This study draws on findings from the European Union Drugs Agency (EUDA) project on hair testing initiated in 2023, and incorporates insights gained from the author's 12 years of expertise in hair analysis. The conference aims to assess the current proposed cut-off values in the field of international hair testing era and to address the challenges of applying these thresholds across different exposure scenarios, including regular, occasional, and single drug use.

A total of 100 drug users were recruited in France as part of the EUDA project. Participants provided consent, completed an online questionnaire, and submitted hair sample segmented into proximal (0–6cm) and distal (6–12cm) sections. Samples were decontaminated, extracted and analyzed by LC-MS/MS and LC-HRMS to screen for traditional drugs of abuse (DoA) and new psychoactive substances (NPS). The questionnaire focused on substance use timelines (<1month, <6months, >6–12months, or never) and frequency (days of use within the past month). Self-reported data were compared with hair concentrations using thresholds (LOQ: 1pg/mg) and Society of Hair Testing (SoHT) cut-offs. The presence of metabolites was also considered. Occasional or regular use were categorized based on hair concentrations, applying international guidelines, the interquartile range approach, or insights from scientific literature.

Cocaine interpretation is presented as an example. Among 78 participants self-reporting cocaine use in the past 12 months, all tested positive using the LOQ approach, but 46% (36 participants) were classified as negative under the SoHT cut-off (500pg/mg for cocaine). Cocaine concentrations for these 36 participants ranged from 6 to 495pg/mg (median: 146pg/mg; mean: 186pg/mg; 1st quartile: 70pg/mg; 3rd quartile: 302pg/mg). Two up to 5 metabolites (benzoylecgonine BZE, norcocaine NC, OH-cocaine, anhydroecgonine methyl ester AEME, cocaethylene CE) were detected in 86% (31 participants) while only one metabolite alongside cocaine was detected in the 5 remaining participants (4 with BZE, 1 with AEME). Additionally, 18 participants who reported never using cocaine had concentrations below 500pg/mg, with up to four metabolites identified. Overall, 96% of the participants tested positive for cocaine, while only three had cocaine concentrations below 20pg/mg and no detectable metabolites.

Cocaine concentrations around 200pg/mg with at least 2 metabolites (excluding BZE/AEME combinations) are strongly indicative of active cocaine use. In contrast, concentrations below 100pg/mg, with no more than two metabolites (typically BZE/AEME and occasionally norcocaine), may point to potential external contamination. However, such cases should be interpreted with caution, considering the broader context and supporting evidence.

On the other hand, single-day exposure to certain DoA and NPS within the last month prior to sampling resulted in very low concentrations detected in hair: MDMA (48pg/mg), 2-CB (8pg/mg), and 3-MMC (3pg/mg). While the MDMA case would result in a negative report if the SoHT cut-off (200pg/mg) was applied, no cut-offs have been officially proposed for NPS.

Key factors influencing interpretation include drug use frequency, metabolites indicating active consumption, elimination kinetics post-abstinence, and external contamination. Although current cut-offs are designed to minimize false positives, they frequently result in false negatives, especially for occasional use. Concentrations below these thresholds may indicate infrequent consumption or drug presence due to dormant hair. However, lowering cut-offs to address these limitations is challenging due to the persistent risk of external contamination.

The present study underscores the need for a more nuanced approach to hair analysis incorporating lower thresholds, metabolite verification, and segmental analysis to accurately capture drug use patterns. It demonstrates the importance of balancing sensitivity and specificity in hair analysis to minimize the risk of misinterpreting drug use.