Swedish researchers say they have devised a blood test that could better diagnose sports-related brain injuries and keep athletes from returning to the field unsafely.

In findings from a study of ice hockey players, the researchers said their method can show just an hour after a head injury how severe the concussion is, whether there is a risk of long-term symptoms and when the player can return to the sport.

“In ice hockey and other contact sports, repeated concussions are common, where the brain has not finished healing after the first blow,” said Henrik Zetterberg of the Sahlgrenska Academy at the University of Gothenburg, who led the study. “This kind of injury is particularly dangerous, but there have not been any methods for monitoring how a concussion in an athlete heals.”

A growing body of evidence, much of it from studies of former football players and boxers, suggests that repeated head knocks can cause chronic traumatic encephalopathy, a brain condition that can lead to loss of cognitive function, dementia, aggression and depression.

While mild concussions don’t generally cause loss of consciousness, they can induce other symptoms such as dizziness, nausea, trouble concentrating, memory problems and headaches. Severe concussions can cause a loss of consciousness. Most concussions get better in days or weeks, but some patients can suffer symptoms more than a year after injury.

The National Football League agreed in August to pay $765 million to settle a lawsuit brought by thousands of former players, many suffering from dementia and health problems. They have accused the league of hiding the dangers of brain injury while profiting from the sport’s violence. (The federal judge hearing the case issued a preliminary rejection of the agreement in January.)

Zetterberg’s team examined all the players in the Swedish Hockey League and found that between September and December of the 2012/2013 season alone, 35 of 288 players had had a concussion.

The players who had a concussion were asked to provide repeated blood samples, immediately after the concussion and then also during the following days. The results were compared with the preseason samples from two full teams, and the scientists found that elevated levels of a nerve cell protein called tau in the blood was a marker of concussion.

By measuring tau levels in regular tests, the researchers could say how severe the concussion was just one hour after the injury and could predict with a high level of certainty which players would have long-term symptoms and needed to rest longer.

Zetterberg said the test could also be used in general emergency medical care to diagnose brain damage from concussions, regardless of how they happened.

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