The Variability of the Blood/Breath Ratio and Its Impact on the Results of Breath-Alcohol Analyses: A Rejoinder

Abstract

The Journal of Chemical Education (JCE) recently published a letter to the editor by Dominick Labianca that was critical of methods of breath alcohol analysis because of biological variability in the blood/breath ratio (BBR) of alcohol (J. Chem. Educ. 2023, 100, 4166). His opinions deserve further comment and discussion. As background information, breath-alcohol instruments are used for two main purposes: (i) as a roadside screening test of driver sobriety and (ii) evidential quality instruments are used to generate evidence for prosecution of traffic offenders. Variations in the BBR are irrelevant in jurisdictions that enforce a statutory breath-alcohol concentration (BrAC) limit for driving, such as 0.08 g/210 L in USA. Such legislation makes it unnecessary to convert BrAC into the coexisting blood-alcohol concentration (BAC) for legal purposes. Variability in the BBR is explained by various physiological factors, such as lung function and the volume of breath discarded before a sample is captured for analysis. Moreover, BrAC is more closely related to ethanol concentration in arterial blood rather than cubital venous blood, which is the specimen used for analysis in forensic casework. In those jurisdictions that might convert BrAC into the coexisting BAC, the use of a 2100:1 BBR as a calibration factor underestimates the true venous BAC in the majority of cases.

Concentration units used to report blood‐ and breath‐alcohol concentration for legal purposes differ between countries which is important to consider when blood/breath ratios of alcohol are compared and contrasted

“…The use of this conversion factor tended to underestimate the true BAC, which gave a certain advantage to a suspect compared with if a sample of blood had been analyzed, and this was considered acceptable from a legal standpoint [21]. Numerous blood-versus breath-alcohol correlation studies, published over many years, have shown that the BBR of alcohol when tests are done in the postabsorptive phase of the blood-alcohol curve is closer to 2300:1 or 2400:1, thus underestimating BAC by about 10%-15% [22,23].…”

“…The BBR of alcohol is not a constant factor, but varies during a prolonged exhalation and also in relation to the time after drinking when blood and breath samples are taken for analysis [22,30]. A lot depends on the volume of breath a person exhales before a sample is captured for analysis of the ethanol content.…”