The researchers suggest that adding measures of blood tau to current clinical assessments might help us make better return-to-play decisions and prevent players resuming contact sports while their brains are still healing.
The researchers - including members from the University of Rochester School Medical Center (URMC) in New York - report their findings in the journal Neurology.
A concussion is a form of traumatic brain injury (TBI) that results either directly from a blow or jolt to the head, or indirectly from a strike to the body that results in the head and brain moving back and forth rapidly.
As the sudden movement shakes the brain inside the skull, it can stretch or damage brain cells and disrupt brain chemistry.
Clinicians often describe concussion as a mild form of TBI because it is not typically life-threatening. However, the effects can be serious.
Jeffrey Bazarian, professor of Emergency Medicine and Physical Medicine & Rehabilitation at URMC, and colleagues explain that although 3.8 million sport-related concussions occur every year in the United States, there are currently no biomarkers for predicting recovery and an athlete's ability to return to play.
If athletes return to play too soon and before the brain has healed, they run the risk of long-term physical and cognitive problems - especially if they suffer another concussion.
At present, doctors and trainers assess return to play based on symptoms the athletes themselves report, together with their performance in memory and attention tests.
Tau is linked to brain damage
In their paper, the researchers describe a serious brain condition called chronic traumatic encephalopathy (CTE), which is linked to playing contact sports that involve repeated head trauma.
One of the hallmarks of CTE is the presence of an altered brain protein called tau and its accumulation into neurofibrillary tangles, which are also hallmarks of other brain disorders, such as Alzheimer's disease.
The researchers note that tau is also linked to brain cell damage following TBI and to sport-related concussions. There is also evidence of higher levels of the protein in professional ice hockey players who are not ready to return to play for at least 10 days.
For the new study, the team evaluated changes in blood tau levels in 46 concussed college athletes from the University of Rochester and Rochester Institute of Technology. The athletes were Division I and III college players of soccer, football, basketball, hockey, and lacrosse.
Levels of the athletes' blood tau were measured from pre-season blood samples and again within 6 hours following concussion. The researchers used an ultra-sensitive technology that can detect single protein molecules.
The researchers divided the athletes into two groups based on their recovery time. One group, the long return-to-play group, took more than 10 days to recover following concussion. The other group, the short return-to-play group, took under 10 days to go back to playing their sport.
Blood tau predicted return to play exceeding 10 days
The results showed that the athletes in the long return-to-play group had higher levels of blood tau 6 hours after concussion, compared with the short return-to-play group.
The long return-to-play group also showed a bigger jump from pre-season blood tau levels to the post-concussion levels, compared with their short return-to-play counterparts.
Using statistical tools, the researchers showed that higher levels of blood tau 6 hours after concussion consistently predicted that athletes would take more than 10 days to return to play.
The results also showed differences between men and women. For instance, female athletes made up 61 percent of the long return-to-play group but only 28 percent of the short return-to-play group. This came as no surprise to the researchers; it is well established that female athletes take longer to recover after concussion than males.
Prof. Bazarian says that their findings suggest that tau may be a useful biomarker for identifying athletes who may take longer to recover after a concussion. He notes that athletes are often keen to return to play as soon as possible and may tell their doctors they are better when they are not. He concludes:
"It may be that tau combined with current clinical assessments could help us make more informed return-to-play decisions and prevent players from going back to a contact sport when their brains are still healing."
He and his colleagues point out that their study is limited by its small size, and more research is needed to confirm tau as a biomarker of concussion severity.
They now want to investigate whether their findings hold true of tau levels immediately following the concussion event - which would involve obtaining blood samples soon after a blow to the head.