The Blood/Breath Partition Ratio in a DWI Case

by Jamison Koehler on August 4, 2010

The science behind the Intoxilyzer 5000EN and other breath test machines all sounds very impressive.  The problem is that the machines are based on certain assumptions about the person being tested.  If the assumptions are faulty, so are the results.

One of the major assumptions used in breath testing is that there is a correlation of 1 to 2100 between the amount of alcohol in a person’s exhaled breath and the level of alcohol in the person’s blood.  (Breath alcohol is used as a surrogate measure for blood alcohol concentration.)  This so-called “partition ratio” is based on averages derived from scientific studies.

The problem is that there is no such thing as an average person.  Lawrence Taylor and Steve Oberman quote one DWI attorney as pointing out that, based on simple averages, his client must be female, Chinese, and dead.  Statistically, there are more women than men, more Chinese than any other nationality, and more dead human beings than living ones.  On this basis, the breath test machine would work great if the defendant were female, Chinese, and dead.  There are serious questions of reliability – certainly enough to generate reasonable doubt – when the defendant is anyone else.

According to scientific studies cited by Taylor and Oberman, the actual ratio of breath to alcohol in any given individual can vary from 1:1300 to 1:3000.  Consider the implications of this.  If the defendant’s actual (but untested) partition ratio was really 1:1500, the breath test machine could, by my calculations, overestimate the blood alcohol concentration by as much as 40%.  In other words, a reading showing that the defendant’s breath alcohol concentrations exceeded the legal limit could really be based on blood alcohol concentrations far below the limit.

Yes, a greater percentage of people will be closer to the 1:2100 ratio used by the breath test machine to estimate blood alcohol concentrations.  But should we be convicting people on the basis that there is a greater than average chance that they have violated a law?  How about a 75% chance that the breath test results are accurate for this particular individual?  This is precisely what the nation’s per se DWI laws do.

If you add in other faulty assumptions, to say nothing of problems with calibrating, maintaining and operating the equipment itself (more on all of these later), you are certainly adding in enough uncertainty to qualify as reasonable doubt.

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