Scientists have been looking for a blood test or other biological marker for post-traumatic stress disorder, which can be difficult to diagnose.

Experimenting in rats, they may have found one.

In a study published this week in the Proceedings of the National Academy of Science, researchers stressed out dozens of laboratory rats by exposing them to used cat litter. Although the rats had never met a cat, they recognized the cat-urine scent as a threat.

Seven days later, the researchers tested the rats for symptoms of PTSD. Animals were deemed to be affected if they cowered in a closed section of a maze – counter to their instinct to explore – and failed to habituate to a loud clapping noise, continuing to jump even after hearing it 30 times.

The researchers then analyzed gene expression in blood and two parts of the brain. The rats with what were seen as PTSD symptoms had a distinct pattern of glucocorticoid receptor signaling.

Glucocorticoids are a class of hormones. The most important is cortisol, which is released in response to stress.

In a second test, rats were injected, after exposure to cat litter, with another hormone that activates the glucocorticoid receptors. Compared with rats that did not get the injections, they showed far fewer PTSD symptoms when they were tested a week later. The lead researcher, Dr. Nikolaos Daskalakis, a neuroendocrinologist at Mount Sinai Hospital in New York, said clinical trials are already underway to see if a glucocorticoid given quickly to survivors of serious car accidents can help prevent PTSD.

In the aftermath of trauma, some people are more vulnerable than others to develop PTSD. Researchers are still trying to figure out why.