A blood test that costs a fraction of the price of a brain scan may flag concussion in patients that pass CT tests.

Researchers want to find out how early trauma appears.

Prions, long studied by UCSF professor and Nobel laureate Stanley Prusiner, build up in the brain to cause Alzheimer's and other dementias. Now they're linked to post-traumatic stress disorder in combat veterans and in the brain damage of athletes who have suffered repeated concussions.

The new guidelines are aimed at protecting the more than 1 million athletes who suffer concussions each year in the U.S.

Researchers found players with a history of concussions had elevated levels of tau protein, a precursor to both Alzheimer's and CTE, which afflicted the late Junior Seau.