NEW YORK (CBSNewYork) — A new study shows a blood test can detect concussions up to a week after a head injury.

CBS2’s Dr. Max Gomez reports that’s important because symptoms can be subtle and undiagnosed, especially in children.

An estimated 250,000 children a year end up in the hospital with concussions from playing sports. Signs of a concussion are often delayed, and if it takes a few days for symptoms to settle in, the only reliable way to diagnose a patients is with a CT scan.

“You really want to minimize the amount of CTs you do to your patients, particularly children, because they are a lot more sensitive to radiation and the bad side effects that come with that,” Dr. Linda Papa, of Orlando Health, said.

Orlando Health researchers developed a simple blood test that can not only detect a concussion, but a new study shows, the results are reliable up to a week after injury.

Scientists studied nearly 600 patients over three years, focusing on biomarkers in the blood known as GFAP.

“The markers that we are looking at are really specific to the brain and are not released through any other parts of the body, which is what make them so unique,” Papa said.

The problem, however, is many patients, especially children, don’t always articulate symptoms at the time of the injury. Since GFAP stays in blood for so long, it could extend the window for diagnosis, so doctors may be able to detect subtle concussions days later.

The study appears in the journal JAMA Neurology.