The Case for Impairment Testing

Employees impaired from cannabis, alcohol, or other drugs put themselves, other workers, and businesses at risk. The solution is impairment testing, not drug testing.



The state of being diminished, weakened, or damaged, especially mentally or physically.

The state of having a physical or mental condition that means that part of your body or brain does not work correctly.

People are tested for drugs and alcohol because these substances can impair their ability to do even simple physical or mental tasks. Substance use which results in slowed reflexes, poor hand-eye coordination, unsteady balance, reduced attention span, or compromised judgment is a major cause of serious workplace injuries and fatalities. 

Companies initiate drug and alcohol testing, not just to comply with government regulations, but primarily to ensure that their workforce is safe, healthy, and productive. Tests are typically conducted during work hours, either at random times or when on-the-job substance use is suspected. Many companies have prospective employees tested before they are hired.

Various bodily fluids – urine, saliva, blood, or sweat – can be used to detect the presence of alcohol, cannabis, or other drugs in a person’s body. Most people have heard of alcohol breathalyzers, which are used to assess a person’s blood alcohol concentration (BAC) and provide on-the-spot results. In contrast, tests for cannabis and other drugs are expensive, time-consuming, and intrusive, and their results may not be known for several hours or even days.

Can companies rely on drug and alcohol testing to help ensure their employees are fit for duty?

In fact, on-site alcohol testing might help businesses achieve this purpose, as BAC can be measured precisely and there is a very strong correlation between a person’s BAC level and their ability to perform either physical or mental tasks. 

The same cannot be said about drug testing. If a drug test detected THC from cannabis use, for example, that result would not necessarily mean that the worker was actually impaired when the test was done. Here’s why: Cannabis’s impairing effects usually last between 3 and 10 hours, depending on but THC will be detectable for three weeks or more thereafter, long after impairment has subsided.+ 

In short, while random drug testing may deter workers from using intoxicating drugs just prior to or during their work shift, these tests cannot determine whether workers are actually impaired.

Drug testing is deficient for another reason: Employees may be impaired and unfit for duty for any number of reasons, not just substance use, including fatigue, concussion or other injuries, acute illness, chronic health conditions, and prescription medications. Clearly, what’s needed is a test that measures impairment directly

Standardized Field Sobriety Test

Law enforcement officers are trained to use the Standardized Field Sobriety Test (SFST) to determine on site whether a motorist is impaired and therefore can be charged with driving under the influence (DUI). Developed in the mid 1970’s, the SFST was developed to measure impairment directly and is admissible as evidence in a majority of US states.

The SFST comprises three tests: Eye Tracking, Walk and Turn, and One Leg Stand. None of these tests provide a scientifically objective measurement of impairment. Instead, officers are tasked with judging the driver’s performance against a set of established standards. By definition, then, the SFST is a subjective test, and this is the primary reason it has been criticized. 

To see why this is the case, consider the Walk and Turn Test. To initiate the test, officers are to read the following instructions:

  • Instruction Stage: Place your left foot on a line, real or imaginary, and put your right heel against the toe of the left foot. Place your arms down at your sides. Maintain this position until I have completed the instructions. Do not start to walk until told to do so. Do you understand?

  • Walking Stage: When I tell you to start, take nine heel-to-toe steps on the line, turn and take nine heel-to-toe steps back down the line. On your turn, keep your front foot on the line and turn by taking series of small steps with the other foot. Watch your feet while you walk, count your steps out loud, and keep your arms by your side. Once you begin to walk, do not stop until the test is completed. Do you understand? 

The Walking Stage instructions are complicated and therefore will often be misunderstood by anyone who is hard of hearing, generally has trouble following verbal instructions, or is simply nervous about having been stopped by a law enforcement officer. Having to repeat the instructions might in some cases bias how officers score the test. 


West Manchester Township Police Officer Adam Bruckhart, left, and York Area Regional Police Officer Mike Georgiou observe a driver suspected of being under the influence of alcohol as he performs a field sobriety test at a DUI checkpoint.

As the Walk and Turn Test proceeds, officers are to check off on a scorecard which of these deviations they have observed, any two of which would mean the person has failed the test:

  • Can’t balance during instructions
  • Starts [walking] too soon
  • Stops while walking
  • Doesn’t touch heel to toe
  • Steps off the line
  • Uses arms for balance
  • Improper turn/loses balance on turn
  • Wrong number of steps

Even if they understand and remember everything, very few people, unless they are well-practiced, will be able to follow the test instructions perfectly. Inevitably, this means that officers must decide whether a deviation they observe is significant enough to for them to check the box on the scorecard– that is, they must make a subjective yes-or-no judgment.

No amount of training can ensure that officers are making these judgments objectively – and therefore accurately – each and every time they administer the SFST. Unsurprisingly, a review of the scientific literature++ concluded that “variations in administration or scoring from one police officer to another will be a substantial source of error,” and that “SFSTs are not validated as tests of impaired driving.”

Even if it were a reliable and valid test, the SFST is obviously impractical for workplace testing. What’s needed is an objective and comprehensive impairment test that is accurate, sensitive, quick, portable, and inexpensive

There’s only one such test now on the market: The Druid® Impairment App.

The Druid® Impairment App

Applying the scientific literature, Dr. Michel Milburn, Druidapp’s inventor, devised four tasks that provide performance data on known indicators of cognitive and psychomotor impairment:

  • Tasks 1-3 are divided-attention tasks that assess reaction time, hand-eye coordination, decision-making accuracy, and time-estimation accuracy. 
  • Task 4 assesses a user’s ability to maintain balance. 

Using a proprietary algorithm, the app statistically integrates hundreds of recorded data points to calculate a total impairment score.

  • Is Druid an objective test? Yes. Note that a supervisor should be present ensure that workers are following the test instructions.  
  • Is it accurate and sensitive? Yes. In a recent scientific paper, Johns Hopkins researchers concluded that Druid® was “the most sensitive measure of impairment” when compared to other cognitive impairment measures and field sobriety tests.+++
  • Is it quick? Yes. The app takes under three minutes to use. The tests are carried out on site and the results are known as soon as a test is finished.
  • Is it portable? Yes. Unlike other cognitive tests of impairment, Druid requires no special equipment, just smartphones or tablets.
  • Is it inexpensive? Yes, especially in comparison to the cost of conducting and analyzing just one drug test, never mind a series of random drug tests. 

Druid has two other advantages. First, unlike drug tests that require a blood, urine, or saliva sample, Druid is a performance-based test and therefor non-invasive

Second, Druid can detect cognitive and psychomotor impairment no matter its cause: substance use, but also fatigue, concussion or other injuries, acute illness, chronic health conditions, and prescription medications.

Is Drug Testing Still Needed?

Druid can be likened to a thermometer that measures body temperature. A thermometer will show how high a fever is, but not its cause. Likewise, Druid can show whether workers are unfit for duty, but ascertaining why they are impaired would require further investigation.

A failed Druid test, when combined with other observations, might warrant having a worker tested for drugs or alcohol, but not necessarily. 

A high impairment score could indicate instead that a worker needs to see a healthcare provider or a mental health counselor, or it could be due to a temporary condition, such as fatigue. 

Whatever the case, a worker who fails a Druid test should be asked to take it again. If the impairment score is still too high, then for at least that day, the worker should be sent home or assigned duties which present fewer health and safety risks.

The goal of protecting workers’ health and safety is paramount, but also important is having fair, transparent, and consistent procedures that protect workers’ rights. Thus, while Druid provides an objective measure of cognitive and psychomotor impairment, how company owners and managers will use this information is for them to determine.

William DeJong, PhD, Director of Research and Evaluation, Impairment Science, Inc.


  +  McCartney, D., Arkell, T. R., Irwin, C., & McGregor, I. S. (2021). Determining the magnitude and duration of acute Δ9-tetrahydrocannabinol (Δ9-THC)-induced driving and cognitive impairment: A systematic and meta-analytic review. Neuroscience & Biobehavioral Reviews, 126, 175-193.

++ Rubenzer, S. J. (2008). The standardized field sobriety tests: a review of scientific and legal issues. Law and human behavior32(4), 293-313.

  +++ Spindle TR, Martin E, Grabenauer M, Woodward T, Milburn MA, Vandrey RAssessment of cognitive and psychomotor impairment, subjective effects, and blood THC concentrations following acute administration of oral and vaporized cannabis. Journal of Psychopharmacology. In press.    

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