A chronic blood clot is a clot that has remained in a blood vessel long enough to harden and scar into the vein wall, typically defined as one that has been present for more than 28 days. Unlike a fresh clot, which is soft and gel-like, a chronic clot undergoes structural changes that make it increasingly difficult for the body to dissolve on its own. While many people recover fully from an acute clot, a chronic one can cause lasting damage to the vein and surrounding tissue.
How a Clot Becomes Chronic
Blood clots are classified by age. A clot less than 14 days old is considered acute. Between 14 and 28 days, it falls into a subacute window. After 28 days, it’s chronic. These aren’t arbitrary cutoffs; they reflect real biological changes happening inside the clot and the vein wall.
In the first days after a clot forms, the body mounts an inflammatory response and begins trying to break down the blockage. But if the clot isn’t fully dissolved during that window, platelets in the blood start driving a process called fibrosis. Essentially, the clot is gradually replaced by tough, collagen-rich scar tissue. Smooth muscle cells invade the clot, and the inner lining of the vein wall thickens. What was once a soft mass becomes a rigid, fibrous obstruction partially or fully incorporated into the vessel. Research published in the Journal of Thrombosis and Haemostasis found that platelets play a central role in this transformation, actively promoting both clot scarring and permanent vein wall remodeling.
This remodeling is what makes chronic clots fundamentally different from fresh ones. A fresh clot can often be dissolved with blood thinners or clot-busting drugs. A chronic clot resists those approaches because it’s no longer made of the same material. It’s more like scar tissue than a blood clot at that point.
How Doctors Tell a Chronic Clot From a New One
Ultrasound is the primary tool for evaluating blood clots in the legs, and two features help distinguish chronic from acute clots. First, chronic clots appear brighter (more echogenic) on ultrasound because the dense, fibrous tissue reflects sound waves differently than fresh, gel-like clot material. Second, veins with chronic clots are less elastic. A healthy vein or one with a fresh clot will compress when the ultrasound probe presses on it. A vein scarred by a chronic clot stays rigid and resists compression.
These differences matter because treatment decisions depend heavily on clot age. Finding a new clot on top of a chronic one (called “acute on chronic” thrombosis) is a particularly important distinction, since it may mean current treatment isn’t working or a new event has occurred.
Post-Thrombotic Syndrome
The most common long-term consequence of a chronic clot in the leg veins is post-thrombotic syndrome, or PTS. This develops because the chronic clot damages the valves inside the vein that normally keep blood flowing upward toward the heart. When those valves fail, blood pools in the lower leg, causing a constellation of symptoms that can range from mild annoyance to serious disability.
About two-thirds of people with PTS experience swelling in the affected leg, often accompanied by a feeling of heaviness or aching that worsens after prolonged standing or walking. Roughly one-third develop visible skin changes: brownish discoloration around the ankle, small clusters of tiny surface veins, thickened or leathery skin, and in severe cases, open sores (venous ulcers) that are slow to heal.
Doctors assess PTS severity using the Villalta scale, which scores both what the patient reports (pain, cramping, heaviness, itching, tingling) and what the doctor observes (swelling, skin changes, new varicose veins, redness). A score above 5 confirms the diagnosis. Scores between 5 and 10 indicate mild disease, 10 to 14 moderate, and 15 or higher severe. The presence of a venous ulcer automatically classifies it as severe regardless of the total score.
Chronic Clots in the Lungs
Chronic clots don’t only affect leg veins. When a pulmonary embolism (a clot that travels to the lungs) fails to fully resolve, the remaining scar tissue can gradually narrow the pulmonary arteries and force the right side of the heart to work harder. This condition, called chronic thromboembolic pulmonary hypertension, develops in roughly 3 to 4 percent of people who survive a pulmonary embolism, based on a study in the New England Journal of Medicine that tracked patients over two years.
Symptoms tend to develop slowly, often months after the original event. Progressive shortness of breath during exercise is the hallmark, sometimes accompanied by chest pressure, fatigue, or fainting spells. Because the onset is gradual, it’s frequently misattributed to deconditioning or anxiety before the correct diagnosis is made. Specialized imaging can reveal characteristic signs of old clot material in the pulmonary arteries, including webs, bands, and areas of narrowing that distinguish it from other forms of pulmonary hypertension.
Long-Term Blood Thinners
Blood thinners won’t dissolve a chronic clot that has already scarred into the vein wall. Their purpose at the chronic stage is prevention: stopping new clots from forming on top of existing damage. The American Society of Hematology recommends indefinite blood thinner therapy for people whose clot was caused by an ongoing risk factor (such as cancer, autoimmune disease, or a clotting disorder) or for those whose clot occurred without any identifiable trigger.
If blood thinners are continued long-term, full-dose anticoagulation is preferred over low-dose aspirin. For people who started on a standard dose, some newer oral blood thinners can be stepped down to a lower maintenance dose after the initial treatment period, which helps reduce bleeding risk while still providing meaningful protection against recurrence. The decision to continue or stop is individualized based on what caused the clot, whether it was a first event, and the person’s risk of bleeding complications.
Stenting and Procedural Options
For people with chronic venous obstruction that causes persistent symptoms despite blood thinners and compression therapy, venous stenting has become an increasingly used option. The procedure involves threading a small mesh tube into the scarred vein to hold it open and restore blood flow. A meta-analysis of medium- to long-term outcomes found that the procedure succeeds technically in over 99 percent of cases, with 83 percent of stents remaining open at one year.
Results vary depending on the underlying cause. In patients whose obstruction came from external compression of the vein (a structural problem rather than clot damage), one-year patency rates reached 99 percent. For patients with post-thrombotic scarring, the one-year rate was 79 percent, dropping to around 59 percent at three years without additional interventions. With follow-up procedures to maintain flow, three-year rates improved to 86 percent. Stenting doesn’t eliminate the need for blood thinners, but it can significantly improve quality of life for people with severe chronic obstruction.
Managing Symptoms Day to Day
Graduated compression stockings are a cornerstone of symptom management for chronic clots in the leg. These stockings apply the most pressure at the ankle and gradually decrease toward the thigh, helping counteract the blood pooling caused by damaged vein valves. Medical-grade stockings typically provide around 18 mmHg of pressure at the ankle, tapering to about 8 mmHg at the upper thigh. Your doctor may recommend a higher compression level depending on symptom severity.
Beyond compression, keeping the legs elevated when resting, staying physically active, and avoiding long periods of standing or sitting all help reduce swelling and discomfort. Walking is particularly beneficial because calf muscle contractions act as a secondary pump that pushes blood upward through the veins. For people with skin changes from chronic venous insufficiency, keeping the skin moisturized and protecting it from injury helps prevent the breakdown that leads to ulcers.