Comparative regression analysis of concurrent elimination-phase blood and breath alcohol concentration measurements to determine hourly degradation rates.

Following the introduction of limit values for blood alcohol and breath alcohol concentrations of 0.5 g/kg and 0.25 mg/L, respectively, as provided under s. 24 a of the German Road Traffic Act the question is whether also breath alcohol concentrations can be back calculated to the time of the traffic offence in cases where it is definite that the person to be examined is in the period of alcohol elimination. To this end, a study was performed in which 56 healthy volunteers consumed 0.5, 0.8 and 1.0 g of ethanol mixed with fruit juice per kilogram of body weight over a period of 10-20 min. Calculations included all 391 pairs of concurrent blood alcohol and breath alcohol concentration values obtained after 2 h following the end of drinking. All volunteers exceeded the peak value of the alcohol curve. The measured values included were above 0.1 g/kg and 0.05 mg/L. For an average intake of alcohol of 0.88 g/kg the following regression lines were calculated for the period starting 2 h after the end of drinking: blood alcohol concentration [g/kg] = 1.318 - 0.172 h and breath alcohol concentration [mg/L] = 0.589 - 0.079 h. Subtracting the simple standard deviation from the mean value yielded hourly degradation rates above 0.1 g/kg and above 0.05 mg/L, respectively. Subtracting two standard deviations, the values fell below this level in both cases. In fact, back calculation of breath alcohol concentrations based on 0.05 mg/h seems to be possible for traffic offences if certain conditions are complied with, such as the use of Evidential 7110, a calibrated breath alcohol analyser approved by the Federal Physical-Technical Laboratory for measuring the breath alcohol concentration.