WASHINGTON - The Institute of Medicine recommended on Friday that troops returning from Iraq and Afghanistan undergo annual screening for post-traumatic stress disorder and that federal agencies conduct more research to determine how well treatments for PTSD are working.

Of the 2.6 million service members deployed to Iraq and Afghanistan, it's estimated that 13 to 20 percent have symptoms of PTSD.

Federal agencies have increasingly dedicated more resources to screen and treat soldiers, but considerable gaps remain, according to the Institute of Medicine, an independent group of experts that advises the federal government on medical issues. Its recommendations often make their way into laws drafted by Congress and policies implemented by federal agencies.

Barely more than half of those diagnosed with PTSD get treatment, often because many soldiers worry it could jeopardize their careers. Also, when soldiers do get care, they're not tracked to determine which treatments are successful in the long-term.

The Defense Department provides medical care to active members of the military and the Department of Veterans Affairs cares for those who no longer serve. Sandro Galea, the chairman of the Institute of Medicine panel, said both departments offer many programs for PTSD.

"But treatment isn't reaching everyone who needs it, and the departments aren't tracking which treatments are being used or evaluating how well they work in the long term," said Galea, a professor and chair of the epidemiology department at Columbia University.

The report concludes only the first phase of the Institute of Medicine study. The panel is hoping to release a second report in 2014 that will provide more specifics about the number of service members and vets who have PTSD and the outcomes and costs of their treatments.

PTSD is triggered by a specific traumatic event, such as being in combat or witnessing death. The symptoms of the illness include a numbing of emotions, difficulty concentrating and exaggerated startled responses to events.