The app users will have to collect their saliva with a small external device. They will allow the saliva to travel onto an assay strip and then take a flash photo of the strip with their phones.

The app will then measure the amount of cortisol (stress hormone) in the saliva.

The users will be able to administer the analysis themselves with smartphone and a cheap, simple tube, says the report.

"Individuals at risk for psychotic depression can use the software to monitor themselves and seek assistance early," lead developer Joel Ehrenkranz, director of diabetes and endocrinology at Intermountain Healthcare in Murray, US, was quoted as saying.