Concentration units used to report blood- and breath-alcohol concentration for legal purposes are important to consider when blood-breath ratios of alcohol are calculated and compared between countries: re-evaluation of a German study with Alcotest 9510 DE evidential instrument

Abstract

This article re-evaluates the results of a controlled drinking study done in Germany to determine inter-subject variation in the blood-to-breath ratio (BBR) of alcohol. Statutory blood-alcohol concentration (BAC) limits for driving in Germany are 0.50 g/kg (administrative offence) and 1.1 g/kg (criminal offence). Mass/mass concentration units (g/kg) are 6 % lower than mass/volume (g/L) units, because the density of blood is 1.06 kg/L on average. The corresponding statutory breath-alcohol concentration (BrAC) limits for driving in Germany are 0.25 mg/L and 0.55 mg/L, respectively. BAC in road traffic cases is determined indirectly by the analysis of serum and dividing by 1.236, which underestimates the true BAC. Using Alcotest 9510 DE evidential breath analyzer, the mean ± SD, median and range of BBRs of alcohol were 2047 ± 150, 2053, and 1571-2394 and 61% were less than 2100:1. After re-calculating BAC assuming a serum/blood distribution ratio of ethanol of 1.14:1 and reporting results in mass/volume units, the corresponding BBRs were 2220 ± 162, 2226, and 1703-2595 and 21% were less than 2100. Care is needed when the results of German studies of the BBR of alcohol are compared and contrasted with studies done in other countries.