What causes deep vein thrombosis in the leg?

8 Causes of Deep Vein Thrombosis

When you get a cut or scrape, your blood clots to stop the bleeding, and that’s a good thing. But sometimes blood clots form in large veins deep in your body, typically in the legs or thighs. Rather than being helpful, these clots can be a serious health problem.

This kind of blood clot is known as deep vein thrombosis (DVT). A blood clot in a vein deep in the body is made up of platelets, which speed the clotting process, and fibrin, a protein that weaves a web to trap blood cells. The real danger develops if a piece of the clot breaks off, travels through the bloodstream, and lodges in the lung, says Nieca Goldberg, MD, a cardiologist and the medical director of the Joan H. Tisch Center for Women’s Health at the NYU Langone Medical Center in New York City. This is called a pulmonary embolism (PE), which can be life-threatening, she says.

According to the Centers for Disease Control and Prevention, every year, between 300,000 and 600,000 Americans have DVT or PE.

What Causes DVT?

Anyone can develop DVT, but some people are at greater risk than others. These situations can raise your risk:

  • Injuring a vein. Injury to a vein can occur from breaking a bone or severely damaging a muscle. Veins can also be injured during major surgery, especially if the procedure involves the pelvis, hips, or legs.
  • Being immobile. If you keep your legs still for an extended period of time — maybe because you’re wearing a cast, recovering from surgery, or even taking a long-distance flight or car trip, for example — your blood flow slows. This can prevent anti-clotting agents in the body from mixing properly in your blood.
  • Taking hormones. If you’re a woman and take certain types of birth control pills or hormone replacement therapy (HRT) after menopause, you may be at higher risk of DVT, says Shahrzad Shareghi, MD, a cardiologist at Los Robles Hospital in Thousand Oaks, Calif. That’s because the hormone estrogen, which is present in some forms of birth control and HRT, may cause the blood to clot more easily. “If you take birth control pills or HRT and you smoke, you are at an even higher risk,” she says.
  • Being pregnant. Pregnancy increases a woman’s risk of DVT, and the risk from pregnancy remains higher up to six weeks after delivery, Dr. Shareghi says. Similar to taking hormones, the risk is related to the increase in estrogen when a woman is pregnant, as well as a function of the body that helps prevent women from experiencing too much blood loss during childbirth.
  • Having cancer. Cancer raises the risk of DVT. “When cancer forms, it sets off clotting reactions that make a person more prone to forming clots,” says Larry Santora, MD, a cardiologist at St. Joseph Health in Orange, Calif. “The danger is mostly with cancers of the abdomen and pelvis, but any kind of cancer can increase the risk of DVT.”
  • Having other health conditions. Chronic health problems that raise the risk of DVT include heart disease, lung disease, and inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis. People who have clotting disorders are also at greater risk of DVT.
  • Having a history of DVT. Once you have DVT or a PE, your risk for another episode is higher, Dr. Goldberg says. Also, if anyone in your family has a history of DVT or PE, this increases your risk as well.
  • Being older or overweight. Your risk of DVT increases with age, particularly for those older than 75. It’s hard to say whether that’s because your veins and blood flow weaken or because you move less as you get older, Shareghi says. And obesity is yet another DVT risk factor, the Society of Interventional Radiology reports.

Know the Signs and Symptoms of DVT

It’s important to recognize the signs and symptoms of DVT because early treatment can save your life. If your leg swells, feels tender or painful, or is red and warm, seek medical attention immediately. An ultrasound of your leg will show if you have DVT. If so, treatment with blood thinners can dissolve the clot and prevent it from getting bigger. If DVT is severe, you may need surgery to remove the blood clot.

Left alone, DVT can lead to PE and potentially death. According to the Society of Interventional Radiology, of every 100 people who develop DVT, one will die of PE. Get medical care right away if you notice any signs or symptoms of DVT, and talk to your doctor if you’re concerned about your risk.

What is Venous Thromboembolism?

Complications of DVT

The most serious complication of DVT happens when a part of the clot breaks off and travels through the bloodstream to the lungs, causing a blockage called pulmonary embolism (PE). If the clot is small, and with appropriate treatment, people can recover from PE. However, there could be some damage to the lungs. If the clot is large, it can stop blood from reaching the lungs and is fatal.

In addition, nearly one-third of people who have a DVT will have long-term complications caused by the damage the clot does to the valves in the vein called post-thrombotic syndrome (PTS). People with PTS have symptoms such as swelling, pain, discoloration, and in severe cases, scaling or ulcers in the affected part of the body. In some cases, the symptoms can be so severe that a person becomes disabled.

For some people, DVT and PE can become a chronic illness; about 30% of people who have had a DVT or PE are at risk for another episode.

Risk Factors for DVT

Almost anyone can have a DVT. However, certain factors can increase the chance of having this condition. The chance increases even more for someone who has more than one of these factors at the same time.

Following is a list of factors that increase the risk of developing DVT:

  • Injury to a vein, often caused by:
    • Fractures,
    • Severe muscle injury, or
    • Major surgery (particularly involving the abdomen, pelvis, hip, or legs).
  • Slow blood flow, often caused by:
    • Confinement to bed
      (e.g., due to a medical condition or after surgery);
    • Limited movement (e.g., a cast on a leg to help heal an injured bone);
    • Sitting for a long time, especially with crossed legs; or
    • Paralysis.
  • Increased estrogen, often caused by:
      • Birth control pills
      • Hormone replacement therapy, sometimes used after menopause
      • Pregnancy, for up to 3 months after giving birth
  • Certain chronic medical illnesses, such as:
    • Heart disease
    • Lung disease
    • Cancer and its treatment
    • Inflammatory bowel disease (Crohn’s disease or ulcerative colitis)
  • Other factors that increase the risk of DVT include:
    • Previous DVT or PE
    • Family history of DVT or PE
    • Age (risk increases as age increases)
    • Obesity
    • A catheter located in a central vein
    • Inherited clotting disorders

Preventing DVT

The following tips can help prevent DVT:

  • Move around as soon as possible after having been confined to bed, such as after surgery, illness, or injury.
  • If you’re at risk for DVT, talk to your doctor about:
    • Graduated compression stockings (sometimes called “medical compression stockings”)
    • Medication (anticoagulants) to prevent DVT.
  • When sitting for long periods of time, such as when traveling for more than four hours:
    • Get up and walk around every 2 to 3 hours.
    • Exercise your legs while you’re sitting by:
      • Raising and lowering your heels while keeping your toes on the floor
      • Raising and lowering your toes while keeping your heels on the floor
      • Tightening and releasing your leg muscles
    • Wear loose-fitting clothes.
  • You can reduce your risk by maintaining a healthy weight, avoiding a sedentary lifestyle, and following your doctor’s recommendations based on your individual risk factors.


Know the Signs. Know your Risk. Seek Care.

Everybody should know the signs and symptoms of DVT/PE, their risk for DVT/PE, to talk to their health care provider about their risk, and to seek care immediately if they have any sign or symptom of DVT/PE.


Downloadable slideshow on DVT

This narrated slideshow describes the risk factors, signs, and symptoms of DVT/PE.

About half of people with DVT have no symptoms at all. The following are the most common symptoms of DVT that occur in the affected part of the body:

  • Swelling
  • Pain
  • Tenderness
  • Redness of the skin

If you have any of these symptoms, you should see your doctor as soon as possible.


You can have a PE without any symptoms of a DVT.

Signs and symptoms of PE can include:

  • Difficulty breathing
  • Faster than normal or irregular heart beat
  • Chest pain or discomfort, which usually worsens with a deep breath or coughing
  • Coughing up blood
  • Very low blood pressure, lightheadedness, or fainting

If you have any of these symptoms, you should seek medical help immediately.

Diagnosis of DVT and PE

The diagnosis of DVT or PE requires special tests that can only be performed by a doctor. That is why it is important for you to seek medical care if you experience any of the symptoms of DVT or PE.

Learn more about diagnosis “

Treatments for DVT and PE

Medication is used to prevent and treat DVT. Compression stockings (also called graduated compression stockings) are sometimes recommended to prevent DVT and relieve pain and swelling. These might need to be worn for 2 years or more after having DVT. In severe cases, the clot might need to be removed surgically.

Immediate medical attention is necessary to treat PE. In cases of severe, life-threatening PE, there are medicines called thrombolytics that can dissolve the clot. Other medicines, called anticoagulants, may be prescribed to prevent more clots from forming. Some people may need to be on medication long-term to prevent future blood clots.

Learn more about treatments “

Did you know?

DVT does not cause heart attack or stroke. There are two main types of blood clots.
How a clot affects the body depends on the type and location of the clot:

  • A blood clot in a deep vein of the leg, pelvis, and sometimes arm, is called deep vein thrombosis (DVT). This type of blood clot does not cause heart attack or stroke.
  • A blood clot in an artery, usually in the heart or brain, is called arterial thrombosis. This type of blood clot can cause heart attack or stroke.

Both types of clots can cause serious health problems, but the causes and steps you can take to protect yourself are different. To learn more about arterial thrombosis, visit CDC’s information about heart disease and stroke prevention.

What is DVT? Deep Vein Thrombosis

Many things can increase your chances of getting DVT. Here are some of the most common:

  • Age. DVT can happen at any age, but your risk is greater after age 40.
  • Sitting for long periods. When you sit or lie down for long stretches of time, the muscles in your lower legs stay lax. This makes it hard for blood to circulate, or move around, like it should. Bed rest and long flights or car rides can put you at risk.
  • Pregnancy. Carrying a baby puts more pressure on the veins in your legs and pelvis. What’s more, a clot can happen up to 6 weeks after you give birth.
  • Trauma: An injury to the leg or other part of the body increases inflammation and also limits peoples mobility. Inflammation and mobility combine to increase the risk for developing DVT.
  • Change in Blood Chemistry: Hormones play a huge part in the risk assessment for DVT. Birth control, estrogen and testosterone injections, supplements or pills increase the risk for DVT.
  • Obesity. People with a body mass index (BMI) over 30 have a higher chance of DVT. This measures how much body fat you have compared to your height and weight.
  • Cancer: It is estimated that nearly half of diagnosed DVT that are asymptomatic are due to an underlying malignancy.

Why Me? A Look at the Risk Factors Behind a Blood Clot

Being diagnosed with a blood clot, whether it appears as deep vein thrombosis (DVT) or pulmonary embolism (PE), is a life changing experience. Patients with DVT or PE must deal with many changes. They may need to adjust what they eat, what medications they’re taking, what hobbies they enjoy, and more. It can cause a great amount of anxiety and uncertainty.

If you’ve been diagnosed with PE or DVT, you might be left with many unanswered questions, feeling very confused about why this happened to you. In many patients, the ultimate cause of thrombosis is still largely unknown. However, there are factors that can make you more likely to develop a blood clot.

Provoked vs. Unprovoked

You may have heard the terms “provoked” and “unprovoked” when discussing your blood clot with your doctors. These terms are related to why your blood clot happened. Determining if your blood clot was provoked or unprovoked can help your doctors decide what treatment is right for you. If it was a provoked blood clot, you had a major clinical risk factor for thrombosis. These risk factors can include surgery, trauma, pregnancy, hormonal therapy, and immobility.

If your blood clot was unprovoked, you have no major clinical risk factors, but could instead have underlying risks. These could include a family history of thrombosis, active cancer, and thrombophilia.

Risk Factors

In order to understand what puts you at risk for a blood clot, it’s important to understand why blood clots occur.

“In deep vein thrombosis (DVT), blood clots develop in the leg or pelvis veins in the absence of obvious damage to the vessel. Within the vessel, a combination of microscopic injury to the vein wall along with abnormal blood flow and a propensity for blood clots all contribute to DVT formation,” explained Dr. Aaron Aday, a Cardiovascular Medicine Fellow at Brigham and Women’s Hospital, in Ask the Expert – Why do Blood Clots Form Where They do.

“Inflammation and underlying genetic factors likely predispose people to DVT, and other risk factors such as cancer or immobility also increase risk for DVT. If a portion of the DVT dislodges, it can travel through the veins and relocate in the pulmonary arteries, thereby becoming a pulmonary embolism (PE).

Here are different risk factors that can predispose patients to blood clots:

  • Genetics – Genetics can make some patients more prone to blood clots. This happens when a genetic mutation has occurred and is then passed down a family line. Two of these mutations include Factor V Leiden and prothrombin gene mutation. Factor V Leiden causes thrombophilia, and the prothrombin mutation causes the body to over produce thrombin.
  • Cancer – Thrombosis is the second most common cause of death in cancer patients. Cancer itself is a risk factor, but so are the chemotherapy medications and the surgeries involved in cancer treatment.
  • Inflammation – Different diseases, such as metabolic syndrome, diabetes, and rheumatoid arthritis, cause inflammation in the body, which can lead to thrombosis. The connection between thrombosis and inflammation is complex and currently being researched.
  • Hormone Therapies – Oral contraceptives, menopausal treatments, and other forms of hormone therapy can put a patient at risk for thrombosis. The estrogen in the medications can cause increased activated protein C resistance, increased thrombin activation, and more. That said, only 1 in 1,000 women per year who are on the birth control pill will develop a clot. Always share any concerns you have about medications with your doctor.
  • Surgery – Your risk for developing a blood clot is higher after you’ve had surgery. Hip and knee replacement surgery patients are especially susceptible, because those surgeries impact the bone marrow cavity. Bone marrow fat can enter the body and cause blood clots to form. Because of this, you should discuss your risk of developing thrombosis with your surgeon before any procedure. Some patients are placed on blood thinners during their surgery, while others wear pneumatic compression boots post-operation.
  • Pregnancy – Thrombosis affects pregnant women up to 10 times more than non-pregnant women of the same age. Specialized treatments for pregnant women at risk of developing thrombosis are available. If you have a history of blood clots and are pregnant (or plan on becoming pregnant) it’s important to talk with doctor about your treatment plan.
  • Immobilization – Patients who are sedentary, or unable to move often, are at risk for thrombosis. This includes people who live sedentary lives, who sit for prolonged periods of time or aren’t very active. Even very active people can be at risk if they sit for a prolonged period, such as for a long plane ride. It also includes people who may be unable to move, such as patients in the hospital on prolonged bed rest.
  • Lifestyle – Certain unhealthy lifestyles can predispose people to blood clots. Smoking is one unhealthy lifestyle choice that increases risk, as is obesity. Eating healthy, exercising regularly, and making health-conscious choices can help decrease your risk of thrombosis.

People develop blood clots for many different reasons. While the eight risk factors listed above can provide some guidance, there are many other risk factors that people face. Some of these risks are known, while others are yet to be discovered. By determining what your risk factors are, you can uncover why you may have developed a blood clot and how to prevent one in the future.

Always talk to your doctor about any concerns you may have. Only they can help guide you through your personal risk factors and future care.

Athletes and Blood Clots

Healthcare providers may delay or miss blood clot diagnoses, including deep vein thrombosis (DVT) and pulmonary embolism (PE), among athletes who exhibit classic symptoms.


Because healthcare providers often do not consider blood clots something that affects athletes. Blood clots are uncommon in young, healthy individuals – and most athletes are young and healthy. So, for that reason, DVT, PE, and arterial clots in athletes are not the norm.

To understand how this problems affects athletes and the health professionals who treat athletes, you must first understand blood clot terminology. Your body is composed of an endless tunnel of arteries and veins through which blood flows throughout the body. Arteries are the blood vessels that carry the blood from the heart into the outside of the body: the brain, the internal organs, the legs, and the arms. Clots in an arteries lead to stroke, heart attack, or limb-threatening peripheral arterial clots, causing a painful, cold, and pale arm or leg.

Many people think of blood clots as a problem that occurs in elderly people and not in young and apparently healthy individuals. Symptoms may, therefore, be misinterpreted as something less serious. Especially in athletes, healthcare providers often interpret the leg symptoms from DVTs as “muscle tear,” a “Charlie horse,” a “twisted ankle,” or shin splints. Chest symptoms from PE are often attributed to a pulled muscle, costochondritis (inflammation of the joint between ribs and breast bone), bronchitis, asthma, or a “touch of pneumonia.”

Veins carry blood back to the heart from the rest of your body. Clots in the deep veins of the legs, arms, pelvis, abdomen, or around the brain are called deep vein thrombosis (DVT). If a piece of the clot breaks off from a leg or arm and travels to the lung, it can cause a clot in the lung. A clot in the lung is called a Pulmonary Embolism or PE. A PE can be a life threatening medical emergency. You need to seek immediate medical attention if you have symptoms of a PE.

Athlete-Specific Risk Factors for Clots

Being apparently healthy and being an athlete does not prevent a person from developing blood clots. Several circumstances put the athlete, as well as the non-athlete, at increased risk for DVT and PE (table 2). Athletes, coaches and trainers should be particularly aware of these risk factors.

  • Traveling long distances to and from a sports competition (by plane, bus, or car);
  • Dehydration (during and after a strenuous sporting event);
  • Significant trauma;
  • Immobilization (brace or cast);
  • Bone fracture or major surgery;
  • Birth control pills and patch, pregnancy, hormone replacement therapy;
  • Family history of DVT or PE;
  • Presence of an inherited or acquired clotting disorder (Factor V Leiden, prothrombin
    20210 mutation, antiphospholipid antibodies, and others);
  • Presence of a congenital abnormality of the anatomy of the veins;
  • May-Thurner Syndrome (narrowing of the major left pelvic vein);
  • Narrowing or absence of the inferior vena cava (the main vein in the abdomen);
  • Cervical rib causing thoracic outlet obstruction

Time Out: Recent Studies Show Important Trends

Recently, two studies have evaluated the risk for blood clots associated with Marathon or endurance athletes. These studies, conducted by Claire Hull, PhD, and her colleagues in the United Kingdom, appear below.

Hull and Harris (2013)_Circulation (2)

Hull et al. (2014)_Scand J SMS

Also, several high profile athletes have been struck by blood clots recently, heightening attention of the potential risk for blood clots among athletes. Read more here.

Go on Offense: Know the Risk Factors for DVT and PE in Athletes

The most common clots occurring in athletes are DVTs of the leg and PE. In addition to the potential risk factors for athletes and non-athletes outlined above, there exist a few unique risk factors that predispose younger people and athletes to DVT and PE include Thoracic Outlet Syndrome or Effort Thrombosis, May-Thurner Syndrome, and Congenital Absence or Malformation of the Vena Cava:

  • Thoracic Outlet Obstruction or Effort Thrombosis. In some individuals an extra (cervical) rib or excess muscle or tendon tissue compresses the big vein in the upper chest (subclavian vein) that drains the blood from your arm. This compression typically gets worse when you lift your arm up. This obstruction, often combined with repeated trauma to your vein (from throwing activities, weight-lifting, or gymnastics maneuvers), may cause a DVT to form in this area, extending into your arm veins. This is called “effort thrombosis” or “thoracic outlet obstruction/syndrome.” If the DVT resolves, for example, after clot buster treatment, you may need a resection of the extra rib or the excess tissue to increase space in the thoracic outlet.
  • May-Thurner Syndrome. This is a common congenital anatomic or mechanical variation that predisposes you to DVTs in the left leg. DVTs form with May-Thurner Syndrome when the main left pelvic vein is compressed by the overlying main right pelvic artery. This increases the risk of clot formation at the site of this narrowing in the left pelvis (hip area) with extension of the clot going down into the left leg. If the DVT resolves, for example, after clot buster (thrombolytic) treatment, the narrowing can be opened up by a radiologist with a balloon angioplasty and then kept open by placing a stent (or tube) to keep the site open.
  • Congenital Absence or Malformation of the Vena Cava. Congenital abnormalities of the anatomy of the big vein in the abdomen (vena cava) or pelvic veins can be a cause of DVT in young people. The abnormal anatomy probably leads to disturbed blood flow and an increased risk of clotting.

Get Your Head in the Game: Recognize Blood Clot Symptoms

It’s important to remain vigilant about the signs and symptoms of blood clots. These include:

Deep Vein Thrombosis – DVT
  • Swelling, usually in your leg (can also occur in your arm especially in weight-lifters, gymnasts, rowers, etc.)
  • Leg (or arm) pain or tenderness, usually described as a cramp or Charley horse
  • Reddish or bluish skin discoloration
  • Leg warm to touch
Pulmonary Embolism – PE
  • Sudden shortness of breath
  • Chest pain-sharp, stabbing; may get worse with deep breath
  • Rapid heart rate
  • Fainting or passing out
  • Unexplained cough, sometimes with bloody mucus

Treatment Considerations

Treatment decisions for people who have blood clots must be individualized. This is particularly true for young, apparently healthy individuals, such as athletes. In the case of unexplained DVT, testing for an inherited or acquired clotting disorder may be appropriate. When first diagnosed with the DVT, clot busting medication (fibrinolytic or thrombolytic therapy) should be considered to quickly dissolve the clot. However, clot busting treatment has not been systematically studied to determine whether it really decreases the risk for long-term damage to the veins of the leg and arm, i.e. the postthrombotic syndrome.

  • Stephan Moll, MD University of North Carolina, Chapel Hill, NC
  • Edward Libby, MD University of New Mexico, Albuquerque, NM
  • William Roberts, MD University of Minnesota Medical School, Minneapolis, MN

Defense Wins Games: How to Prevent Blood Clots

  • Take breaks and stretch legs when traveling long distances
  • Stay well hydrated (during and after a strenuous sporting event and travel)
  • Know the symptoms of DVT and PE and seek early medical attention if they occur
  • Realize that DVT and PE can occur in the athlete
  • Know the risk factors for blood clots
  • Know whether you have a family history of blood clots
  • In case of major surgery, trauma, prolonged immobility, or when in a cast: ask your doctor whether you should receive DVT prophylaxis and, if yes, for how long

The Clotting Process

Your body is designed with a natural balance between factors in your blood that cause your blood to clot and other factors that cause your blood to dissolve clots.

Blood clots can occur when:
  • You have an imbalance between the two systems that keep the clotting process in your blood in check; either (A) too much activity of the proteins and blood platelets that form clots (the procoagulant system), or (B) too little activity of the system that dissolves blood clots as they form (the fibrinolytic system);
  • You have trauma to a blood vessel wall, like you might have after a bone fracture or in thoracic outlet obstruction (see discussion below);
  • Your blood return from your arms and legs to your heart is impaired or not functioning properly, like when you sit with your legs bent in a cramped positions for a prolonged period of time;
  • Your blood is “thicker” than usual, which occurs when athletes are dehydrated, using the drug erythropoietin (EPO), or receiving excessive blood transfusions (blood doping).

Unfortunately, few studies exist that investigate the influence of physical training on blood clot formation and dissolution. So, we don’t know the exact net effect of training on this clotting balance mechanism. We do know, for example, that blood levels of the clotting protein Factor VIII increase with exercise and that the elevation persists during recovery. Theoretically, this could lead to an increased risk of blood clots in athletes. However, data also indicate that the fibrinolytic system that dissolves blood clots is overactive in people who exercise. With this over activity, athletes would be protected from having a blood clot. Yet, we do not know the net effect of these changes in athletes.

You can find a detailed scientific discussion of the coagulation issues relevant to exercise and training in a published review listed as reference 1. However, the conclusions are sparse and vague, because of a lack of data and conflicting results from different studies.

Psychosocial Implications

Athletes need to appreciate that significant deconditioning can occur after a DVT or PE. Depression can also set in after such a life-changing event. This is not surprising, given that athletes often view themselves as healthy and, from a health point of view, invincible, and now suddenly realize that they are vulnerable, sick, and sometimes even disabled. Patient support groups may be helpful in this situation, as may antidepressants.

Selected References for the above article: (More resources below)

El-Sayed MS et al: Exercise and training effects on blood haemostasis in health and disease: an update. Sports Med 2004;34(3):181-200.

Shrier I, Kahn SR: Effect of physical activity after recent deep venous thrombosis: a cohort study. Medicine and Science in Sports and Exercise 2005;37: 630-634.

Roberts WO, Christie DM: Return to training and competition after deep venous calf thrombosis. Medicine and Science in Sports and Exercise 1992;24:2-

Resources for Athletes

    1. Clot Buster – Our friend Roland Varga has an incredibly inspirational blog that follows athletes return to their sport after blood clots.
    2. Venous thrombosis in athletes. (2013) Article from the Journal of the American Academy of Orthopaedic Surgeons. (Abstract only) Authors: Grabowski G, Whiteside WK, Kanwisher M.
    3. Skater Tara Lipinski Speaks Out About DVT (2011) from NIH Medline Plus National Institutes of Health
    4. Venous Thromboembolism and Marathon Athletes (2013) A patient oriented article that discusses why athletes are at risk of blood clots, signs and symptoms of blood clots, and advice to returning athletes From the American Heart Associations’ Circulation Journal
  • Hypercoagulability in athletes (2004) Article from Current Sports Medicine Reports. Discusses the conditions athletes face that can result in exposure to several risk factors for blood clots, how to address the risks, prevent blood clots and return to sport. (Abstract only) Authors: Meyering C, Howard T.
  • Thromboembolic disorders: guidance for return-to-play (2011) Article from Current Sports Medicine Reports. Discusses VTE recovery and the structured step by step return-to-training program with progressive increase in intensity for the road back to play for athletes. (Abstract only) Author: Depenbrock PJ.
  • Deep Vein Thrombosis in Athletes: Risks of Racing and Resting (2010) Article from the American Medical Athletic Association Journal that discusses how blood clots form, the impact of exercise on the clotting process, misdiagnosis of blood clots in athletes, and blood clot risks common to athletes.
  • Blood Clots and the Athlete: A Review of Deep Vein Thrombosis in Sports (2007) Article discusses the formation of blood clots, prevention of blood clots, risk factors including thrombophilias or hypercoagulability disorders, and return-to-play issues. From American Medical Society for Sports Medicine’s Athletic Therapy Today.
  • Deep Vein Thrombosis in Athletes: Prevention and Treatment (2012) A chapter from a book “Sports Injuries” that discusses the risk factors, diagnosis, prevention, and treatment of DVT and PE in athletes. (Abstract only) Authors: Faik Altıntaş, Çağatay Uluçay.
  • How To Prevent Blood Clots After Destination Races (2013) Useful tips from Women’s Running to prevent blood clots after the race when you are at risk during travel. Read the National Blood Clot Alliance’s blood clot travel safety tips
  • Runners and Blood Clots: What You Need to Know (2013) Useful tips from Runner’s World on the risks of blood clots for runners, what signs and symptoms of clots to look for, and strategies for prevention of blood clots.
  • Athletes and Anticoagulation: Return to Play After DVT/PE (2016) Article from The American College of Cardiology discusses whether or not athletes need to be prevented from competing in contact sports while being anticoagulated. Authors: Josh Berkowitz, MD; Stephan Moll, MD

Read Stories of Athletes who have had Blood Clots

  • Eric O’Connor, Blood Clot Survivor, Marathon Runner, and NBCA Board Member, Treads 24
  • Tim Allen Tells His Blood Clot Survival Story
  • In Memory of Kyle Baca: His Blood Clot Story as Told by His Mother
  • Rebekah Bradford’s Blood Clot Story
  • Dan Capobianco Tells His Blood Clot Survival Story
  • Jim Fenton’s Blood Clot Survival Story
  • Hope’s Story of Blood Clots as a Warning Sign for Cancer
  • Michelle Winters’ Blood Clot Story

Many factors can lead to excessive blood clotting, leading to limited or blocked blood flow. Blood clots can travel to the arteries or veins in the brain, heart, kidneys, lungs and limbs, which in turn can cause heart attack, stroke, damage to the body’s organs or even death.

Acquired Risk Factors

  • Smoking
  • Overweight and obesity
  • Pregnancy
  • Prolonged bed rest due to surgery, hospitalization or illness
  • Long periods of sitting such as car or plane trips
  • Use of birth control pills or hormone replacement therapy
  • Cancer

Genetic Risk Factors

The genetic, or inherited, source of excessive blood clotting is less common and is usually due to genetic defects. These defects often occur in the proteins needed for blood clotting and can also occur with the substances that delay or dissolve blood clots.

You’re more likely to have a genetic cause of excessive blood clotting if you have:

  • Family members who have had dangerous blood clots
  • A personal history of repeated blood clots before the age of 40
  • A personal history of unexplained miscarriages

Other Risk Factors – Diseases and Conditions

Many diseases and conditions can cause excessive blood clotting, or hypercoagulation. Certain diseases and conditions are more likely to cause clots to form in specific areas of the body.

Conditions that can trigger excessive blood clotting in the heart and brain:

  • Atherosclerosis is a disease in which a waxy substance called plaque builds up inside your arteries. Over time, the plaque may rupture. Platelets clump together to form clots at the site of the damage.
  • Vasculitis is a disorder that causes the body’s blood vessels to become inflamed. Platelets may stick to areas where the blood vessels are damaged and form clots.
  • Diabetes increases the risk of plaque buildup in the arteries, which can cause dangerous blood clots. Nearly 80 percent of people who have diabetes will eventually die of clot-related causes.
  • Heart failure is a condition in which the heart is damaged or weakened. When the heart can’t pump enough blood to meet the body’s needs, blood flow slows which can cause clots to form.
  • Atrial fibrillation, the most common type of arrhythmia, or irregular heartbeat. Atrial fibrillation can cause blood to pool in the upper chambers of the heart and can cause clots to form.
  • Overweight and obesity refer to body weight that’s greater than what is considered healthy. These conditions can lead to atherosclerosis, which increases the risk of clots.
  • Metabolic syndrome is the name for a group of risk factors that increases your chance of having heart disease and other health problems, including an increased risk of forming blood clots.

Conditions That Can Trigger Excessive Blood Clotting in the Limbs

  • Deep vein thrombosis (DVT): Blood clots can form in the veins deep in the limbs, a condition called deep vein thrombosis or DVT. DVT usually affects the deep veins of the legs. A blood clot in a deep vein can break off and travel through the bloodstream. If the clot travels to the lungs and blocks blood flow, the condition is called pulmonary embolism.
  • Peripheral artery disease (PAD): A narrowing of the peripheral arteries, most commonly in the arteries of the pelvis and legs. PAD is similar to coronary artery disease (CAD) and carotid artery disease. All three of these conditions are caused by narrowed and blocked arteries in various critical regions of the body. Hardened arteries (or atherosclerosis) in the coronary artery region, restricts the blood supply to the heart muscle (View an illustration of coronary arteries). Carotid artery disease refers to atherosclerosis in the arteries that supply blood to the brain.
  • Atherosclerosis: A disease in which plaque builds up in the wall of an artery. PAD is usually caused by atherosclerosis in the peripheral arteries (or outer regions away from the heart). Plaque is made up of deposits of fats, cholesterol and other substances. Plaque formations can grow large enough to significantly reduce the blood’s flow through an artery. When a plaque formation becomes brittle or inflamed, it may rupture, triggering a blood clot to form. A clot may either further narrow the artery, or completely block it.


Medicines can disrupt the body’s normal blood clotting process. Medicines containing the female hormone estrogen are linked to an increased risk of blood clots. Examples of medicines that may contain estrogen include birth control pills and hormone therapy.

Other Factors

Many other factors can lead to excessive blood clotting:

  • Smoking raises the risk of unwanted blood clots and makes it more likely that platelets will stick together. Smoking also damages the lining of the blood vessels, which can cause clots to form.
  • Increased homocysteine levels, linked to a high risk of vascular disease. Increased levels of this substance may damage the inner lining of the arteries.
  • Pregnancy. Women are more likely to develop blood clots when they’re pregnant due to an increase in platelets and clotting factors. The uterus can also compresses the veins slowing blood flow, which can lead to blood clots.
  • Prolonged bed rest. Several days or weeks in bed from surgery or illness can increase risk of excessive blood clotting.
  • Use of birth control pills or hormone replacement therapy. These can slow blood flow and cause clotting.
  • Cancer. Some types of cancer increase the proteins that clot your blood.
  • HIV and HIV treatments. The risk of blood clots is highest in HIV patients who have infections, are taking certain medicines, have been hospitalized, or are older than 45.
  • Dehydration, a condition in which your body doesn’t have enough fluids. This condition causes blood vessels to narrow and blood to thicken, raising risk for blood clots.
  • Organ transplants and implanted devices, such as central venous catheters and dialysis shunts. Surgery or procedures done on blood vessels may injure the vessel walls. This can cause blood clots to form. Also, catheters and shunts have a man-made surface that may trigger blood clotting.

Learn more:

  • Quit smoking
  • Losing weight
  • Family history and heart disease, stroke


Isolated calf deep vein thrombosis (ICDVT), defined as thrombosis confined to the infra-popliteal veins of the lower limbs, is a frequent finding in symptomatic out- and in-patients when the ultrasound examination is extended to the whole deep leg veins. Studies based on a complete investigation of deep veins in the whole leg, reported a prevalence of ICDVT of 7–11% in cases with suspected PE, 4–15% in cases with suspected DVT, and 23–59% in patients with diagnosis of DVT (1). Notwithstanding these high figures, many and clinically relevant aspects of ICDVT are still controversial; in fact, that of ICDVT is currently one of the most debated issues in the field of venous thromboembolism (VTE). First of all, whether an extended ultrasound examination of calf deep veins is necessary in all suspected subjects is still matter of discussion and the American College of Chest Physician guidelines on VTE in the last edition (2) propose a rationale for not routinely examining the distal veins, based on the facts that: (I) other assessment (e.g., low clinical probability and/or negative D-dimer) may help guiding those in whom distal examination is not necessary; (II) a repeat ultrasound of the proximal veins can be done after a week to identify those patients with a risky proximal DVT; and finally, (III) false-positive findings for DVT may occur with a subsequent unnecessary and risky anticoagulant treatment to a number of subjects. Moreover, even in the case that the calf veins are imaged and ICDVTs are diagnosed, the above mentioned guidelines suggest two different management options as equally suitable in clinical practice: (I) to treat patients with anticoagulant therapy; or (II) to not treat patients with anticoagulant therapy unless extension of their DVT is detected on a follow-up ultrasound examination (e.g., after 1 or 2 weeks). However, important differences on this issue are present among currently available international guidelines on VTE; these differences reflect the broad variability in clinical practice between the strategies on how to manage patients with suspected leg DVT and even on how to treat ICDVT after diagnosis. The treatment for ICDVT is even not mentioned at all by the National Clinical Guideline Centre (last published in June 2012) since the guideline “…focused on proximal DVT rather than isolated calf vein DVT as the latter is less likely to cause post-thrombotic syndrome than proximal DVT and also less likely to embolize to the lungs.” (3). In contrast, the International Consensus Statement on prevention and treatment of VTE affirms that evidence “…indicates that oral anticoagulants should be given to all patients with symptomatic isolated calf DVT and that three months seems to be sufficient.” (4). It is really evident that the diagnostic and therapeutic approaches to suspected or diagnosed ICDVT vary greatly among guidelines as well as among even expert professionals and in clinical practice. This seems mainly attributable to the fact that the natural history of calf-limited DVTs, their potential risk and optimal treatment have, to date, not been sufficiently investigated. Thus, different options and clinical decisions are possible and equally justified. Evidence on the natural history of ICDVT is currently insufficient especially because in most studies ICDVTs, once diagnosed were treated with anticoagulants and, therefore, their natural history was modified by the treatment. Evidence on clinical evolution of diagnosed ICDVT left untreated is scarce. The proximal extension rate of untreated ICDVT was reported to range between 10% and 15% in recent reviews (5,6). The CALTHRO study showed that 90% of untreated ICDVTs, diagnosed in patients well monitored with serial CUS, did not reach the proximal veins and/or embolize; the proximal extension rate at 7 days after diagnosis was as low as about 3% (7), in line with results (1–5.7%) of studies based on serial proximal ultrasound evaluations (8). These data support the view that the need of anticoagulant treatment in all patients with ICDVT has not been proved for sure. At last, one randomized, placebo controlled, clinical trial on the need of anticoagulation in patients with ICDVT has recently been published (9). The CACTUS study randomized patients with a first ICDVT to receive therapeutic nadroparin dose (170 UI/kg) or placebo for 42 days. There was no significant difference between the groups in the composite primary outcome: 3% in the nadroparin group and 5% in the placebo group; whereas bleeding occurred in 4% of patients in the nadroparin group and in no patients in the placebo group (P=0.0255). These data support the conclusion that not all IDDVT should receive full-dose anticoagulation. A practical therapeutic approach has recently been proposed (10), based on giving therapeutic anticoagulation for 3 months, as for proximal DVT, in patients with an unprovoked event or with other high-risk factors for VTE. A shorter treatment (4–6 weeks) with LMWH, even at lower anticoagulant doses, can be enough in patients who have low-risk conditions (11). Unfortunately, no data are still available on the use of DOACs in this clinical condition. Of notice, recent studies, based on long follow-up after stopping anticoagulation in patients with a first IDDVT, showed an incidence of recurrent VTE that was similar to that of patients with proximal DVT (12,13). Whether the occurrence of these long-term complications of ICDVT can be influenced by the initial treatment (anticoagulation yes or not, its type, dose, duration) remains to be assessed.

An interesting clinical study on treatment of patients with diagnosed ICDVT has recently been published (14). In the study the authors have retrospectively examined the cases of patients who had an ICDVT diagnosis with duplex ultrasonography during 4 years activity at the Vascular Laboratory of the University of California. After the exclusion criteria, 384 patients were available for analysis (57.8% males; mean age 60±16 years), 222 of whom (57.5%) were inpatients. Therapeutic anticoagulation was prescribed to 243 patients (63.3%), the remaining non-treated patients were evaluated as controls. Significantly less patients received anticoagulation if admitted to a medical-surgical unit, had an operation or traumatic injury within prior 30 days, were in non-ambulatory status, or had received prophylactic anticoagulation during the 7 days before diagnosis of ICDVT. In contrast, the presence of acute medical illness, use of hormonal medications, presence of cancer and history of VTE were conditions associated with more prescription of anticoagulation. Proximal DVT or PE occurred in 13 control group patients (9.2%) and 8 anticoagulation group patients (3.3%). Intention to administer therapeutic anticoagulation was associated with a lower likelihood of proximal DVT or PE, with an RR of 0.36 (95% CI, 0.15–0.84). Clinically significant bleeding occurred more frequently in patients who received a prescription of therapeutic anticoagulation (8.6%) than in controls (2.2%; adjusted OR, 4.87; 95% CI, 1.37–17.3). On the basis of the high rate of clinically significant bleeding events associated with therapeutic anticoagulation the authors’ conclusions are rather conservative. The conclude that: “…therapeutic anticoagulation of patients with isolated calf DVTs may be warranted to reduce the risk for proximal venous thromboembolism. However, randomized studies are needed to draw firmer conclusions. Because the benefits of anticoagulation seem modest, we recommend attention to the risk for bleeding when determining whether anticoagulation is appropriate.” I agree with this cautious conclusion. I am convinced that not all ICDVT are associated with the same risk of complications and not all deserve anticoagulation. The problem is that currently it is not sufficiently ascertained which ones are those at high or low risk. Furthermore, if anticoagulation is the preferred option, its intensity and duration are still uncertain. For sure prospective, controlled studies are urgently needed to reduce the risk of insufficient or excessive treatment in the high number of patients who present with ICDVT.

Deep vein thrombosis – causes, symptoms, prevention

Blood tests may be done to check for irregularities in the blood clotting system or for inherited disorders

If a pulmonary embolism is suspected a range of additional tests may be used.


The immediate goal of treatment for a DVT is to limit the size and movement of the clot, and to prevent complications. Treatment will depend on the location and severity of the clot. Some small clots may resolve spontaneously without treatment but a DVT is generally treated intensively. Admission to hospital for treatment and observation for signs of complications may be required. Treatment may include:



These medications “thin” the blood, reducing its ability to clot. They prevent an existing clot from getting bigger and reduce the risk of developing more clots.

Anticoagulant medications such as heparin may be administered initially as a continuous infusion into a vein (intravenously) because it acts quickly to prevent further clotting. After initial treatment, anticogulant medication may be given in tablet form (e.g. warfarin) or as an injection under the skin.

Regular blood tests will usually be required to monitor the effectiveness of the medication and to adjust dosage. Anticoagulant treatment is usually maintained for at least three months to be fully effective in treating a DVT. In some cases, it may be required on a long-term basis.

Thrombolytic agents:

In some cases, these medications are given by intravenous injection to help dissolve the clot. However, they can cause side effects, such as severe bleeding, so are usually used only in life-threatening situations, eg: the presence of a large pulmonary embolus.

Compression stockings

Elasticised compression stockings give support to the lower legs and encourage the return of blood to the heart and helps to reduce swelling. It is generally recommended that compression stockings are worn in situations where immobility is likely.


In high-risk cases, where there have been recurrent or severe DVTs, or where anticoagulant medication is not appropriate or has not worked, surgery to insert a small filter or sieve into the main vein leading to the heart (the vena cava) may be recommended. This traps any blood clots travelling through the blood stream thus preventing the clot travelling to the heart and lungs.


General measures to help lower the risk of developing a DVT are to quit smoking, maintain a healthy body weight, and take regular exercise. When certain medical conditions or inherited disorders are present, long-term anticoagulant treatment to minimise the risk of DVT may be recommended.

Measures that can reduce the risk of DVT associated with long-distance travel include:

  • Compression stockings
  • Drinking plenty of non-alcoholic fluids to avoid dehydration
  • Leg and ankle exercises to encourage blood flow in the legs
  • People at high risk of DVT may be prescribed aspirin or anticoagulant tablets or injections whilst travelling.

Anyone at risk of DVT is advised to see their doctor prior to travelling to discuss preventative measures. Measures that can reduce the risk of DVT associated with being bedridden as a result of surgery or illness include:

  • Compression stockings
  • Anticoagulant medications
  • Specific leg and breathing exercises to promote blood flow.

Long-term complications

After a DVT some people may develop a long-term condition called “chronic venous insufficiency” or “post-phlebotic syndrome”. This is due to damage and scarring to the veins and is characterised by swelling, discomfort, and skin pigmentation in the affected area. It can increase the likelihood of subsequent DVT.

Repeated pulmonary emboli can lead to a condition called pulmonary hypertension, which is where the blood pressure within the lungs is increased. This can cause serious problems with the functioning of the heart. Certain medications, compression stockings, and, in rare cases, surgery, may be recommended to help treat these long-term complications.

Last reviewed – August 2019

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