A comparison of blood alcohol levels as determined by breath and blood tests taken in actual field operations.

Smith, Thomas J · 1972 · ROSA P / Virginia Transportation Research Council (VTRC)

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Summary

This study addresses the validity and reliability of breath alcohol testing as a legal alternative to blood testing for determining blood alcohol content (BAC) in driving while intoxicated cases. Motivated by the passage of Virginia Senate Bill 104 in 1972, which authorized breath tests as admissible evidence, the research aimed to compare breath test results against established blood test methods under actual field conditions. The study was conducted in conjunction with the Fairfax Alcohol Safety Action Project, which increased police enforcement against drunken driving. The methodology involved collecting paired data from 104 arrested drivers who submitted to both blood and breath tests. Breath tests were administered using Intoximeter Mark II devices, which utilize gas chromatography, mounted in mobile police vans. A mandatory 15-minute waiting period after arrest was enforced before breath testing. Blood samples were split between the Commonwealth’s laboratory (State Medical Examiner) and private laboratories chosen by the defendants. Statistical analysis, specifically t-tests for paired data, was used to compare the average differences between the three measurement sources: breath vs. Commonwealth blood, breath vs. private blood, and Commonwealth blood vs. private blood. The results demonstrated that breath test readings were statistically significantly lower than both blood test methods. Compared to the Commonwealth laboratory, the breath test averaged 0.00836% lower, a difference significant at the 1% level (t-value of 3.256). Compared to private laboratories, the breath test averaged 0.00721% lower, significant at the 5% level (t-value of 2.109). In contrast, no significant difference was found between the two blood testing methods (t-value of 0.613). However, the breath tests exhibited much larger standard deviations (0.02615% and 0.03487%) compared to the comparison between the two blood tests (0.01916%), indicating greater variability and less precision in breath testing. The authors attributed this variability to potential operator inexperience, calibration issues, or equipment instability in mobile vans. The significance of these findings lies in the legal implications for defendants. Because breath tests consistently yielded lower readings, they generally benefited defendants, though the high variability meant that individual results could deviate significantly from true BAC levels, particularly near legal presumptive limits. The study concluded that while breath testing is viable, the large standard error poses risks for accuracy near legal thresholds. It recommended further replication to assess whether operator training and fixed-location usage could reduce variability and improve the reliability of breath testing as a forensic tool.

Key finding

Breath test results were statistically significantly lower than both Commonwealth and private laboratory blood test results, with average BAC readings of 0.19077% for breath versus 0.19913% and 0.19798% for the respective blood tests.

Methodology

field_study

Sample size: 104

Provenance

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• alcohol