On the Possibility of retrograde calculation of blood alcohol concentrations below 0.15‰

It is widely recognised in forensic medicine that retrograde extrapolation of blood alcohol concentrations (BAC) is generally avoided once the ethanol concentration falls below 0.15‰. In Germany, retrograde calculation is commonly applied to estimate a minimum BAC in favour of the subject and a low elimination rate of 0.1‰/h is legally accepted; however, expert practice typically refrains from back-calculation when the measured BAC is below 0.15‰ ("Rückrechnungsverbot"). This pragmatic threshold is rooted in historical concerns about uncertainty near the end of the BAC curve and the increasing influence of analytical and kinetic variability at very low concentrations. A re-analysis was conducted using datasets from four controlled drinking trials (n = 87), in which measurements were continued until complete ethanol elimination. The analysis demonstrated that a linear relationship of ethanol elimination persists below the 0.15‰ threshold and that elimination kinetics remain largely unaffected by the type of alcoholic beverage consumed or by the concentration intervals examined (0.3-0‰; 0.6 - 0.3‰; 0.9 - 0.6‰; cmax-0.6‰; cmax-0.9‰). Algorithms were developed to perform regression analyses across these distinct segments of the blood alcohol concentration curve and to derive parameters for an overall linear relationship from cmax to 0.0‰. The arithmetic mean of the average elimination rates - both across different beverage types and across the respective BAC intervals - was approximately 0.13‰/h. Within the 0.3-0‰ segment, all mean elimination rates exceeded 0.11‰/h. The integrative algorithmic analyses further indicated that the slope representing ethanol elimination below the 0.15‰ threshold averaged approximately 0.083‰/h, remaining relatively close to the conventionally applied minimum rate of 0.1‰/h. Based on these findings, an optimised rule for retrograde extrapolation was developed. The modified extreme model for BAC values ≤ 0.15‰ allows a simple and practicable estimation of minimum BAC by applying the low elimination rate of 0.1‰/h after subtracting a corrective time adjustment dependent on the initial measured BAC. To ensure forensic reliability, initiation of retrograde extrapolation from a conservative lower threshold of 0.06‰ is recommended. Hence, as an alternative strategy for calculating the minimum BAC at the time of the incident, it is proposed that, for measured BAC values between 0.06 and 0.14‰, a one-time deduction of 43 min be subtracted from the conventional retrograde calculation time interval, after which back-calculation may proceed using an elimination rate of 0.1‰/h. Retrograde extrapolation should then be omitted entirely for measured BAC values below 0.06‰.