The courts have recently decided that single numerical values for blood and breath test results without a measure of uncertainty are inadmissible as they preclude objective interpretation.

Alcohol is one of the most widely tested chemicals in forensic laboratories, and the numerical test results carry probative weight as evidence. Many jurisdictions have imposed statutory law defining alcohol intoxication as having a blood alcohol concentration (BAC) of 0.080g/dl or greater. Because of this, forensic toxicologists are frequently faced with the question “What was the BAC of the defendant at the time he/she was operating the vehicle?” The problem with answering this question is that the BAC is typically measured at a time long after the incident occurred and does not reflect the alcohol level at the time in question. Therefore, to answer this, toxicologists must extrapolate the BAC at the time in question. Alcohol extrapolations are typically conducted using empirically derived formulae which account for the manner in which the alcohol is metabolized. Alcohol metabolism consists of three key processes (absorption, distribution, elimination) which occur simultaneously in the human body and each affecting how much and how fast alcohol accumulates in the blood stream.

Alcohol metabolism is well understood, but highly variable amongst the human population.^1,2 Forensic toxicologists must have a keen understanding of these variable factors and incorporate them during the extrapolation and interpretation of the BAC results. To accomplish this, forensic toxicologists typically rely on population averages for the rate of absorption, manner of distribution, and rate of elimination. Extrapolated BAC results, then, are typically reported as singular numerical values which are compared to the statutory limit of 0.080g/dl by the courts to determine guilt or innocence. Recently, however, the National Academies of Science (NAS), in their report Strengthening Forensic Science in the United States:A Path Forward, recognized that “methods for measuring the level of blood alcohol in an individual...can do so only within a confidence interval of possible values.”³ By continuing to report BAC results as a single numerical value without accounting for the uncertainty associated with the metabolic process and calculations precludes objective interpretation of, and testimony concerning, the result which interferes with the quest for truth in the courtroom and the jury’s ability to execute justice. In the last few months, courts have recognized this and have decided blood and breath test results without a measure of uncertainty are inadmissible under Evidence Rules (ER) 702 and ER 403.^4,5 With regard to ER 702, the court found:

If an expert testifies that a particular blood alcohol content measurement is value A, without stating a confidence level, it is this court’s opinion that the evidence is being represented as an exact value to the trier of fact... [and] that presenting to the trier of fact the result of a blood test as an exact numerical value without stating a confidence level, is not generally acceptable in the scientific community and misrepresents the facts to the trier of fact... This court holds that the result of the blood test in this case is not admissible under ER 702 in the absence of a scientifically determined confidence level because it misrepresents the facts and therefore cannot be helpful to the trier of fact.⁴