Let's take a look:

Researchers at Loughborough University and Imperial College in London say they've come up with a breath test for stress. They say they've identified six compounds in a person's breath to determine if they are under stress.

For medical purposes, it might aid patients who are having trouble verbalizing being under stress, perhaps an Alzheimer's patient. Researchers activated stress in test subjects by administering what they called a tricky math test. So what they found is that two of the compounds increased with stress and four of them decreased.

They say the stress level is pretty mild in this test so further testing will be needed.

But think about it. Usually anyone suspected of a crime and is being interrogated by law enforcement officials is going to be under stress. So giving them the breath test might not indicate anything other than being under stress. Having a calm conversation with someone who is hiding something might show them also under stress. Again, no clear indication they did anything wrong.

There are breath tests that have been developed to check for various diseases, but as far as having a breathalyer for the truth, I think we're way down the road on that.

Conclusion: (Take a deep breath) Fiction