Materials and methods

A total of 100 consecutive blood samples submitted to the Institute of Legal Medicine in Munich for solely BAC analysis in November 2022 were retrospectively examined for PEth and EtG.

The samples were collected in the federal state of North Rhine-Westphalia and included cases of driving under the influence (DUI) as well as other offenses suspected to involve alcohol impairment. Upon delivery to the Munich laboratory, the blood samples were aliquoted with permission from the Ministry of the Interior of North Rhine-Westphalia (LZPD NRW) and the state authorities.

One aliquot of each sample was analyzed for BAC at the Institute in Munich according to the German forensic guidelines. Four independent measurements were conducted using both gas chromatography with a flame ionization detector and enzymatic (alcohol dehydrogenase, ADH) methods, and the arithmetic mean of the four concentrations was, after converting from g/L plasma to g/kg whole blood (‰), reported as the BAC [10].

A second aliquot of each sample was frozen to -80 °C and sent cooled to the MVZ Medizinische Labore Dessau Kassel GmbH, a laboratory in Dessau for quantification of PEth and EtG. Samples were stored at − 80 °C after arrival until analysis. Quantification of PEth and EtG was performed with two different LC-MS/MS methods. Both LC-MS/MS methods were accredited according to ISO 15,189 and ISO 17,025 and were used for clinical and forensic routine samples. Measuring range for PEth was between 10 ng/mL and 5,000 ng/mL with a lower limit of quantification (LLOQ) of 9.1 ng/mL. The measuring range for EtG in whole blood was between 0.6 and 5,000 ng/mL with a cutoff of 1 ng/mL Samples above the measuring range were tested again after dilution.

General sociodemographic data such as sex, age, offense type, and vehicle category were extracted from the accompanying police request forms submitted with the samples.

To prevent potential re-identification of cases, BAC values were grouped. In Germany, several thresholds apply to DUI. A BAC of ≥ 1.1 g/kg constitutes a criminal offense regardless of observable signs of impairment and results in the loss of the driver`s license. For BACs between 0.5 g/kg and 1.1 g/kg, a criminal offense may apply if the driver shows signs of impairment or alcohol-related driving errors; otherwise, the act is considered an administrative offense. When BACs exceed 1.6 g/kg, offenders are required to demonstrate their fitness to drive.

The dataset included 13 cases of suspected drinking after driving, in which two blood samples of the same individual were collected at intervals of 30 to 40 min. In these cases, the first sample was used for this evaluation, with one exception, where BAC determination failed in the first sample due to sample quality. In this case, the second sample was used for evaluation. The remaining 13 samples were excluded, resulting in 87 blood samples included in the analysis.

Ethics statement

Ethical approval for this study was granted by the Ethics Committee of Ludwig-Maximilians-University of Munich, Germany (approval number 22–0664). The use of forensic blood samples was additionally approved by the Ministry of the Interior of North Rhine–Westphalia. The analysis was performed in accordance with the Declaration of Helsinki and all relevant guidelines and regulations. Individual consent to participate was not required.

Statistical analysis

All data were anonymized prior to analysis and evaluated using standard spreadsheet software (Xact, Hamburg 2004), Microsoft Office LTSC Professional Plus, excel Version 2408 (Microsoft® Excel® LTSC MSO (Version 2408 Build 16.0.17932.20602) 64 Bit), and IBM SPSS Statistics, Version 29.0.0.

Descriptive and explorative statistics like cross tables for frequencies and calculations of median, minimum, maximum, and mean were performed depending on variable scale. Pearson correlations coefficients between continuous variables are independently calculated and presented in addition. Scatterplots are used to show the common distributions of BAC values with EtG and PEth values, respectively; bar charts are used for sample number description and frequencies of PEth groups among BAC groups, and boxplots are utilized to visualize BAC and EtG value distributions for PEth groups.