Most people who get a concussion seem to regain normal brain function within a month or two at most. But doctors have no way to predict which patients are in that group and which will suffer long-term cognitive problems.

A team from the University of Pennsylvania and Baylor College of Medicine seeks to solve that riddle with a simple blood test.

In a new study in the journal Frontiers in Neurology, the team reported that a protein called SNTF is a promising indicator of which patients with concussions are likely to experience chronic brain deficits.

Researchers tested the brain processing speed of 13 concussed patients soon after their injury and also measured the amount of SNTF in their blood. The cognitive test was administered again three months later.

Among five patients with elevated levels of the protein, none showed improvements in processing speed three months later. But in six of eight patients in whom the protein was not detected just after injury, scores on the brain test did improve three months later.

The researchers also reported that patients with elevated levels of the protein were more likely to have abnormal brain scans. The patients underwent a special kind of MRI.

Lead author Robert Siman, a neurosurgery professor at Penn’s Perelman School of Medicine, warned that while the findings were statistically significant, the study was small and needs follow-up. “We don’t have a way to identify at an early and potentially treatable stage those concussed individuals who are not going to get better,” he said.

The study also included a set of 13 patients with a broken bone or other orthopedic injury. Levels of the protein were elevated in the blood of three of those patients, suggesting that they may have suffered an undiagnosed concussion, said study author and Penn professor Douglas H. Smith.

“It highlights the important thing, that concussion is really poorly diagnosed and defined,” said Smith, director of Penn’s Center for Brain Injury and Repair.

When someone goes to the ER with symptoms of a concussion but no internal bleeding, they are examined and sent home. It is up to the patient and family to follow up with a doctor.

If the Penn-Baylor blood test holds up in larger studies, it would help doctors determine who needs follow-up care, Smith said. “It would be crucially important to weed out those who really need the call-back.”

Such a test also would help researchers test drugs and other therapies for concussions, he said. Now it is hard to tell if drugs are helping, since most people get better anyway.

The type of protein in the blood test was chosen because past studies had shown it was secreted by dying brain cells.

The concussion patients were defined as having suffered a “mild” traumatic brain injury, meaning that a CT scan did not reveal internal bleeding. But Smith dislikes the term mild, since some of the patients’ cognitive symptoms were anything but.