What is a cut-off level?

Regularly anti-drug testing individuals will complain that they are being punished for "having drugs in their system, there's not a number like roadside breath testing".


Well, let's look into that, for the TLDR millennials; yes, there is a level in workplace drug testing (as mentioned in my article about police testing) and just like the alcohol limit, it is a flat level for everyone.


When you drive a vehicle, you cannot have any illicit drugs in your system and you can have alcohol up to the following concentrations in NSW;

  • Learner / P1 / P2 Drivers/riders: 0.000 grams in 210 Litres of Breath.

  • Trucks with a GVM >13 tonne, Taxis etc.: 0.019 grams in 210 Litres of Breath.

  • Unrestricted licence holders: 0.049 grams in 210 Litres of Breath.

Does that indicate impairment? Not always, but it is a general guess. Let's look at a person with the build of Hagrid from Harry Potter, he could probably drink 6 standard drinks and be fine to drive (to an extent), But let's look at a small woman, in some cases 0.20 may be enough to cause impairment to the point they are unsafe to drive. Experiments have determined the average reading for impairment. In some countries it is 0.080 (US, UK etc.) in others like Australia it is 0.050 (it used to be 0.080).


We have done the same for drugs. Whilst some people may not feel impaired at the cut off levels, some may actually be impaired under the cut off level.


Let's look at the following failed drug test;

In this case, the donor tested positive for THC as the lab reported they had 10ng/mL of THC in their saliva and the cut off is 5 ng/mL.


Now let's look at a negative drug test;

In this case, there is no doubt that this individual is using Methamphetamine, however they are under the cut off limit (25ng/mL) so they are negative and their employer would be advised that, legally speaking, they have never failed a drug test.


How are cut offs determined? Based on how quickly it leaves the body and how much is needed to start impairing you is usually how it is determined.


So there you have it, drug tests are not just "finding drugs in your system" and as you can see in the second sample, even if we find them they may still be under the limit.


Additionally, field tests are required to be 2 - 3 times more lenient than lab tests. For instance the field test cut off for THC is 15 ng/mL (3 times higher than a lab cut-off) and the field test cut off for Oxycodone is 40ng/mL (twice that of a laboratory). So if you are testing not-negative on a field test, it is likely you will be over the limit at the laboratory.


So the next time someone states that field tests are inaccurate and don't test for impairment, feel free to point out they actually are detecting at least double what a lab may deem is impairment.

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