Limitations of Workplace Drug Testing Methods for Proving THC Impairment

Employees in Canada can consume medical cannabis legally and soon will be allowed to use recreationally too. 

Recreational use presents challenges for employers who are concerned about the safety and productivity of their workplace. The principal concern is that employees could be impaired at work.  

Having a policy and educating employees can go a long way to ward off unwanted behaviour. However, there is still a risk that even an educated employee may be impaired at work. 

Measuring Impairment

This risk exists with alcohol too and breathalyzers are an effective means for assessing active impairment. This is because alcohol levels in the blood and brain tend to be directly correlated. As a result, alcohol impairment is closely tied to blood (and breath) alcohol levels. 

This is not the case for cannabis, where THC is stored in the fatty tissues and retained even as blood levels fall quickly. 

The courts have commented on this, saying “alcohol tests are usually conducted with a breathalyzer, which provides an immediate result concerning present alcohol impairment in a minimally invasive manner. Though drug-testing technology has advanced, it does not provide an immediate detection of drug impairment” (CEP Local 30 v Irving Pulp & Paper).

The tests commonly used today in workplaces to detect past drug use fall short of measuring active THC impairment. 

First, let’s explore how cannabis is metabolized by the human body. THC is the psychoactive, impairment-causing component in cannabis. When cannabis is consumed, THC is absorbed into the bloodstream and is stored in fatty tissues. THC remains active in the body for several hours and begins to be metabolized into carboxy-THC (also called THC-acid). Carboxy-THC is an inactive metabolite, so it doesn’t affect brain function the way that THC does. THC re-enters the blood from the fatty tissues throughout the body in small quantities over time. (Institute for Behavior and Health Inc.). 


Urinalysis is a common test administered for workplace drug testing but it has limitations. Urinalysis identifies carboxy-THC, the inactive metabolite. Carboxy-THC is not an indicator of active THC, rather that THC has been in the system at some time in the past hours, days, weeks or even months. While the test does detect recent use it doesn’t detect impairment. 

The courts have been clear on this limitation, saying “urinalysis does not demonstrate current impairment. It does indicate use, but not the quality, quantity or time of use” (Suncor Energy Inc v Unifor Local 707A).

Oral Fluid Analysis 

Oral fluid analysis is another test that has gained popularity over the past decade. This test uses fluid from the mouth and identifies  THC in the liquid. Oral fluid tests have a shorter window of detection than urine tests, detecting THC 12-24 hours after cannabis consumption. An important distinction from urinalysis is that it detects active THC, not carboxy-THC. 

Other benefits of oral fluid tests include ease of use, non-invasiveness, observable sample collection, difficulty to adulterate, and demonstration if recent drug use (Psychopharmacology (2012) 223:439-446).  

Oral fluid testing devices can have noteworthy limitations. In a study involving the Drager Drug Test 5000 and the Securetec Drugwipe 5, the Drager “generally performed well” with false negatives ranging from 0-10%. However, the Securetec “was less accurate” (Psychopharmacology (2012) 223:439-446). Accuracy limitations can be a serious problem with the efficacy of oral fluid tests. 

Measuring active THC in the system is a more accurate assessment of recent use. As discussed, THC rapidly breaks down into carboxy-THC over 3-6 hours from consumption. A test that positively indicates that a person has THC in their system does confirm recent use. However, this test too falls short of confirming impairment. 

Impairment by THC is a complicated matter. In studies “performance impairment has repeatedly been shown to last for 3-4 hours after smoking THC” (Psychopharmacology (2012) 223:439-446). However, frequent users will develop behavioral tolerance to THC-induced impairment and build up of THC in the fatty tissues. 

Cut Off Limits 

Another issue in both urinalysis and oral fluid tests is the cut-off limit. Cut off limits with alcohol (typically .08 percent for driving infractions) are appropriate because of the direct correlation between alcohol blood levels and impairment. Cut-off levels for cannabis have commonly been set at 50ng/ml (carboxy-THC) for urine tests and 5ng/ml (THC) for oral fluid swabs. 

It’s been shown that in new users THC concentrations rapidly decrease to 1-4 ng/mL within 3-4 hours of consumption (Iranian Journal of Psychiatry. 2012 Fall; 7(4): 149–156). However, this is not the case infrequent users because of the build up in the fatty tissues. 

A study published in 2015 by Forensic Science International found that after 24 hours of cannabis abstinence that 43% of participants who had consumed cannabis heavily over the previous three months had active THC levels over 5ng/ml. While the performance-impairing window of 3-4 hours had passed, these individuals would have still tested positive for THC. 

Our longstanding practice of testing for THC has obvious limitations and it is evident that it's not a reasonable measure of impairment. 

The risk is that we are punishing employees that are not actually impaired. The punishment can come in the form of termination of employment, affecting their ability to earn an income.

So How Do We Measure Impairment?

This is not an easy answer but workplaces should be training supervisors to identify signs of impairment and using this in conjunction with oral fluid (or urine) tests to establish a totality of evidence. In the absence of a test for THC impairment we must assess the big picture or we risk being guilty of punishing employees that we cannot unequivocally prove are impaired.