Ethanol as a psychoactive substance and the importance of external quality assessment (EQA) in clinical and forensic laboratories

Background: Ethanol is a widely consumed psychoactive substance with significant forensic and public health implications. The accurate determination of blood alcohol concentration (BAC) is critical in both clinical and forensic settings, necessitating rigorous quality control measures. External Quality Assessment (EQA) programs play a fundamental role in standardizing analytical performance across laboratories. This study evaluates the effectiveness of the EQA program in monitoring and improving BAC determinations among diagnostic laboratories in the Lombardy region from 2020 to 2024. Methods: A total of 58 ethanol-spiked whole blood samples were analyzed by participating laboratories over a five-year period. BAC was quantified using Headspace Gas Chromatography-Mass Spectrometry (HS-GC–MS) and Headspace Gas Chromatography-Flame Ionization Detection (HS-GC-FID). Statistical assessments included calculation of the coefficient of variation (CV), z-scores, and interlaboratory comparisons following ISO 13528 standards. Results: The study demonstrated a statistically significant reduction in BAC measurement variability over time ( p = 0.0008). While most results complied with forensic guidelines, variability was higher in samples with BAC near the legal threshold of 0.5 g/L, with 49.3 % of measurements exceeding and 50.7 % falling below the threshold. Methodological differences between HS-GC–MS and HS-GC-FID were noted, although both techniques remained within acceptable performance limits. Conclusions: EQA programs confirmed to improve BAC measurement reliability. However, interlaboratory variability, particularly around forensic cutoffs, highlights the need for further methodological harmonization. Future efforts should focus on refining calibration protocols, integrating advanced quality assurance strategies, and expanding proficiency testing at the national level to ensure consistent forensic and clinical BAC determinations.