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Severe asthma increases the risk of blood clots in lung

By Anna Sayburn

Having severe asthma increases the chances of getting a potentially fatal blood clot in the lung. The risk is nine times higher than for people without asthma. However, the overall risk remains less than 1 in 1,000 people per year.

What do we know already?

Blood clots sometimes form in blood vessels within deep veins of the legs. This is more likely when people are sitting for a long time - for example, during a long flight or if they are confined to bed after surgery. These blood clots can be dangerous if they block blood vessels carrying blood to your organs. Parts of the clot can break off and travel to your lungs, which can be dangerous. A blood clot on the lung is called a pulmonary embolism, and can be fatal.

We know that inflammation in the body plays a part in causing blood clots. People who have diseases that cause inflammation, such as rheumatoid arthritis and inflammatory bowel disease, are more likely to get blood clots. Asthma also causes inflammation but no-one has looked at whether people with asthma are more likely to get blood clots in their legs or their lungs.

In this new study, researchers compared the rates of blood clots in the legs or lungs of people who had mild to moderate or severe asthma, with people of the same age and sex who did not have asthma.

What does the new study say?

People with severe asthma had a much bigger risk of getting a blood clot in the lung (a pulmonary embolism) than people without asthma. The study showed a smaller increased risk for people with mild or moderate asthma but, because of the small numbers of people who had a pulmonary embolism, this could have been down to chance.

In total, 13 of the 283 people with severe asthma in the study had a pulmonary embolism. Taking into account the length of time they’d had asthma, that worked out as a risk of just under 1 blood clot in the lung in every 1,000 people with severe asthma each year.

Perhaps surprisingly, the numbers of people who’d had a blood clot in their legs (a deep vein thrombosis) was not greater for people with asthma than for people without. This suggests that asthma only affects the likelihood of blood clots in the lungs, not all types of blood clots.

How reliable is the research?

This is a good-quality study and the results were clear for people with severe asthma. However, pulmonary embolism is fairly rare, and the total numbers of people in the study who had one were small (19 out of 762 people). This means it is hard to be precise about the exact level of risk.

This study only looked at adults with asthma. We don’t know what the risk is for children with asthma.

What does this mean for me?

If you have severe asthma, it may be alarming to hear that you have an increased risk of a blood clot in the lung. However, the overall risk is still not high. There are other things that affect your chances of getting blood clots, such as your weight, how much exercise you do, and if you take certain types of medication.

If you are concerned about your overall risk of blood clots, you could discuss this with your GP.


Majoor CJ, Kamphuisen PW, Zwinderman et al. Risk of deep-vein thrombosis and pulmonary embolism in asthma. European Respiratory Journal. Published online 20 December 2012.

Dec 20, 2012