Dilemma of a Constant Blood/breath Ratio of Ethanol in Chemical Test Evidence of Intoxication

The notion of a constant blood/breath ratio of ethanol for all subjects under all conditions of testing continues to cause litigation and debate when results of breath analysis are used as evidence of impairment. A person's apparent blood/breath ratio depends on a host of factors that cannot be adequately controlled in forensic practice. Among others, the source of blood analyzed; artery, capillary or vein, the sampling site, the subject's breathing technique and the phase of an exhalation captured for analysis will contribute to variations in the blood/breath ratio. Whether the concentration of alcohol in blood is rising or falling at the time of testing is also an important consideration. The magnitude of random errors in the methods of alcohol analysis in specimens of blood and breath, and any systematic bias in instrument calibration will seriously distort the calculated blood/breath ratio. This paper explores some problems and solutions to the use of a constant blood/breath ratio of ethanol in traffic law enforcement.