Results of roadside drug testing have been called into question by a study that found tests often failed to detect THC but delivered false positives up to 10 per cent of the time.

The study, published in the journal Drug Testing and Analysis, used the two most common tests the police use to detect drugs, the Securetec DrugWipe and the Draeger DrugTest 5000.

False-negative rates in the presence of high concentrations of THC were nine and 16 per cent respectively. The devices were also sometimes giving positive results when saliva THC concentrations were very low or negligible, giving a false positive rate of five and 10 per cent respectively.

The accuracy, specificity and sensitivity of the two tests falls below recommended levels by the European Union.

Problems with cannabis testing in court

In July 2019 New South Wales Magistrate David Heilpern found a man not guilty of drug driving after the government’s own website claimed that THC is not detectable after 12 hours – which the man had checked before driving.

But, apparently, THC can stay detectable in saliva for days or weeks, leading the Magistrate to rule in favour of the man, who had admitted to sharing a joint two days before being pulled over on his way to work.

Magistrate Heilpern said: “The 12-hour advice is nothing more than a cruel underestimation that gives people specious information, lulls them into a false sense of security, and leads to greater levels of detection, criminalisation and loss of licence.”

Magistrate Heilpern also accepted a passive smoking defence of a driver who denied having smoked cannabis in the weeks leading up to her roadside drug test but said she had been visiting her terminally ill neighbour who was smoking medical cannabis in her presence.

Sydney University’s Iain McGregor, the study program leader, said you can take cannabis oil in a capsule and it totally bypasses your saliva, while you may have had a very high dose. Someone with THC in their saliva may not be intoxicated at all, making these tests fraught. McGregor suggests better tests for impairment, rather than THC testing.

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