Most defendants in drunk driving cases are charged with two offenses: (1) driving under the influence of alcohol and (2) driving with a blood-alcohol level in excess of .08 percent ( the so-called per se statute.) the prosecution usually offers the test result as evidence of driving impaired as well as proof of the per se violation. A presumption of being impaired may be shown based on the results of the test. That evidence is, however, a rebuttable one — and one of the factors that should be used in rebuttal is the question of individual tolerance.
Scientific studies have repeatedly confirmed the existence of variability in individual tolerance. In a study reported in Sullivan, Hauptman, and Bronstein, Lack of Observable Intoxication in Humans with High Plasma Alcohol Concentrations, 32 Journal of Forensic Sciences 1160 (1987), for example, hospital patients with a history of alcohol abuse were clinically observed and tested after consuming significant amounts of alcohol. The researchers found essentially no correlation between the patients' BACs and their assessed levels of intoxication. This lack of correlation was attributed to the high tolerance developed from long-term alcohol abuse. See also Redmond, Alcohol Blood Levels, 16 Annals of Emergency Medicine 374 (1987), in which 20 patients studied in a detox unit of an English hospital showed no ill effects after being admitted with BACs above the lethal limit.
In another illuminating study, researchers investigated the effect of intoxication on memory. The study found that increased blood-alcohol levels generally impaired the individual's ability to recall — but went on to find that the relationship between an individual's BAC and his physical performance was not significant. The scientists concluded that BAC as determined by breath analysis was a poor predictor of a person's ability to perform; in theory, this was due to individual differences in absorption and metabolism rates. Maylor and Rabbit, The Effect of Alcohol on Rate of Forgetting, 91 Psychopharmacology 230 (1987).
Research at the Alcohol Research Center at the University of Colorado confirms that the blood-alcohol level of an individual is a poor measure of intoxication due to individual tolerance. The following December 20, 1983, release from the University's Public Information Office summarizes the results of that research:
Defining a "drunken driver" by measuring blood alcohol levels gives little indication of how impaired the individual's judgment or physical responses actually are, according to studies at the Alcohol Research Center at the University of Colorado, Boulder.