Compounds present in exhaled breath can act as a "fingerprint" for individuals, scientists say.
These "metabolites" represent the waste products of the body's chemistry - but their uniqueness had never been shown.
A study in PLOS ONE suggests they could be as useful to medical diagnosis as those found in urine or blood.
Because a breath test is non-invasive and the results are instantaneous, it could prove even more convenient for example in anaesthesia or doping tests.
"I don't understand why breath hasn't been a widely used [means of] medical science diagnosis," said the study's lead author Renato Zenobi of the Swiss Federal Institute of Technology (ETH) in Zurich.
"In traditional Chinese medicine, they feel your pulse, look at your tongue and smell your breath," he told BBC News. "There are trained dogs who can sniff cancer with a fairly good hit-rate - but the dog doesn't tell you what the compounds are."
Prior work has shown that the precise type of bacteria responsible for lung infections or even the presence of stomach cancer could be discerned in the breath.
What remained to be seen was whether the breath's metabolic contents varied enough between people - and varied little enough within an individual - to be diagnostic - to act as a real and repeatable "breathprint".
"You need to show there is a core individual signal that is stable over time," Prof Zenobi said. "If it changes a lot during the course of the day or after you've had a coffee or smoked a cigarette, you can just forget about it."
source - http://www.bbc.co.uk/news/science-environment-22013700