Deep vein thrombosis, or DVT, is a blood clot that forms in one of the body’s deep veins, usually in the leg. Up to 900,000 people in the United States develop a blood clot each year, and an estimated 60,000 to 100,000 die from complications. DVT is both common and potentially dangerous, but it’s also preventable and treatable when caught early.
How a Clot Forms
Blood clots aren’t random. They form when three conditions overlap: slow blood flow, damage to the vein wall, and blood that clots more easily than normal. These three triggers work together, and you don’t necessarily need all three for a clot to develop.
Slow blood flow is the most intuitive factor. When blood pools instead of moving steadily back toward the heart, it becomes oxygen-deprived. Research in animal models has shown that blood sitting in the small pockets near vein valves can become hypoxic within hours, and clot formation on the valve itself can begin in as little as two hours of stagnant flow. This is why sitting for long stretches, whether on a plane or in a hospital bed, raises your risk.
Damage to the vein’s inner lining is the second trigger. This can happen during surgery, from a catheter, or from an injury. When the lining is disrupted, the body releases clotting signals as if it’s trying to seal a wound, even when no external bleeding exists. Cancer cells growing near a vein wall can cause the same response.
The third factor is blood that’s more prone to clotting than usual. Certain conditions push the balance toward clotting: cancer, chronic inflammation, dehydration, pregnancy, and inherited clotting disorders. Some people carry genetic mutations that make specific clotting proteins overactive, keeping them at elevated risk even without surgery or immobility.
Common Symptoms
DVT most often develops in the lower leg, though it can also form in the thigh or pelvis. The classic signs include swelling in one leg (not both), pain or cramping that typically starts in the calf, skin that turns red or purple in the affected area, and warmth you can feel when you touch the skin. Doctors look for a calf that measures more than 3 centimeters larger than the other leg as one of their diagnostic clues.
Here’s the tricky part: DVT can also develop without any noticeable symptoms at all. Some people discover they have a clot only after it causes a more serious complication. That’s why understanding your risk factors matters as much as knowing the symptoms.
Who Is Most at Risk
Several situations significantly raise your chances of developing DVT:
- Recent surgery or immobility. Being bedridden for more than three days or having major surgery within the past four weeks are among the strongest risk factors.
- Cancer. Active cancer increases clotting risk both from the disease itself and from certain treatments.
- Pregnancy and the postpartum period. Hormonal changes and pressure on pelvic veins make clots more likely.
- Paralysis or a leg cast. Any condition that prevents normal leg movement reduces blood flow and promotes pooling.
- Long travel. Flights or car rides lasting several hours keep your legs in the same position, slowing circulation.
- Inherited clotting disorders. Some people are born with blood that clots more aggressively. These conditions often go undiagnosed until a clot occurs.
Many people who develop DVT have more than one risk factor at the same time. A long flight alone might not cause a clot, but a long flight combined with recent surgery, dehydration, or an inherited clotting condition changes the equation considerably.
How DVT Is Diagnosed
Doctors use a scoring system called the Wells score to estimate how likely it is that your symptoms are caused by a clot. It assigns points for factors like active cancer, recent surgery, leg swelling, tenderness along a deep vein, and whether the entire leg is swollen. If another diagnosis (like a muscle strain) seems equally likely, points are subtracted. A low score combined with a normal result on a blood test called a D-dimer can effectively rule out DVT without further imaging.
If the score suggests a clot is likely, or if the D-dimer comes back elevated, the next step is an ultrasound of the leg. This painless imaging test can visualize the clot directly and determine its location and size. It’s the standard diagnostic tool and takes only a few minutes.
The Danger: Pulmonary Embolism
The most serious complication of DVT is a pulmonary embolism, or PE. This happens when part of the clot breaks free from the vein wall, travels through the bloodstream, and lodges in the lungs. A PE can block blood flow to part of the lung, and in severe cases, it’s fatal. About 25% of people who have a PE experience sudden death as their first symptom, with no warning signs beforehand.
Warning signs of a PE include sudden, unexplained shortness of breath, sharp chest pain (especially when breathing in), coughing up blood, and fainting. These symptoms require emergency medical attention.
Post-Thrombotic Syndrome
Even after a clot is treated, the affected vein can sustain lasting damage. Between 20% and 50% of people develop post-thrombotic syndrome within two years of their DVT diagnosis. The vein valves that normally keep blood flowing upward become scarred and stop working properly, leading to chronic swelling, skin discoloration, thickened skin, new varicose veins, and in severe cases, open sores on the lower leg called venous ulcers. This is a long-term condition, not a temporary recovery phase, which is one reason preventing DVT in the first place is so important.
Treatment and What to Expect
The cornerstone of DVT treatment is blood-thinning medication. These drugs don’t dissolve the existing clot. Instead, they stop it from growing larger and prevent new clots from forming, giving your body time to gradually break down the clot on its own.
Newer oral blood thinners have largely replaced older options for most patients. They work by blocking specific steps in the clotting process and have been shown to be as effective as older medications with a similar or lower risk of bleeding complications. The older approach, which required frequent blood tests to adjust the dose, is still used in some situations but is no longer the default for most people.
Treatment duration varies. Some people take blood thinners for three to six months. Others, particularly those with recurring clots or ongoing risk factors like cancer, may need to stay on them indefinitely. Your doctor will weigh your clotting risk against your bleeding risk to determine the right timeline. While on these medications, you’ll bruise more easily and bleed longer from cuts, so you’ll need to be mindful of activities that carry injury risk.
Preventing DVT During Travel and Immobility
If you’re facing a long flight, car ride, or period of reduced mobility, simple leg exercises can help keep blood moving. The National Blood Clot Alliance recommends walking for 30 minutes before boarding a plane and performing seated exercises during the flight:
- Ankle circles. Lift your feet off the floor and draw circles with your toes for 15 seconds in each direction.
- Foot pumps. Keep your heels on the floor and lift the front of your feet as high as possible, hold briefly, then press the balls of your feet down and raise your heels. Continue for 30 seconds.
- Knee lifts. With your leg bent, pull one knee toward your chest, lower it, and repeat with the other leg. Do 20 to 30 repetitions per side.
- Forward flex. Keep both feet on the floor, slowly bend forward, and reach for your ankles. Hold for 15 seconds.
Beyond exercises, staying hydrated, avoiding alcohol during flights, and wearing compression stockings all reduce your risk. If you’ve had a DVT before or have known risk factors, talk to your doctor before long trips about whether a short course of blood-thinning medication makes sense.