DUI Breathalyzer Accuracy

Breath analysis is by far the most commonly used method of testing for blood alcohol in DUI cases. While not as accurate or reliable as blood tests, it has generally been regarded as acceptably accurate for use in DUI investigations, if administered correctly. This regard is beginning to erode, however. An example of this is the comment of Dr. Michael Hiastala, Professor of Physiology, Biophysics and Medicine at the University of Washington:

Breath testing, as currently used, is a very inaccurate method for measuring BAC. Even if the breath testing instrument is working perfectly, physiological variables prevent any reasonable accuracy.... Breath testing for alcohol using a single test method should not be used for scientific, medical or legal purposes where accuracy is important. [Hlastala, Physiological Errors Associated with Alcohol Breath Testing, 9(6) The Champion 19 (1985).]

A number of scientists who have conducted studies of breath-alcohol analysis have concurred with Dr. Hlastala in concluding that breathalyzer accuracy is inherently unreliable. Thus, for example, one study determined that breath readings vary at least 15 percent from actual blood alcohol levels. (Simpson, Accuracy and Precision of Breath-Alcohol Measurements for a Random Subject in the Postabsorptive State, 33(2) Clinical Chemistry 261 (1987)). Furthermore, at least 23 percent of all individuals tested will have breath results in excess of true blood-alcohol levels. The author concluded that, "[g]iven the choice, it would seem that if a conclusion is to be made about the BAC of a random subject, especially when the conclusion can have serious consequences, it would be far preferable to make it on the basis of a direct [blood] measurement...."

In another study, conducted by members of the toxicology section of the Wisconsin State Laboratory of Hygiene, only 33 percent of the breath test results correlated with corresponding blood tests. Reported in 32(4) Journal of Forensic Sciences 1235 (1987), the study involved a survey of 404 actual drunk driving cases in Wisconsin in which DUI defendants had been tested on a Breathalyzer (either the Model 900 or 900A) as well as by blood analysis. The two tests were considered to correlate when there was a difference of .01 percent or less.

One interesting aspect of the study was that in 11 of the DUI cases, the defendant was shown to be intoxicated using one of the tests but not intoxicated when using the other.

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