The science behind Roadside Drug Testing (RDT)

Despite the fact drug testing has been a thing in Australia for over 18 years now, there is a lot of misinformation coming from politicians and social media. It is very common to see comments like these:

Well, the good news for Peter and people who share his belief is; there is a 'level'.

Now before we unpack what the NSW Police level is, we have to first understand what the levels are. Let's focus on THC which is the one that gets the most misinformation on social media.

In Australia we have a THC cut-off of 5 ng/mL under AS/NZS 4760:2019, this means it is legal for any person, company, or government agency to treat you as a risk/offender.

Now it was held for a long time that impairment starts at around 2 ng/mL but this was in an era where we had the following limitations:

1. Cheap plastic (or worse, glass) test tubes, and

2. LC-MS/MS was not well developed to detect THC, and

3. No preserving reagents.

We now have fantastic ways to preserve analytes in saliva that we did not have when the 2ng/mL level was set in the US. What this likely meant is that the level was likely 3 or 4 as we know that THC loves binding with plastic/glass unless a surfactant is present, so when we were finding levels of 2 this was probably after we lost 30% to 50% of the THC in transit to the lab.

Accordingly, Australia did not decide to adopt the American 2 ng/mL level and instead set it to 5 ng/mL which creates fairness. Now, is everyone going to be impaired at this level? No, just like not everyone is drunk at 0.050 grams of alcohol per 210 Litres of breath, but the line has to be drawn somewhere. Just as there are alcoholics that can function at high levels like 0.080g/210L, there are probably stoners that can function at 8 ng/mL without impairment.

But, there are also people who get drunk with as little as 0.020g/210L in their system just as some people may be impaired by cannabis levels as low as 1 or 2ng/mL

NSW Police testing

The roadside devices themselves have a cut-off of 10 ng/mL. This is double what the Australian Standard cut-off is.

Now, are these devices 100% accurate? No. No immunoassay is, nor are they required to be.

They are required to be about 90% accurate under Australian Standards (Appendix C3 of AS/NZS 4760:2019)

And that is why police do not start proceedings based on the roadside device. They wait for the lab. The lab also has a cut-off of 10 ng/mL, here is an extract of their scientific method:

And for those who are going to get confused by ng/mL and mg/L:

What does a lab actually see?

Well, let's look at the actual thing a lab looks at. When they run your saliva they run a sample of the drug (left screen) against your saliva (right screen) and if they match - the drug is present.

Here is an example of someones saliva being run as methamphetamine, no match - correctly reported as negative (remember, the left screen is the actual methamphetamine sample used for comparison):

Now, let's see that exact same saliva run as THC (once again, the left screen is the known standard - the positive sample):

If you overlapped them, they are almost perfect copies - so yes, there is THC here. This person is positive.

You would then run it again and report the level. If it exceeds 10 ng/mL, it is positive (in NSW Health labs only, elsewhere - 5 ng/mL).

I hope that helps the "There needs to be a level". There is. There always has been.