The test works by measuring the levels of proteins, known as UCH-L1, and GFAP, that are released from the brain into blood and measured within 12 hours of the head injury. Levels of these blood proteins can help predict which patients may have intracranial lesions visible by CT scan, and which won’t. In a statement announcing the approval, the F.D.A. said that the brain trauma indicator was able to predict the presence of intracranial lesions on a CT scan 97.5 percent of the time, and those who did not have such lesions 99.6 percent of the time.

The possibility of testing an athlete on the sidelines could also be used in all sports, but particularly football, which includes high-speed collisions on every play. While professional and collegiate athletes have access to trainers and doctors, players on high school teams and in youth leagues often make do with a volunteer physician or an emergency medical technician, if at all.

Far more athletes play football at younger ages. More than one million boys play football in high school, about the same as those who play baseball and basketball combined. Many more play football in youth leagues, including Pop Warner, one of the most established organizations.

These organizations have seen their insurance costs rise in part because parents of injured players have sued them for not doing enough to protect their children.

Putting an athlete with a concussion back on the field can also have grave consequences. A player who suffers a concussion is susceptible to second-impact syndrome, which occurs when the brain swells after a second concussion, but before the first concussion has been diagnosed.

The F.D.A. approved the test for use in adults, but Henry Nordhoff, chief executive of Banyan Biomarkers, based in San Diego, the company that makes the device, said he thought it might also appeal to physicians evaluating children for concussions. While that would initially be an off-label use, the company plans to soon start a clinical trial evaluating injured children, Mr. Nordhoff said.

There is also a smaller device for the blood test in the works, which Banyan is working on with the French company bioMérieux, and a hand-held one they are developing with Abbott Laboratories. The tests could eventually be used in other countries besides the United States. Worldwide, about 10 million people a year are treated for concussion-related injuries, according to bioMérieux.