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Drug-Impaired Driving: The Contribution of Emerging and Undertested Drugs

Date Published
May 16, 2024

Impaired driving is often associated with alcohol use and frequently leads to accidents, injuries, and fatalities. According to the National Highway Traffic Safety Administration, one person was killed every 39 minutes in an alcohol-related crash in 2021.[1] But alcohol is not the only concern; the use of illicit drugs, legalized drugs such as cannabis, and the abuse of prescription medications may also impair a driver’s abilities. In 2022, an estimated 13.6 million people drove under the influence of illicit drugs during the prior year.[2]

In 2007, the National Safety Council (NSC) introduced testing scope and cutoff standardization for impaired driving cases and traffic fatalities to improve testing consistency. Since 2013, it has recommended that forensic toxicology labs regularly test blood for 35 of the most often encountered drugs and metabolites. Referred to as Tier I drugs (Figure 1), they are now included as a testing standard in many forensic toxicology labs.[3] Furthermore, these compounds can be detected and confirmed with commonly used analytical instrumentation.

Figure 1. List of Tier I and Tier II drugs. Tier II drugs can be both individually named drugs and classes of drugs (e.g., atypical antipsychotics).

NSC also created a second drug category with significant impairment potential, termed Tier II drugs. These drugs include emerging novel psychoactive substances, prescription drugs, and traditional drugs of abuse with limited or regional prevalence, many of which require advanced instrumentation for detection. Most laboratories test for Tier I drugs, but only test for select Tier II drugs when they are regionally relevant. Therefore, the frequency and the types of Tier II substances contributing to drug-impaired driving cases and fatal crashes is not well understood.

NIJ-funded researchers from the Center for Forensic Science Research and Education examined blood samples from over 2,500 driving under the influence of drugs (DUID) cases. The goal was to create a detailed picture of both Tier I and Tier II drugs that contribute to impaired driving cases and compare results to the NSC’s recommended testing scopes. Researchers also analyzed drug presence at various blood alcohol concentrations to assess the operational impact of different testing thresholds and stop limit testing.

What is Stop Limit Testing?

If a sample meets or exceeds a pre-determined blood alcohol concentration threshold, some labs will not perform any additional drug tests. This cutoff is most commonly either 0.08% or 0.10%.[4] The proscribed per se blood alcohol level in the U.S. across every state is 0.08% (except Utah, where it is 0.05%). Labs that adhere to this practice will not detect other drugs that may cause or contribute to driving impairment. 

This stop limit testing can interfere with a comprehensive understanding of drug involvement in impaired driving. Why do so many labs use it?

  • Toxicology labs have limited budgets and resources.
  • Driving impairment can be explained by the blood alcohol concentration alone.
  • A lack of enhanced penalties for drug use means there is no need to measure beyond the blood alcohol level.
  • Agencies that use the laboratories’ services have requested this limit.

National Safety Council Recommendations Are Supported

Researchers estimated the frequency with which drugs contribute to the national DUID problem by testing 2,514 cases using a scope of 850 therapeutic, abused, and emerging drugs. They examined deidentified blood samples randomly selected from a pool of suspected impaired driving cases. The samples were collected from NMS Labs in Horsham, Pennsylvania, between 2017–2020.

Of the 2,514 suspected DUID cases examined:

  • The overall drug positivity (Tier I or Tier II drugs) was 79%, nearly double the 40% positive for alcohol (Figure 2).
  • A smaller portion of cases (23%) tested positive for both drugs and alcohol.
  • Only 17% of the cases were positive for alcohol alone.
  • Naturally occurring cannabinoids experienced a statistically significant increase in positivity over the four years.

Figure 2. The frequency of cases with (a) no drugs or ethanol detected (4%), (b) ethanol detected (40%), (c) drugs and ethanol detected (23%), and (d) drugs detected (79%).

Alcohol use in combination with drugs spanning multiple categories was common, as was multiple drugs used in combination. THC (the primary psychoactive component of marijuana) was most often found with ethanol (n=359), and it was frequently found with amphetamine/methamphetamine (n=146).

Samples with a blood alcohol content of 0.08% or higher that were also positive for either Tier I or Tier II drugs occurred 19% of the time (n=478). Cases with blood alcohol content of 0.10% (the cutoff used most frequently by toxicology labs) were also positive for Tier I or Tier II drugs 17.3% of the time (n=434). This suggests that laboratories employing stop limit testing may miss many drug-positive cases.

“Limiting testing based on alcohol results precludes information of drug involvement in several cases and leads to underreporting of drug contributions to impaired driving,” said Mandi Moore, one of the researchers involved in the study.

The research supported NSC’s recommendations for Tier I and Tier II testing. Tier I drugs were found in 73% of suspected impaired driving cases while only 3% contained just Tier II drugs. This suggests that Tier I testing captures the vast majority of drug-involved DUID cases. However, some Tier II drugs (diphenhydramine, gabapentin, hydroxyzine, and two novel psychoactive substances) were found as often or more often than some Tier I drugs, potentially indicating their increased prevalence and a need to re-examine guidelines.

Study Limitations

The cases used in this analysis were exclusively from Pennsylvania. Therefore, they provide a geographically limited snapshot rather than a comprehensive characterization for the entire U.S. population. However, the sample size of over 2,500 cases was “suitable to meet the research goals outlined” by the researchers.

Because Tier II and novel psychoactive substances were found in relatively low frequencies, the researchers did not develop or validate additional confirmatory methods as they had previously planned.

Filling in the Big Picture Details

This work increases awareness of drugs that labs are less likely to test for and labs’ role in addressing the DUID problem. It also demonstrates how frequently DUID cases involve drugs other than alcohol. Although stop limit testing can be justified, data on both alcohol and drug use creates the clearest picture of DUID contributing factors. Current estimates of drug frequency in DUID cases are likely to be inaccurate and actual usage is likely to be higher than previously believed due to stop limit testing. Equipping labs with sufficient resources could encourage labs to eliminate stop limit testing.

About This Article

The work described in this article was supported by NIJ award number 2020-DQ-BX-0009, awarded to the Frederic Rieders Family Renaissance Foundation.

This article is based on the grantee report “Assessment of the Contribution to Drug Impaired Driving from Emerging and Undertested Drugs” (pdf, 26 pages), by Amanda L.A. Mohr and Barry Logan, The Center for Forensic Science Research and Education (CFSRE) at the Frederic Rieders Family Renaissance Foundation.

Date Published: May 16, 2024