A fingerprint can show if someone has taken cocaine or just touched it

A single fingerprint can distinguish whether someone has recently touched cocaine or actually ingested it. This test can be completed in less than 2 minutes, far quicker than blood tests, and could be used for forensic investigations or drug testing.

Melanie Bailey at the University of Surrey in the UK and her colleagues have developed a technique that detects trace amounts of cocaine, as well as signs of cocaine use, on human skin.

“You can use it to say either somebody’s touched the drug or they’ve ingested it,” says Bailey.

As well as cocaine, the test picks up on a molecule called benzoylecgonine, which is excreted through the skin after a person has ingested cocaine. The chemical is also present as an impurity in some street samples of cocaine. “Benzoylecgonine in street cocaine actually washes off the fingers,” says Bailey.

But a person who has ingested cocaine will continue to excrete the molecule through their sweat, so even after washing their hands it is detectable in a fingerprint.

Bailey and her team took fingerprints from people who had touched samples of cocaine of 99 per cent purity as well as street samples that were far less pure. They took fingerprints immediately after the drug had been handled and again after participants had washed their hands.

They also took fingerprints from 26 people at a drug rehabilitation clinic who reported taking cocaine in the past 24 hours.

For the test, the individual presses their finger onto a piece of specialised paper for 10 seconds. The paper is then analysed using a technique called mass spectrometry to detect the presence of cocaine or benzoylecgonine.

In the 86 samples, the fingerprinting technique was 95 per cent accurate. The team found that detection was possible up to 48 hours after contact or ingestion.

Unlike blood tests, which are the current standard for testing cocaine use, the fingerprint analysis can be completed in less than 2 minutes.

The technique is now commercially available and could be used for drug testing. “If someone’s ingested cocaine, you wouldn’t want them to fly a plane or drive a bus,” says Bailey.

It may be used in future as a forensic tool to determine the presence of cocaine in fingerprints left at a crime scene, although it may need further validation before then, says David Berry, an independent toxicology consultant in the UK.

Whether someone has broken drug-driving laws, for example, currently depends on the concentrations of illegal drugs in the blood. Regulation may be needed for this new technique to be used in legal settings.