The frequency of apparent acetone in a group of breath alcohol data: statistical treatment and forensic implications

The potential presence of interfering substances (specifically acetone) is a concern in the forensic reporting of evidential breath alcohol analysis. As a result, manufacturers have designed instruments to monitor its occurrence through various hardware and software features. This paper is a retrospective study where 35,945 duplicate breath samples from BAC Verifier DataMaster instruments are evaluated for the frequency of 'interferant' values > or = 0.010 g per 210 l ethanol equivalent. A total of 264 (0.74%) of the duplicate samples had an interferant value on the first sample only, while 235 (0.66%) had interferant values on the second samples only. A total of 77 (0.21%) of the duplicate samples had in interferant values on both breath samples and only in these cases could the presence of measurable acetone even be considered. The occurrence of interferant results appeared also to be instrument-dependent with 55.7% of the interferant values on the first breath sample occurring on nine (13%) of the instruments displaying such results. The occurrences of interferant values on the first breath sample did not conform to the Poisson distribution (P < 0.0001) for the instrument with the largest number of occurrences, while there was conformance for other instruments evaluated. Finally, approximately 23 cases (0.064%) remained where the presence of acetone is a possible consideration. Several issues are presented that the forensic scientist should consider when attempting to explain an apparent interferant result in an individual case. It should be remembered that measurement results need to be interpreted in their context, and data analysis concerning an instrument's performance should be considered.