.
If you arrive with chest pain, shortness of breath, or a racing heart, and a pulmonary embolism is suspected, the pathway is similar but more urgent. A high Wells Score may skip blood work entirely and go straight to a CT scan. A low or moderate score triggers a D-dimer first, with a CT scan to follow if the blood test is elevated. The entire diagnostic process, from the first blood draw to imaging results, can happen within a few hours in most hospital settings.