Abstract
Background
This study examines trends in delta-9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) positivity rates in pre-employment urine drug screenings at a single university-based hospital occupational medicine clinic from 2017 to 2022, following California’s recreational cannabis legalization in 2016, with sales beginning officially on January 1, 2018.
Methods
Retrospective analysis of 21,546 de-identified urine drug screenings from 2017 to 2022 was conducted. Initial screening used instant urine drug immunoassays (50 ng/mL cutoff for THC-COOH), followed by confirmatory gas chromatography-mass spectrometry (15 ng/mL cutoff). Positivity rates were calculated annually, and descriptive statistics summarized age and gender distributions among positive cases. The Cochran-Armitage trend test assessed temporal trends.
Results
Of 21,546 screenings, 92 (0.44%) were THC-COOH-positive. The positivity rate increased from 0.12% (4/3,215) in 2017 to 0.94% (45/4,784) in 2022 (Cochran-Armitage, Z = 5.19, p < 0.001), a 683.33% relative increase (absolute change: 0.82%). Among positive cases, 76% were aged 20–39 (mean: 29.00 years, SD = 9.5), and the female proportion rose from 25.00 to 62.22%. Median THC-COOH levels ranged from 60.00 to 176.50 ng/mL (overall: 145.0, IQR: 309.5).
Conclusion
THC-COOH positivity rates increased significantly post-legalization, potentially influenced by increased cannabis use. Urine tests detect past use, not impairment, highlighting the need for impairment-focused testing per Assembly Bill 2188 (2024). Missing total sample demographic data limit age/gender subgroup analyses.
Keywords: Cannabis, Drug screening, Occupational medicine, Recreational cannabis legalization, THC-COOH
Introduction
Pre-employment drug screening for cannabis has been a longstanding practice in many U.S. workplaces, with federal drug testing programs formally established in the 1980s. A single study has found that cannabis-positive employees face higher risks of turnover, accidents, injuries, and disciplinary actions compared to non-users [1]. In California, pre-employment cannabis screening has been widely implemented for decades, particularly in safety-sensitive industries and healthcare settings. Despite evolving legal and societal norms, urine drug screening remains standard practice in many states, with the majority still allowing pre-employment cannabis screening in some capacity [2].
Tetrahydrocannabinol (THC), the psychoactive component of cannabis, impairs reaction time, coordination, and cognitive function, with effects varying based on method of consumption, frequency of use, and THC potency [3]. However, standard urine drug tests detect the non-psychoactive metabolite delta-9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH), which can remain in the body long after psychoactive effects have subsided, meaning a positive test does not indicate current impairment [3].
Cannabis is the most commonly used federally illegal substance in the U.S., with 18% of adults reporting past-year use in 2020, with increased prevalence in states with legalized recreational cannabis [4]. Cannabis use is associated with health risks, including respiratory, cognitive, and psychiatric impairments [3, 5, 6]. The legalization of personal cannabis use and cultivation for adults aged 21 and older in California was established with the passage of Proposition 64 in 2016 [7, 8]. Additionally, the proposition established regulatory, licensing, and taxation frameworks for legal cannabis use, with subsequent retail sales beginning in 2018 [4, 7]. Without a control group, changes in positivity rates cannot be directly attributed to legalization.
This study examines trends in THC-COOH positivity rates in pre-employment urine drug screenings at a single university-based hospital occupational medicine clinic from 2017 to 2022, post-recreational cannabis legalization. The primary aim was to assess temporal trends in positivity rates. Secondary aims included describing positive case demographics and THC-COOH levels, though missing total sample demographic data limit subgroup analyses. Pre-employment drug screening remains standard in safety-sensitive industries, using multi-panel tests to detect substances including the non-psychoactive cannabis metabolite delta-9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH), as well as other drugs [3, 9].
Materials and methods
Data collection
Pre-employment urine drug test results non-negative for delta-9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) from 2017 to 2022 were extracted from scanned paper records at Loma Linda University Occupational Medicine Clinic and de-identified for analysis. The clinic primarily serves healthcare workers (e.g., nurses, medical technicians, resident physicians, janitorial staff, administrative personnel) employed or seeking employment at the affiliated university hospital, and a smaller number of community workers from safety-sensitive (e.g., transportation, construction) and non-safety-sensitive (e.g., retail, clerical) industries, reflecting its role as a regional occupational health provider. Records were transferred to an Excel spreadsheet and cross-checked for accuracy. Each screening represented a unique individual, verified by clinic drug screen records.
Initial screening used instant urine drug immunoassays during pre-employment physicals, with a 50 ng/mL cutoff for THC-COOH per U.S. Department of Health and Human Services (HHS) standards [9]. Non-negative samples underwent confirmatory testing via gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS) at an HHS-certified laboratory, using a 15 ng/mL cutoff. Confirmed positive tests were certified by a Medical Review Officer (MRO). The testing protocol remained consistent from 2017 to 2022, with no changes until after January 1, 2024, when Assembly Bill 2188 prompted exploration of impairment-focused testing methods [10]. The number of screenings varied annually, as shown in Table 1.
Table 1.
Summary of pre-employment drug screening data (2017–2022)
| Year | THC-COOH-Positive Cases | Total Screenings | Positivity Rate (%) | Among Positive Cases | |||
|---|---|---|---|---|---|---|---|
| Mean Age (Years, SD) | Male (% Year) | Female (% Year) | Median THC-COOH (ng/mL, IQR) | ||||
| 2017 | 4 | 3,215 | 0.12 | 28.00 (2.2) | 3 (75.00%) | 1 (25.00%) | 60.00 (18–105) |
| 2018 | 6 | 3,240 | 0.19 | 30.30 (7.8) | 4 (66.67%) | 2 (33.33%) | 176.50 (41–3773) |
| 2019 | 15 | 3,372 | 0.44 | 31.70 (10.5) | 11 (73.33%) | 4 (26.67%) | 165.00 (17–512) |
| 2020 | 10 | 2,756 | 0.36 | 25.10 (6.8) | 5 (50.00%) | 5 (50.00%) | 66.50 (18–245) |
| 2021 | 12 | 4,179 | 0.29 | 27.70 (6.3) | 5 (41.67%) | 7 (58.33%) | 151.50 (33–317) |
| 2022 | 45 | 4,784 | 0.94 | 29.70 (11.8) | 17 (37.78%) | 28 (62.22%) | 151.50 (77–640) |
| Total | 92 | 21,546 | 0.44 | 29.00 (9.5) | 50 (54.35%) | 42 (45.65%) | 145.0 (309.5) |
Age gender, and THC-COOH level data apply to positive cases only
For positive cases (n = 92), the age distribution (continuous, in years) was described using means, standard deviations (SD), and medians, while the gender distribution (males, females) was described using proportions. Due to missing information about age and sex in the overall sample, these analyses were restricted to positive cases, and no positivity rates by age and sex were computed. Total screening counts were extracted from billing data.
Statistical analysis
This retrospective study analyzed de-identified data stored in a password-protected database. Descriptive statistics summarized positive case counts, annual positivity rates, and THC-COOH levels (ng/mL) by year. For positive cases (n = 92), age (continuous, in years) and gender were described using means, standard deviations (SD), medians, and proportions. Due to missing total sample demographics, analyses were restricted to positive cases, and no subgroup-specific positivity rates were computed.
The Cochran-Armitage trend test evaluated the temporal trend in THC-COOH positivity rates from 2017 to 2022, applied to binary test outcomes (positive vs. negative) across all 21,546 screenings. The test was conducted with p < 0.05 indicating significance. P-values are reported precisely (e.g., p < 0.001, p = 0.083). Exploratory descriptive analysis examined the proportion of positive cases by gender, limited by small counts. Spearman’s rank correlation assessed relationships between median THC-COOH levels and years (2017–2022) among positive cases. All analyses used SAS® OnDemand for Academics.
Ethical considerations
As a secondary analysis of de-identified data without human subject interaction, this study was exempt from Institutional Review Board (IRB) approval per retrospective research guidelines. Documentation of exemption status is available upon request.
Results
Table 1 provides an overview of yearly positivity rates, age, gender distribution, and THC-COOH levels. From 2017 to 2022, 92 THC-COOH-positive cases were identified out of 21,546 pre-employment drug screenings, yielding an average positivity rate of 0.44% (92/21,546). The positivity rate increased from 0.12% (4/3,215) in 2017 to 0.94% (45/4,784) in 2022, a 683.33% relative increase (absolute change: 0.82%; Cochran-Armitage, Z = 5.19, p < 0.001). Positivity rates declined to 0.36% (10/2,756) in 2020 and 0.29% (12/4,179) in 2021.
Among positive cases, 70 (76%) were aged 20–39 (median: 27.00 years, range: 16–65; mean: 29.00 years, SD = 9.5). The female proportion among positive cases rose from 25.00% (1/4) in 2017 to 62.22% (28/45) in 2022, with 50 males (54.35%) and 42 females (45.65%) overall. Descriptive analysis showed no clear gender-specific trends, limited by small counts and missing denominator demographics.
THC-COOH levels (median: 145.0 ng/mL, IQR: 309.5) varied across years, with medians ranging from 60.00 ng/mL in 2017 to 176.50 ng/mL in 2018. Spearman’s rank correlation showed no significant trend in median THC-COOH levels over years (ρ = 0.37, p = 0.47). The proportion of positive cases with THC-COOH levels > 100 ng/mL was highest in 2018 (66.67%) and 2022 (60.00%).
Discussion
This study found a significant increase in delta-9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) positivity rates in pre-employment urine drug screenings at a single university-based hospital in California, from 0.12% in 2017 to 0.94% in 2022. Without a control group, changes in positivity rates cannot be directly attributed to legalization [7]. The average positivity rate during the study period was 0.44%, with 76% of positive cases aged 20–39. The proportion of females among positive cases rose, potentially reflecting shifts in cannabis use or workforce demographics, though missing total sample demographics limit interpretation. A potential trend in THC-COOH levels was suggested, but conflicting findings and small sample sizes warrant caution. Nationally, Quest Diagnostics reported a 30% increase in cannabis positivity in safety-sensitive workforces (1.0% in 2018 to 1.3% in 2022), based on a press release due to limited peer-reviewed sources [11]. This aligns with our findings and suggests that the observed rise may reflect broader trends in cannabis use following recreational legalization in California under Proposition 64 (2016) [1, 7]. Additionally, legalization has been associated with a ~ 10% increase in workplace injuries among 20–34-year-old workers in U.S. states where recreational cannabis is legal, including an 8.4% rise in injury rates per 100 workers, underscoring ongoing safety challenges [12].
Positivity rates declined in 2020 and 2021 (0.36%, 0.29%, respectively), possibly due to reduced cannabis use, particularly via inhalation, stemming from concerns about lung health during the COVID-19 pandemic [13]. The surge in 2022 (0.94%) may reflect increased cannabis accessibility, though this could not be directly assessed.
Urine drug tests detect non-psychoactive THC-COOH, indicating past cannabis use up to weeks prior but not workplace impairment [3, 14]. Blood testing for active delta-9-THC is more accurate for recent use and correlates with driving deficits but is invasive [15, 16]. Oral fluid testing detects use within 13.5–30 h but does not confirm impairment [17, 18]. Given these limitations, the American College of Occupational and Environmental Medicine recommends prohibiting cannabis use in safety-sensitive jobs for at least 8 to 12 h before reporting to work [2], while the Occupational & Environmental Medical Association of Canada advises refraining from use for at least 24 h prior to safety-sensitive tasks to mitigate risk—acknowledging that no validated test for impairment currently exists [19]. California’s Assembly Bill 2188 (effective January 1, 2024) prohibits employment discrimination based on non-psychoactive metabolite tests, rendering urine testing largely incompatible with state law, except in federally regulated sectors [10]. Unlike California, where Assembly Bill 2188 (2024) restricts urine testing for non-psychoactive metabolites, most U.S. states lack such legal restrictions, allowing its continued use in workplace policies [2]. Post-AB 2188, our clinic began transitioning away from traditional urine testing for non-federally regulated roles, exploring impairment-focused methods like oral fluid testing and real-time assessments, while continuing urine testing for federally regulated sectors. As of 2025, the clinic has not implemented oral fluid testing but is pursuing compliance with AB 2188.
Future research should evaluate the impact of AB 2188 on workplace drug testing, the efficacy of impairment-focused testing methods, and the role of workforce demographics in positivity trends. Multi-site studies with comprehensive demographic and impairment data are needed to enhance generalizability. Exploring real-time impairment assessments and the influence of cannabis potency or societal attitudes could further inform occupational health policies.
Limitations
Data from one clinic may not generalize to other settings. The lack of positivity rates by occupation limits occupational subgroup analyses. The absence of pre-2017 denominator data and a control group prevents direct attribution of positivity rate changes to legalization. Due to missing total sample demographic data, age and gender findings may reflect workforce demographics rather than use patterns, and no subgroup-specific positivity rates were computed. Urine tests detect past cannabis use, not impairment, limiting safety conclusions [3]. Explanatory variables such as cannabis potency, societal attitudes, and occupational differences (e.g., healthcare vs. transportation workers) may explain observed trends but were not assessed. The impact of Assembly Bill 2188 (effective January 1, 2024) could not be evaluated due to the study’s 2017–2022 scope. Potential post-COVID shifts in cannabis use patterns after 2024 could not be assessed.
Conclusion
In conclusion, the statistically significant but practically limited rise (0.82%) in THC-COOH positivity rates suggests increased cannabis use post-legalization, though urine tests detect past use, not impairment or workplace use [3]. Employers in California must adopt impairment-focused testing to comply with Assembly Bill 2188 (2024), which prohibits employment discrimination based on non-psychoactive metabolite tests, except in federally regulated sectors, while other states may continue using urine tests [2, 10].
Acknowledgements
None.
Authors’ contributions
Akbar Sharip prepared the majority of the manuscript, conducted the data analysis, preparing data table, refining manuscript the structure, and enhancing the clarity of the paper. Roshan Razavi made initial contributions to the document by assisting with manuscript organization. Both authors reviewed and approved the final version of the manuscript.
Funding
No funding was received for this study.
Data availability
The datasets analyzed during the current study are available from the corresponding author upon reasonable request.
Declarations
Ethics approval and consent to participate
This study used de-identified data and was exempt from IRB review per retrospective research guidelines. Documentation is available upon request.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets analyzed during the current study are available from the corresponding author upon reasonable request.