PTSD and TBI have many overlapping symptoms. Now, a brain study of 20,000 people has found how to tell the two conditions apart.

In 2008, Capt. Patrick Caffrey of the 2nd Battalion, 7th Marines’ Combat Engineer Platoon was deployed in Afghanistan.

The combat engineer’s goal was to detect and remove mines, improvised explosive devices (IEDs), and other hazards from roads, clearing the way for troops and supplies to move through.

By the time Caffrey left Afghanistan, he had survived three explosive blasts that caused concussions.

These weren’t the first concussions in his life. He’d already had five or six such head injuries from playing sports and from other incidents.

Still, when he first got home, he felt fine. Not just fine, but fortunate that he’d survived Afghanistan unscathed.

Or so he thought.

Over the weeks and months following his return, Caffrey began to deteriorate. He began experiencing headaches and difficulty sleeping, as well as problems concentrating, focusing, and remembering. He had frequent angry outbursts, which he’d never experienced before.

“I was rude and nasty to people, and the worst part was that I didn’t really know just how much I had changed,” said Caffrey.

But when Caffrey sought medical help, his diagnosis was unclear. A history of surviving explosive attacks, coupled with his symptoms, pointed to two different possible conditions: traumatic brain injury (TBI) and post-traumatic stress disorder (PTSD).

So which was it?

Read More: The Health of U.S. Soldiers 10 Years After the Invasion of Iraq »