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So far, studies investigating the comparability of breath alcohol concentration (BrAC) with blood alcohol concentration (BAC) have focused on the accuracy of BrAC testing instruments. The presented study, conducted with cases from the district of the Middle Hessian Police Headquarters, is to the best of our knowledge the first to compare both methods under real-life conditions in normal policing situations. For a 1-year period, alcohol-impaired drunk-driving suspects, who were by criminal procedure required to give a blood sample, were offered a voluntary, additional BrAC test with a ''Dräger Alcotest 7110 Evidential''. The BrAC test was to be administered as soon as possible after the suspect had been apprehended, without, however, delaying the collection of the blood sample. Ninety-two cases could be included in our study. In 30 cases, a blood sample was not taken; in 11 cases, a BrAC test could not be performed. In the remaining 51 cases, we found the following pairings of BrAC and BAC results: BrAC ≥ 0.55 mg/l and BAC ≥ 1.1% (n = 39); 0.25 mg/l ⩽ BrAC < 0.55 mg/l and 0.5% ≧ BAC < 1.1% (n = 5); BrAC ≥ 0.55 mg/l and BAC < 1.1% (n = 4); BrAC < 0.55 mg/l and BAC ≧ 1.1% (n = 3). The mean value for the conversion factor, Q, was 2.12% l/mg. In accord with numerous other studies, our study results would suggest a value of 2.1% l/mg to German legislature as a new statutory value for Q. In borderline cases, of which there were already 7 in our study with 51 cases, suspects could benefit both from a BrAC test or a BAC test, with the benefit lastly depending more on early testing time than on the test method used. Our results support the call for the earliest possible measurement of alcohol concentration values after a drunk driving offense was committed. In some situations, this can probably only be accomplished with BrAC testing. A supplementary blood sample and BAC testing could compensate for the known weaknesses of BrAC testing. Thus, the complementary use of both methods might be a viable option. [ABSTRACT FROM AUTHOR]