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Friday, 14 September, 2001, 07:02 GMT 08:02 UK
Winter 'increases blood clot risk'
There are seasonal differences in DVT risk
Patients are more likely to suffer dangerous blood clots during winter, and less likely during summers, says research.

Deep vein thrombosis, in which clots form usually in the veins of the lower legs, often affects older people and people who have had major surgery.

Occasionally, small bits of the clot break off and travel to the the lungs, where they can lodge and cause potentially fatal blockages called emboli.

The latest research, published in the British Medical Journal, found that elderly people were 18% more likely to be admitted to hospital with a thrombosis in February than average, and 18% less likely in August.

Although this is a phenomenon which has been noticed since the 1950s, this is one of the largest studies which demonstrates it.

The study authors, from a hospital in Nice, France, suggest that respiratory tract infections more common in winter might make patients more vulnerable to blood clots.

Cold weather effects

They also suggested that chilly weather might make the blood vessels constrict, making it more likely that blood clots will form.

However, Professor Roger Greenhalgh, a consultant vascular surgeon from Hammersmith Hospital in London, who studied the link in the 1980s, told BBC News Online that another explanation was feasible.

He said: "We found that blood appears actually more likely to clot if diluted with saline.

"In summer, people tend to sweat more, and the reverse is true in winter.

"In the summer months blood may well be less dilute, and less likely to clot."

And another London surgeon, Professor George Geroulakos, said that there might be a simpler explanation.

He told BBC News Online: "People, particularly elderly people, who get infections during the winter months are far less likely to move around.

"We know that keeping still makes you more vulnerable to thromboses."

See also:

11 May 01 | C-D
Deep vein thrombosis
04 Sep 01 | Health
Heart op clot risk warning
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