This study aims to explore the mechanisms behind external contamination in hair drug testing. Specifically, it investigates how external contamination can be detected, quantified, and differentiated from internal drug use. Ultimately, the study addresses whether contamination continues to pose a significant concern in forensic toxicology.

Hair drug testing is commonly used in forensic toxicology to detect long-term substance abuse, offering the advantage of a prolonged detection window for drugs and metabolites. A major challenge in hair analysis is external contamination, which occurs when drug residues from environmental exposure or direct contact with substances like cocaine or cannabis are deposited onto the hair. This can lead to false-positive results, complicating test interpretation. The debate continues on whether external contamination is a real concern or just an artifact of testing. Some argue that proper washing and metabolite-to-parent drug ratios can distinguish contamination from internal use, while others believe contamination obscures the true source of the substances detected.

The study analysed hair samples from individuals with occupational drug exposure and judicial cases where drug use was suspected. These were tested using validated methods for detecting substances like cocaine, amphetamines, and opiates. The hair was segmented, washed with dichloromethane, water, and methanol to remove external contamination, then dried, pulverized, and extracted with acidified methanol using ultrasonic assistance. The extracted drugs were filtered, evaporated, reconstituted, and analysed using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

In the study, 96% of hair samples from individuals with occupational exposure to drugs tested positive for external contamination. Even after multiple washes with dichloromethane, water, and methanol, contamination remained, indicating that external sources may significantly contribute to the drug levels detected in hair samples. One notable case involved a hair sample from an individual who had a single environmental exposure to cocaine. This sample showed a concentration up to 292pg/mg of cocaine, suggesting that even a single exposure to a drug could lead to detectable contamination in hair.

Among 596 judicial hair samples tested, 53% exhibited signs of external contamination. In 6.7% (12 samples) of these contaminated cases had more than 10% of the drug concentration remaining in the final methanol wash, pointing to the predominance of external contamination. In these cases, the presence of metabolites and additional corroborative biological tests (such as urine or blood tests) helped support the hypothesis that drug ingestion had occurred. However, in 3% of cases with significant external contamination, there was insufficient additional evidence to confirm internal drug use. This issue was particularly apparent in cases involving amphetamine and MDMA, substances that lack distinctive metabolites that could definitively indicate internal drug use.

Despite the application of comprehensive washing protocols, completely eliminating contamination from hair samples remains a challenge. This study emphasizes the importance of interpreting hair drug test results carefully, considering not only the washing procedures used but also the presence of metabolites, which can help distinguish internal drug use from external contamination. Relying solely on cut-off values without accounting for the possibility of contamination could lead to misinterpretation, especially in laboratories with limited experience in hair analysis. The findings indicate that while external contamination is a real concern, its effects can be mitigated through the use of comprehensive testing protocols and careful interpretation of results in the context of additional biological and case-specific information.

Future research should focus on standardizing contamination detection methods across laboratories to reduce variability and improve consistency in results which will also help laboratories with less experience in hair analysis avoid misinterpretations that could have serious legal and forensic implications.