Drug Recognition Expert Testing

A Critical Review of the Literature

Counterpoint Volume 8; Issue 1 - Article 6 (May 2024)

An article in the upcoming Mastery Skills I Program

David Rosenbloom, Pharm. D.
​Pharmaco-Legal Consultant

Article information:

Article - 2500 words (approximately 12-14 minutes)
The tests used in the Drug Recognition Expert (DRE) or Drug Influence Evaluation (DIE) testing are based on the initial work by Tharp, Burns and Moskowitz first published in 1981. The raw data from that study have never been published. However, a number of studies have investigated the various parameters used in DRE testing and the results of these studies raise considerable doubt on the validity of this form of evidence.
Most recently, Marcotte et al. published a randomized trial describing that the roadside tests for THC (cannabis) are neither specific nor sensitive enough to be used for roadside testing for impaired driving when compared to placebo. This paper, the first in a series of three, provides the background as to why this was predictable based on prior studies and the pharmacology of THC compared to alcohol.

Part One

An Introduction to the DRE Testing Procedures
​It is claimed that the tests used during the Drug Recognition Expert (DRE) testing procedure can distinguish if a person is impaired by a particular drug, including alcohol, and can identify the drug responsible for the alleged impaired driving. These articles review the scientific evidence underpinning these claims.
It should be borne in mind that the various tests encompassed in DRE testing were developed as tests specific to alcohol. They were not developed with marijuana in mind. The evidence for the accuracy of these tests for both alcohol and marijuana is reviewed below.
Most recently, a publication by Marcotte et al. demonstrated that roadside testing (FST’s), in a randomized control with THC 13.9% vs. placebo came to these conclusions:
​CONCLUSIONS AND RELEVANCE This randomized clinical trial found that when administered by highly trained officers, FSTs differentiated between individuals receiving THC vs placebo and driving abilities were associated with results of some FSTs. However, the high rate at which the participants receiving placebo failed to adequately perform FSTs and the high frequency that poor FST performance was suspected to be due to THC-related impairment suggest that FSTs, absent other indicators, may be insufficient to denote THC-specific impairment in drivers. (Marcotte, et al, 2023).
The background as to why this is not surprising is presented in this paper.

The Battery of Tests in the DRE Examination

Although there are 12 tests in the battery, there are three principle ones – the Horizontal Gaze Nystagmus test (HGN), the Walk and Turn test (WAT) and the One Leg Stand test (OLS). These are also known as the Standardized Field Sobriety Tests (SFSTs) and were developed in an attempt to measure alcohol effects that might apply to driving impairment. This paper will address the other nine tests but these first three are the principle ones used. The scientific basis for the selection of these tests will be addressed first, followed by a discussion of the limitations of the reasoning behind these tests. The studies that have been carried out to explore the validity and reliability of these tests will then be reviewed.
It should be understood that the construct that a test of balance is considered a surrogate for impairment.  This is a pharmacological construct, but the Courts may interpret poor balance as evidence for impairment, which is only true under certain circumstances, and these are reviewed here.
There are many potential explanations for poor balance that have nothing to do with impairing substances; for example, there is a natural variation in how well an individual balances normally, and, further, different individuals will vary compared to each other. Moreover, the measurement of balance, if done without appropriate instruments, is subjective. Hence it is important to explore these differences to determine how changes in balance in people consuming various substances vary from a normal degree of balance or sway.
The issue is separating the signal – what one is looking to detect – from the background noise - that can mask this signal.

WAT and OLS

As stated, the WAT and OLS are predicated on an individual’s ability to balance and how this might be affected by alcohol. These were originally tested for the effect of alcohol on humans and later these tests were applied after the consumption of marijuana. They were not tested initially for subjects consuming marijuana but were grafted on for that purpose. The studies that later addressed how well they work for marijuana are reviewed below.
Judge Mary A. Celeste has pointed out that other tests, such as the Romberg test (the subject stands erect with feet together and eyes closed- sometimes with the added feature of finger to nose) have not been sanctioned by NHTSA to date. (Celeste, 2017)
These tests were initially studied by Goldberg in 1943 (Goldberg, 1943). He compared alcohol abstainers, moderate drinkers and heavy drinkers. While there was an increase in sway with alcohol use in all three groups Goldberg tested, the abstainers began to show an increased degree of sway at blood alcohol concentrations (BAC) exceeding 0.050 g/dL, moderate drinkers beginning at a BAC of 0.075 g/dL and heavy drinkers at a BAC of at least 0.110 g/dL. However, there was considerable variance with some abstainers not demonstrating sway at a BAC of 0.075 g/dL, nor in moderate drinkers at a BAC of 0.100 g/dL and not in heavy drinkers at a BAC of 0.150 g/dL. Hence the degree of sway is a poor predictor of an individual being impaired.
Perrine et al. (1993) found that over 50% of drivers with zero BAC failed Walk and Turn and there was no correlation with standing on one leg with the BAC. This was in a study of 361 drivers and 119 dosed subjects.
Cole and Nowaczyk (1994) two groups of seven police officers viewing videos of 21 sober subjects reported that these experienced police officers judged 46.5% of completely sober subjects performing field sobriety tests to be impaired, again, identifying unimpaired drivers failing SFSTs almost as often as impaired drivers.
Shinar and Schechtman performed a double blind trial where police officers were not allowed to interview the subjects but could only perform the psychophysical tests (SFTSs) (2005). These subjects were given specific drugs but not alcohol. There was a placebo group. It should be borne in mind that the definition of impairment was the police officer’s interpretation without knowledge of any valid testing to prove impairment and they may have been testing for drug effect rather than impairment, as the concentrations of drug may have been too low to cause impairment.
Without the history from the subject, the false alarm rate (specificity) was 57% for those not on any drug. The sensitivity for amphetamine was only 10% (i.e. they could only identify 10% of people who had taken amphetamine) and for cannabis was 49% or, only moderate agreement at best, as they could only identify half of the people taking cannabis as actually having taken it. In other words, the tests did not work for cannabis and were of very limited usefulness for other substances.
The authors stated that the officers used psychophysical signs, such as HGN even when the Drug Evaluation and Classification Program (DECP or DRE) guidelines state that HGN is only relevant for depressants (e.g. alcohol, alprazolam) not for narcotics, stimulants, or cannabis. There are two problems with the authors’ and the DRE classification around this issue that are described in the HGN section of this paper.
The importance of blinding of police officers to the history of the suspect was demonstrated in a study by Smith et al. (2002). Seventy cases were selected in written form with the statements and physical evidence (drugs or paraphernalia found) and toxicology results being disclosed to the officers. Interestingly, the stated accuracy in specifying the drug category was 81% for cannabis, 94% for narcotic analgesics, 78% for central nervous system (CNS) stimulants and 69% for CNS depressants.
Considering that all but the stimulant categories are CNS depressants, the specificity is surprising and speaks to the lack of blinding of the officers. ​This is because it is important to note that over 80% of a medical diagnosis is made on history alone, (Summerton, 2008), and the lack of blinding of the officers to the history in this study makes the role of confirmation bias a strong contender for explaining these results.
Five more studies on the effects of alcohol on balance were located (Black 1982, Schukit 1985, Kubo 1990, Nieschalk 1999, Wu 2017). Each of these studies, while demonstrating that alcohol does worsen balance, all identified a fundamental problem in that it is essential to measure the baseline balance of an individual to determine by how much balance is affected by alcohol.
In 1982, Black et al. concluded that, “Tests repeated over a period of five consecutive days yielded results with large variances within normal limits and no systemic individual or group trends.” (Black, 1982). In other words, the balance tests were too variable to be of predictive value for the effects of alcohol on balance both from day to day and within the same day. Individuals would vary from day to day in their balance and the differences in balance between individuals were very large both with and without alcohol.
​Schuckit (1985) found that men with a family history of alcoholism swayed a little less than men without such a history when administered alcohol. However, the correlation of sway with alcohol dose, while statistically significant, was moderate at best (r = 0.64, p<0.001) (Schukit, 1985).
Kubo et al. (1990) compared nystagmus with body sway before and after alcohol use. In other words, did body sway and nystagmus correlate with each other under the influence of alcohol? While they did find a correlation between degree of sway and the appearance of positional alcohol nystagmus (PAN-1), again the r value was only 0.5 despite a p< 0.01, and this was using posturography with instruments to measure balance, as opposed to subjective observation by a police officer assessing balance. In other words, the correlation was moderate and not beyond a reasonable doubt.  
This lack of correlation may well be related to the wide scatter of data points as to how alcohol affects sway. In their words, “In contrast to PAN, determining the threshold for an increase in body sway after alcohol ingestion proved difficult, since posturographic measurements vary considerably among individuals."
Nieschalk et al. (1999) established the amount of baseline sway in sober subjects before dosing them with different quantities of alcohol (BAC 0.022-0.159 g/dL). The baseline measures of sway were little different from the amount of sway following alcohol ingestion. While it appears that having the eyes closed or open made only a small difference to the amount of sway, the correlation between the increase of both sway path and sway area was only r=0.652 (p<0.01) for sway area and r=0.545 for eyes closed, a very modest correlation. For chance, r=0.5. These correlations were even lower for eyes closed for both sway path and sway area.
This is because for a person to understand where they are, they require working proprioception. Proprioception is knowing where one is and requires seeing where one is in relationship to the environment. It also requires balance mediated through the vestibular apparatus in the ear and the ability to sense the body’s movements in space. Closing the eyes takes away one of these senses and one would expect more sway when that happens. In a sense, this is stacking the test in evaluating impairment.
Furthermore, the tests do not consider the baseline ability of the subject to maintain balance when entirely substance free. These shortcomings call into question the value of these tests as a diagnostic instrument. Nor do the DRE balance tests account for variability in tolerance to alcohol. 
Wu et al. (2017) studied the effects of alcohol on postural stability in older people (ages 68-80). They measured the amount of time a person could stand on the dominant foot and the pressure exerted by that foot on their centre of gravity. They also measured a baseline degree of ability to stand on one leg and this was worsened by alcohol. But again, the point was made: “Our findings also suggest that alcohol-related declines in older adults’ postural stability after moderate AC [alcohol consumption] may differ according to level of baseline...”
In other words, one cannot use the degree of sway as a marker of alcohol consumption without knowing how much the individual sways without consuming alcohol. This increased variability of sway with age as a potential confounding factor is not referred to as a potential source of error in the DRE training manual.
Thus it is clear that the fundamental premise that alcohol worsens body sway affecting two of the fundamental principles of DRE testing is affected by poor correlations with the amount of alcohol in the blood and by tolerance to alcohol, and, importantly, is affected by the baseline ability in maintaining physical equilibrium.
There is one further point worth emphasizing: the degree of body sway, while affected by alcohol, not only correlates poorly with BAC, but it is not proven to be related to actual driving ability in a sober person. There is a conflation of two issues here – body sway in and of itself has not been shown to affect driving. Alcohol does increase body sway in different amounts in different people, but only when this degree of sway is extreme, such as staggering or falling, can it be related to alcohol consumption. In that extreme case, the sway is a surrogate marker for impairment because
​Hence it is extremely difficult to separate the signal from the noise of these measurements because of the high degree of variability both between and within subjects.

Marijuana & Balance

The marijuana literature is also informative on the issues of marijuana and balance. Bolbecker et al. (Bolbecker, 2018) investigated sway in healthy volunteers compared to chronic cannabis users. While they found that the cannabis group had a higher degree of sway when not taking cannabis at the time of the study, they felt this was due to the peripheral effects of cannabis on skeletal muscle rather than an effect mediated through the brain. Liguori et al (Liguori, 1998) found that a modest dose of marijuana (3.95% THC) did increase body sway but that the effects on braking were “marginal” (they did not reach statistical significance).
For clarity, the subjects were swaying even without cannabis. In other words, whether due to cannabis or not, there was either a residual effect of cannabis on sway, or, they swayed a lot before smoking cannabis and this may or may not have been related to cannabis use. However, interestingly, this was interpreted to be related to the effects of cannabis on skeletal muscle and not the brain. ​
In other words, the premise that marijuana affects sway, and by inference driving ability, is predicated on its effect on the brain. This premise appears not to hold out in this study. Moreover, the change in body sway they did measure was not correlated with impaired driving on a driving simulator, further taking away from the concept that sway due to marijuana is a surrogate marker for impairment.
​In a second study by Liguori et al. (Liguori, 2003), these investigators found no effect of 2% marijuana on standing balance or brake latency, even when subjects were sleep deprived. Similarly, subjects receiving 3.5% marijuana increased body sway but not brake latency, even when subjects were sleep deprived. Finally, in a study by this group reported in 2002, they concluded:
Although field sobriety tests are often used to determine driving impairment, these results suggest that impaired balance following marijuana use may not coincide with slowed reaction time. Conversely, braking impairment from low doses of alcohol may not be revealed by tests of balance. (Liguori, 2002)
Thus, it can be concluded that while alcohol and marijuana both influence balance, it appears that alcohol’s effects are mediated through its action on the brain whereas marijuana’s may not be. Second, it is impossible to use body sway as a marker of impairment without knowing the baseline degree of sway, which can vary widely from person to person. Third, even when marijuana does impair sway, this is not reflected in standardized field sobriety tests and there is no correlation of sway to driving impairment when marijuana does impair driving.
End of Part One

​In Part Two, Dr. Rosenbloom will discuss marijuana and balance, and the psychopharmacology of cannabis and alcohol. He will examine issues in assessing driving impairment, and continue with the HGN test.

About David Rosenbloom:

Dr. Rosenbloom was formerly a Clinical Professor in the Department of Medicine, McMaster University, Hamilton, Ontario, Canada. He has served as an expert witness on various aspects of pharmacology in both civil and criminal trials. He may be reached at the following address:

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