Breath-alcohol analysis as a surrogate for blood-alcohol concentration depends on assuming a constant blood/breath ratio of alcohol.

The ratio of blood-alcohol concentration (BAC) to breath-alcohol concentration (BrAC), which is commonly referred to as the blood/breath ratio (BBR), is an important concept in forensic science and legal medicine. For example, the BBR serves as the calibration factor used when a breath-alcohol test result is converted into the coexisting BAC for clinical, research, and forensic purposes. Furthermore, when legislative bodies established statutory BrAC limits for driving, they divided the statutory BAC limit by an assumed population average BBR, hence BrAC = BAC/BBR. However, jurisdictions opted to use different BBRs when calculating statutory BrAC limits for driving, and values of 2000:1, 2100:1, 2300:1, and 2400:1 were used in different countries. Under in vitro conditions, the blood/air partition ratio of ethanol can be determined with high precision (coefficient of variation CV ~2%), whereas in vivo the BBR of alcohol depends on numerous physiological factors, such as lung physiology, breathing pattern, and other biological variables; BBRs in vivo have CVs ranging from 8-12%, depending on the type of breath analyzer used. BrAC increases during a prolonged exhalation into an evidential breath-alcohol analyzer and the BBR therefore decreases as a person reaches a vital capacity exhalation. The BBR of alcohol also depends on whether arterial (A) or venous (V) blood samples were used for ethanol analysis, because A-V differences are continuously changing during the absorption, distribution, and elimination stages of the blood-alcohol curve. This article reviews the historical background and wisdom of assuming a constant BBR of alcohol for legal purposes when breath test results are used as a proxy for venous BAC. Discussion and debate about a person's BBR should be irrelevant in those jurisdictions that enforce a statutory BrAC limit for driving.