More accurate and reliable than current US test, Belgian evidence suggests
States where marijuana use is lawful should take a leaf out of Belgium’s book and consider roadside mouth swabs to test drivers who might be under the influence of the drug while behind the wheel, say experts in the journal Injury Prevention.
The test is more accurate and reliable than the currently used visual assessment and subsequent blood test, which screens for the active ingredient of marijuana, tetrahydrocannabinol, or THC for short, they say.
And it would most likely be constitutionally permissible, argue the authors from the Center for Injury Research and Policy, at The Johns Hopkins Bloomberg School of Public Health, Baltimore.
Driving under the influence (DUI) of marijuana is becoming more of an issue now that eight states—Massachusetts, Maine, Colorado, Washington, Oregon, California. Nevada, Alaska, and the District of Columbia— have legalised the drug for recreational use, they point out. Twenty eight states have already legalised medicinal use of the drug.
And while the number of fatal road traffic collisions in the US fell between 2005 and 2014, these rose by 7 per cent in 2015, resulting in a death toll of more than 35,000.
In 2010 alone, the direct costs of road traffic collisions amounted to US$242 billion, but if quality of life measures are included, that figure rises to US$836 billion.
There are specific issues around taking blood tests to pick up DUI, say the authors. In Colorado, for example, the law stipulates that THC blood levels above 5 ng/ml signify impaired driving.
But the average time between sample collection and analysis is around 2.5 hours, which not only falls outside the legally permissible window, but also the period for detecting peak levels of the drug in the bloodstream.
By contrast, a mouth swab is minimally invasive and can very quickly establish whether someone recently used marijuana.
The test is already used in nine European countries, plus Australia and New Zealand, and the reliability of the tests has improved considerably since concerns were raised about this in the mid-2000s, say the authors.
They cite a recent two year hospital study of blood samples taken from Belgian drivers injured in car crashes. This found evidence of illicit drug use in one in 20 of the drivers, with marijuana accounting for half of this total.
In 2009 the Belgian government updated its DUI legislation to include a mouth swab test and a reduction in the cut-off limit of illicit drug to 1 ng/ml.
Subsequent evaluation of the amended legislation showed that just 8.5 per cent of roadside mouth swab test results were falsely positive.
Following a Supreme Court ruling in the case of Maryland vs King, in which a cheek swab without the prior issue of a warrant was permitted for a lawful arrest in respect of a serious crime, a mouth swab may provide “a constitutionally permissible alternative to blood testing,” conclude the authors.
“As US states consider legalising marijuana for recreational use, policy makers should take steps to guard against foreseeable injury risks associated with this policy change,” they write.
“There is a need for an objective and timely field test to ascertain marijuana-related DUI,” and roadside mouth swabs might just fill that need, they say.
Brief report: Oral fluid testing for marijuana intoxication: enhancing objectivity for roadside DUI testing https://ip.bmj.com/lookup/ip/doi/10.1136/injuryprev-2016-042264
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