Most everyone sheds a tear from time to time. Now, there’s a good reason to do just that: Tears may hold clues about whether someone has Parkinson’s disease.

Researchers at the University of Southern California say tears are a reliable, noninvasive and inexpensive biological marker of the neurodegenerative disorder. They focused on tears because they contain proteins produced by the tear gland, which secretes the proteins based on nerve signals. The scientists believed changes in nerve function caused by Parkinson’s disease would ultimately be revealed at the protein level.

Using samples from Parkinson’s disease patients and people who were disease-free, researchers analyzed the levels of four proteins in their tears. They found differences in one protein in the Parkinson’s patients compared with the control patients. They also found different levels of a related protein in the Parkinson’s disease patients, one that has been linked to nerve damage.

The differing protein levels indicate tears can help distinguish between those who have Parkinson’s disease and those who don’t, the researchers concluded. Their findings were presented at an American Academy of Neurology meeting.

Because Parkinson’s disease can begin years before symptoms appear, researchers said having an identifiable biological marker is a potential tool for earlier diagnosis. Next, they want to test larger groups of people to determine just how early the crucial protein changes can be detected.

Before long, it’s quite possible that tears can involve more than just a romantic movie, an ex-partner or a country music song.