Establishing national decision rules for forensic blood alcohol analysis in Italy: A dynamic framework incorporating measurement uncertainty and interlaboratory performance

This article introduces a dynamic framework for reporting blood alcohol concentration (BAC) by integrating measurement uncertainty and interlaboratory variation. This approach is considered necessary to ensure more consistent decision-making rules when analytical results are used in a legal context. Although these rules have been developed within the Italian context, they are adaptable to other countries as well. Included in the framework are critical challenges stemming from variability in laboratory performance and method uncertainty. Data were collected through a national interlaboratory proficiency program involving up to 50 laboratories since 2017. Robust statistical methods were applied to determine consensus values and combined uncertainties. Laboratories meeting inclusion criteria were classified into four performance tiers (Q1-Q4) based on the quartiles of the guard band (GB) distribution, derived from their own uncertainty values. GB values were used to set standardized decision limits, ensuring that the maximum false positive risk remains within 5 % across all laboratories. Results highlight significant performance discrepancies, with GB values varying widely and leading to fragmented decision limits. By clustering laboratories into quartile groups, the framework reduces fragmentation and harmonizes assessments while at the same time maintaining a transparent, performance-based classification. Additionally, targeted training programs and periodic recalibration of uncertainty and GB values promote continuous improvement and enable laboratories to advance to higher tiers as their performance improves. This approach enhances overall reliability, accuracy, and fairness of BAC assessments, and provides a scientifically rigorous approach to uniform reporting results from forensic laboratories.