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A blood clot normally forms around damaged tissue, like a wound or a cut. ((iStock))

What is a blood clot?

A blood clot is a semi-solid or gelatinous mass of blood that may block the flow of blood. A blood clot, or thrombus, is how our bodies stop bleeding when cut, for example.

How does blood clot?

Blood contains many substances, including platelets and clotting factors, that will pile up around damaged tissue. Threads called fibrin form and stick to the sides of a wound, eventually closing it. When the wound heals, the clot dissolves into the bloodstream.

(eMedTV)

When do blood clots become a problem?

The simple answer is, when they form in the bloodstream not in response to a wound. Then it's called thrombosis.

Deep-vein thrombosis (DVT) usually occurs in the leg. DVT clots can block the flow of blood through the veins. It's a problem that occurs most often in the legs because it is harder to maintain blood flow in the lower parts of the body.

When a clot breaks away and travels through the body, it causes a pulmonary embolism (PE) if it blocks an artery.

Venous thrombosis (VTE) is simply the umbrella term for DVT and PE.

When a clot forms in the arteries supplying the heart, and blood flow is blocked, it may result in a heart attack. If a clot forms in the arteries supplying the brain, blocking blood flow, it may cause a stroke.

What are the risk factors?

Some people have a genetic predisposition to blood clots, just as other people are at risk of their blood not clotting properly when necessary (hemophiliacs, for example).

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Immobility, especially during flights of six hours or more, is a risk factor for venous thrombosis. ((iStock))

There is also a long list of things that increase the probability of VTEs:

  • Hospitalization: the risk of VTE is 10 times higher when hospitalized, a result of the combination of risk factors listed below. One in five VTEs occurs in hospital, according to the National Blood Clot Alliance (NBCA) in the U.S. In England, VTEs in hospital result in more than 25,000 deaths per year.
  • Age: risk increases as individuals  get older.
  • Obesity or being overweight: doubles the risk.
  • Infections and disease: these raise the levels of clotting factors in the blood.
  • Immobility: especially after air flights of six hours or longer.
  • Injury and/or trauma.
  • Surgery, intravenous catheters or any treatment that damages blood vessels. (Hip replacement patients have a 60 per cent chance of DVT.)
  • Pregnancy.
  • Varicose veins.

How would taking a drug lead to a VTE?

Some drugs may add to the risk of unwanted blood clots. "A cascade of factors leads to a lower threshold" for blood clots, according to Dr. Karl Kabasele, a public health physician and medical specialist for CBC News.

For example, oral contraceptives (and other hormone-based treatments) increase the risk of blood clots, some brands significantly more than others.

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Human arteries and veins are portrayed in a 3D rendered image. ((iStock))

What happens when you get a blood clot?

If it's DVT, the symptoms can include:

  • Pain, tenderness and swelling of the leg (usually the calf, but if the clot occurs in the thigh, the swelling may affect the whole leg).
  • Discoloration, with the leg appearing a pale, blue or reddish purple colour.
  • Leg feels warm when touched.

People with DVT can also be asymptomatic.

If it is PE, symptoms can include:

  • Shortness of breath, either severe and sudden or gradual onset.
  • Sharp, stabbing chest pain, which may be worse when breathing deeply.
  • Sudden collapse.
  • Rapid heart rate.
  • Unexplained cough, sometimes with bloody mucus.
  • DVT symptoms (see above).

PE is a medical emergency.

How is thrombosis treated?

The most common treatment is blood thinners (anticoagulants). These drugs block the action by the clotting factors rather than literally thinning blood. The most extensively used blood thinner, warfarin, was first used as a rat poison. It was introduced in 1948, but after a 1955 suicide attempt using warfarin, researchers began looking at other possible uses. U.S. President Dwight Eisenhower was one of the first people to be treated with warfarin.

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Age and poor fitness increase the risk of blood clots, but even young athletes are vulnerable. Tennis star Serena Williams, pictured at Wimbledon on July 3, 2010, received emergency treatment for a blood clot in her lungs in February. ((Alastair Grant/Associated Press))

In the U.S. today, warfarin is No. 20 on the list of most-prescribed drugs, with about 20 million prescriptions per year.

Newer blood thinners include low-molecular-weight heparin, made from animal sources and injected once or twice a day, and fondaparinux (arixtra), a synthetic drug that is also injected but lasts longer than heparin.

How can VTEs be prevented?

  • When travelling by plane, train or bus, get up and move around at least every hour, especially on trips over four hours. If you cannot move around, exercise and stretch your legs and feet.
  • Stop every two hours when driving to get out and move around.
  • Drink lots of water and wear loose-fitting clothing when travelling.
  • Lose weight if overweight, stay active, exercise and avoid long periods of immobility.
  • Discuss the risks of blood clots with your doctor if taking hormones, going into hospital or planning a long trip. For people at higher risk, doctors may recommend wearing elastic compression stockings or taking a single dose of a blood thinner.

Kabasele recommends against taking aspirin to prevent blood clots unless recommended by a doctor.

Hospitals especially need to take preventive measures with their patients.