After a hit to the head or rapid whiplash, millions of Americans develop traumatic brain injuries (TBIs) each year. TBIs can range from mild concussions, causing only a headache or temporary blurred vision, to much more severe injuries—causing seizures or even coma. These symptoms, whether mild or more severe, are generally caused by damaged brain cells.

Until now, most physicians have relied on CT scans and patients’ symptoms to determine whether to take extra precautions. However, CT scans can only detect bleeding in the brain, not damage to brain cells, which can happen without bleeding.

“A typical situation is that someone comes to the emergency department with a suspected TBI, we get a CT scan and, if the scan shows no bleeding, we send the patient home,” says emergency medicine physician Frederick Korley. “However, these patients go home and continue having headaches, difficulty concentrating and memory problems, and they can’t figure out why they are having these symptoms after doctors told them everything was fine.”

Now, Korley and his team have developed a new blood test that could help emergency deparatment doctors quickly diagnose a TBI and determine its severity. The blood test, taken within 24 hours of a head injury, measures levels of brain-derived neurotrophic factor (BDNF).

“Compared to other proteins that have been measured in traumatic brain injury, BDNF does a much better job of predicting outcomes,” says Korley, first author on the study that appeared in the July 10 Journal of Neurotrauma.

The scientists followed 300 patients with TBI over six months, finding that those with the highest levels of BDNF, as measured soon after their injury, had the best recovery. The results suggest that a test for BDNF levels, administered in the emergency room, could help stratify patients.

“The advantage of being able to predict prognosis early on is that you can advise patients on what to do, recommend whether they need to take time off work or school, and decide whether they need to follow up with a rehab doctor or neurologist,” Korley says. In addition, it could help doctors decide which patients to enroll in clinical trials for new drugs or therapies targeting severe TBIs.