MOTORISTS have been warned not to rely on DIY breathalysers to test whether they are safe to get behind the wheel at Scotland's new drink-drive limit as a new study casts doubt on their accuracy.

Researchers found considerable variations in the ability of some breathalysers on sale at major retailers and pharmacies to detect potentially unsafe levels of breath alcohol for driving.

The devices have been marketed recently in Scotland as a means for drivers to carry out their own readings following the reduction in the legal drink-drive threshold from 80mg per 100ml of blood to 50mg, with reports of the self-test kits - priced at around £60 - selling out in some stores.

However, the authors of the study, published by the online journal BMJ Open, have questioned the regulatory process for approving these sorts of devices for personal use. They stress that the false reassurance they provide could have "catastrophic safety implications" for road users.

The researchers compared the diagnostic accuracy, or "sensitivity", of three personal use breathalysers against the UK limit - which remains 80mg - on a sample group of 208 adults drinking in college bars and pubs in the centre of Oxford in late 2012 and early 2013.

Breath tests were conducted at least 20 minutes after drinking using the commercially available Alcosense Single, Alcosense Elite, and the Dräger Alco-check, as well as the Dräger Alcotest 6510, which is used by the police.

Using the police breathalyser, 35 participants tested over the limit.

However, the Alcosense Elite detected only 90 per cent of these while the Dräger AlcoCheck had a sensitivity of just under 95 per cent, meaning that around one in 20 people over the legal driving limit would be falsely reassured.

The Alcosense Single had an even lower sensitivity of only 26 per cent, compared with the police breathalyser, meaning that the device would pick up only around one in four people over the legal limit, shortly after drinking.

However, the researchers acknowledge that the study has limitations. For example, the manufacturers of the Alcosense Elite stipulate that it should be used 30 minutes after drinking, whereas the study took samples at around 20 minutes.

The profile of the drinkers - aged 20 on average - may not be typical of those who are likely to buy breathalysers for personal use, they add. Similarly, the researchers did not test the accuracy of the devices for "morning after" scenarios.

Nonetheless, they state: "Our research suggests that at least some personal breathalysers available for sale to the public are not always sufficiently sensitive to test safety to drive after drinking alcohol, where use of inaccurate information from breathalysers, thought to be accurate, could have catastrophic safety implications for drivers."

They add: "Our research raises worrying questions about the level of scrutiny that medical tests intended for sale to the public undergo in Europe, and raises wider concerns about how diagnostic accuracy, in particular, is evaluated."

Police and the Institute of Advanced Motorists have previously warned drivers not to rely on DIY breathalysers.

Neil Greig, the IAM's director of policy, said he was pleased to see their concerns backed up by science.

He added: "The problem is not just about accuracy, it's that people might use them as a means to drink and drive because they think it's a way round the new limit. But your reading can change later.

"Our fear is that if people have a self-test kit they are more likely to take the risk, when the consistent message has to be 'if you plan to drive, don't drink anything at all'.

"I can see that they are maybe more useful for the morning after, which is a growing issue with the new limit, but even then there is a risk of false readings giving a false sense of security."