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Hospitals are using the wrong kind of stocking to prevent blood clots in patients, a study suggests.

Researchers found that mid-calf stockings, which are similar to flight socks, do little to prevent deep vein thrombosis (DVT) in stroke patients.

The study from the University of Edinburgh compared the use of both thigh-length stockings, which cover the whole leg, and short stockings, in more than 3,000 stroke patients from 112 hospitals in nine countries.

However, the research found that the clot rate in stroke patients was higher among those fitted with the shorter stockings than for those with longer stockings.

Stroke patients fitted with below-the-knee, stockings were 30 per cent more likely to develop deep vein thrombosis than patients fitted with thigh-length stockings.

This could be because the most serious type of blood clots tend to be in the thigh, researchers suggest.

Guidelines introduced earlier this year by the National Institute for Clinical Excellence recommend that stockings are no longer used for UK stroke patients. This followed on from a previous University of Edinburgh study that questioned their effectiveness for stroke patients.

However, stockings are used by the NHS to prevent blood clots in million of patients who undergo surgery each year. In Scotland, about three-quarters of stockings used by the NHS are short. Their main use is to prevent DVT in patients undergoing surgery, but they are also fitted to patients with medical conditions who have become immobile and are at increased risk of blood clots.

Clinicians worldwide most commonly use short stockings, which are cheaper and easier to fit than thigh-length stockings, to prevent DVT - a life threatening form of blood clot that can travel up into the heart or lungs. The stockings tighten the blood vessels and increase blood flow.

Martin Dennis, Professor of Stroke Medicine at the University of Edinburgh, said: "Although we have shown in previous work that stockings are not very effective in reducing the risk of DVT after a stroke, we believe that the results of this trial may have important implications for the millions of patients undergoing surgery each year.

"Unless reliable evidence emerges that short stockings do actually reduce the risk of DVT, long stockings should always be used in preference."

Although trials have shown that stockings reduce the risk of DVT in patients undergoing surgery, these have only tested long stockings.

David Clark, Chief Executive of Chest Heart & Stroke Scotland, said, "More than 150,000 people a year have a stroke in the UK and it is vital they receive the best possible treatment. This important research, which was seed funded by Chest Heart & Stroke Scotland , shows conclusively that short compression stockings do not benefit stroke patients. We can now focus our efforts and research funding on finding a treatment which will reduce the risk of clots. "

The CLOTS - Clots in Legs or sTockings after Stroke - trial was funded by the Medical Research Council (UK), the Chest Heart & Stroke Scotland charity and the Scottish Government Chief Scientist Office. The research was published in the journal The Annals of Internal Medicine.