Long‐Term Stability of Ethyl Glucuronide in Hair: A 10‐Year Retrospective Analysis of 909 Samples by LC–MS/MS

Monitoring long-term alcohol consumption is critical in forensic and public health contexts. Hair analysis of ethyl glucuronide (EtG), a direct metabolite of ethanol, has become a standard method for detecting chronic alcohol use. While the reliability of EtG hair testing is well established for short- and medium-term analyses, its stability in hair stored over extended periods has not been comprehensively evaluated. This limitation is especially relevant in retrospective investigations, postmortem evaluations, and long-term epidemiological studies, where archived samples may be analyzed years after collection. In this study, we assessed the long-term stability of EtG in human hair stored for up to 10 years. A total of 909 samples originally analyzed between 2013 and 2022 were re-tested in 2023 using a previously published and validated liquid chromatography–tandem mass spectrometry (LC–MS/MS) method. When the results of the old and the new analyses were compared, EtG concentrations showed no significant degradation over time, with more than 80% of the samples displaying matching values when analytical uncertainty was considered. Only a small fraction of samples (4.4%) dropped below the commonly used interpretive threshold for chronic alcohol use (30 pg/mg) after 10 years of storage. These findings provide robust evidence that EtG remains chemically stable in hair under standard storage conditions over a decade, confirming the reliability of archived samples for assessing alcohol use history and expanding the utility of EtG analysis in long-term toxicological and forensic investigations. The demonstrated stability strengthens confidence in hair as a matrix for retrospective substance use evaluation across scientific disciplines.