Deep vein thrombosis (DVT) most commonly shows up as swelling, pain, or tenderness in one leg, not both. The tricky part is that roughly half of DVT cases cause minimal or no obvious symptoms, which is why knowing your risk factors matters just as much as recognizing what it feels like. Here’s how to spot the signs and what to expect if you need testing.
The Four Main Symptoms
DVT typically affects one leg, and the contrast between your two legs is one of the most telling clues. The core symptoms are:
- Swelling in one leg. This is the most common sign. If your calf or thigh looks noticeably puffier on one side, that’s significant. A difference of more than 2 centimeters in calf circumference between your legs is considered a red flag. You can check this yourself with a flexible tape measure around the widest part of each calf.
- Pain or cramping. It often starts in the calf and can feel like a deep soreness or a charley horse that doesn’t go away. The pain typically gets worse when you stand or walk.
- Skin color changes. The skin over the affected area may turn red or purplish, depending on your skin tone.
- Warmth. The skin on the affected leg may feel noticeably warmer to the touch compared to the other leg.
None of these symptoms on their own confirms a DVT. But if you have two or more of them in the same leg, especially with risk factors (more on those below), you should get evaluated promptly.
What DVT Can Be Confused With
A painful, swollen leg has several possible causes, and even experienced clinicians can’t always tell them apart without imaging. The most common conditions that mimic DVT include cellulitis (a skin infection that causes redness, warmth, and swelling), a ruptured Baker’s cyst (a fluid-filled sac behind the knee that can burst and send fluid down the calf), and simple muscle strain or hematoma.
One way to start telling them apart: cellulitis usually causes spreading redness with a clear border on the skin surface and may come with fever. A Baker’s cyst often causes sudden pain behind the knee that radiates downward, and you may have a history of knee problems. DVT pain tends to be deeper, more like an ache through the muscle, and the swelling is more diffuse rather than localized to a red patch of skin. But none of these distinctions are reliable enough to skip getting tested if you’re concerned.
Risk Factors That Raise Your Odds
Your symptoms matter more in context. A sore, swollen calf after a long flight is more concerning than the same symptom after a tough leg workout. The major risk factors include:
- Recent surgery or immobilization. The risk of a blood clot peaks around three weeks after an inpatient surgery, with risk remaining substantially elevated for up to 12 weeks. During the third week post-surgery, the risk is roughly 110 times higher than baseline. Bed rest, a leg cast, or any period of reduced mobility raises your risk similarly.
- Hormonal birth control. Combined oral contraceptives increase the risk of venous blood clots about 3.5 times compared to non-use. The absolute risk is still low for most young, healthy women, but it’s worth knowing.
- Active cancer or cancer treatment. Cancer itself and chemotherapy both increase clotting risk.
- Long travel. Sitting for four or more hours, whether flying, driving, or at a desk, slows blood flow in the legs.
- Pregnancy and the postpartum period. Blood clots more easily during pregnancy as a natural protective mechanism, but this also raises DVT risk.
- Previous DVT. If you’ve had one before, your risk of another is significantly higher.
How DVT Is Diagnosed
If you go to a doctor or emergency room with suspected DVT, the evaluation typically follows two steps: a blood test and an ultrasound.
The first step is often a D-dimer blood test. D-dimer is a protein fragment that appears in your blood when a clot is breaking down. A level below 500 ng/mL in someone with a low or moderate likelihood of DVT effectively rules it out, and no further testing is needed. The catch is that D-dimer can be elevated for many other reasons: recent surgery, infection, pregnancy, cancer, even aging. So a positive result doesn’t confirm DVT. It just means you need imaging.
The gold standard imaging test is a compression ultrasound. A technician presses an ultrasound probe against your leg veins. Healthy veins collapse under gentle pressure; a vein with a clot inside won’t compress. For clots in the thigh and upper leg (proximal DVT), this test has about 97% sensitivity and 98% specificity, meaning it catches nearly all clots and almost never gives a false positive. It’s less reliable for clots in the smaller calf veins, which is why a repeat ultrasound a week later is sometimes recommended if the first one is negative but suspicion remains high.
When a Clot Becomes an Emergency
The most dangerous complication of DVT is pulmonary embolism (PE), which happens when a clot breaks free from the leg and travels to the lungs. This is a medical emergency. About 25% of PE cases present with sudden death as the first symptom, which is why recognizing the warning signs early matters.
The symptoms of PE include sudden shortness of breath that seems out of proportion to your activity level, sharp chest pain that worsens when you breathe in, a rapid heartbeat, coughing (sometimes with blood), and feeling lightheaded or faint. In a large PE, you may feel a sense of doom, turn bluish around the lips or fingertips, or collapse. A smaller PE might cause only mild, transient breathlessness that’s easy to dismiss.
If you have leg symptoms consistent with DVT and then develop any combination of chest pain, sudden breathlessness, or rapid heart rate, call emergency services immediately. If you already have heart failure or lung disease, worsening shortness of breath that you can’t explain may be the only sign.
What Happens if You Have DVT
If an ultrasound confirms a clot, treatment usually starts with blood-thinning medication. Most people begin treatment the same day. You’ll typically take blood thinners for at least three months, though the duration depends on what caused the clot and whether it’s your first one. During treatment, you can usually continue your normal activities. In fact, walking is encouraged because it helps blood flow.
The main thing to watch for during treatment is unusual bleeding, since blood thinners reduce your body’s ability to clot. Nosebleeds, bleeding gums, blood in urine or stool, and excessive bruising are all signs to report. Most people tolerate treatment well and the clot gradually dissolves on its own with the medication preventing it from growing.
After a DVT, some people develop long-term swelling, aching, or skin changes in the affected leg. This is called post-thrombotic syndrome, and it happens because the clot can damage the vein’s internal valves. Wearing compression stockings and staying physically active can help reduce this risk.