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DWI/DUI Defense

What is a partition ratio and how does it affect a breath test?


I just got arrested for DWI. What should I do?
How does a DWI investigation work?
The breath test said I had a blood alcohol content over the legal limit. Do I have a defense?
What is a partition ratio and how does it affect a breath test?
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I cannot afford to have my license suspended or revoked. Is there anything I can do?
Beware the cheap lawyer!

Did you know that a breath test could overestimate the blood alcohol content of approximately one out of four people?

A breath measures the amount of alcohol in a person’s breath, not in the person’s blood. The breath test estimates the blood alcohol content by assuming that there is 2100 times as much alcohol in the person’s blood than in the person’s breath. Thus, for a given amount of breath, the test measures the amount of alcohol present and multiplies by 2100. This ratio of the amount of alcohol in a person’s blood versus the amount in the person’s breath is known as a partition ratio.

The problem is that not everyone has the same partition ratio. If you have a partition ratio less than 2100:1, then the breath test will overestimate the amount of alcohol in your blood. The Missouri Department of Health believes that the breath test overestimates the blood alcohol content in approximately 3% of cases. The training materials given to law enforcement mention a couple of studies referencing this figure.

The problem is that there are other studies from well-respected scientists studying partition ratios showing that a partition ratio of 2100:1 may overestimate the blood alcohol content in approximately 22.8% of cases (see Kurt M. Dubowski, Absorption, Distribution and Elimination of Alcohol: Highway Safety Aspects, 10 J. Stud. Alcohol Suppl. 98-105 (July 1985)). In this study, Dr. Dubowski found that the partition ratios are normally distributed with an average ratio of 2280:1 and a standard deviation of 241.5. Statistically, then, 22.8% of the population would have a ratio less than 2100:1.

To see what difference this makes, consider that one out of twelve people (one person in the jury pool) would likely have a partition ratio of 1946:1. If this person blew a 0.085, he would be arrested and charged with a DWI. However, if we adjust for his partition ratio, his blood alcohol content should only be 0.079, which is legal.

This is one potential defense to a breath test among many. If you have been arrested or charged with a DWI, you need to find a lawyer who understands the science behind DWI investigations and can identify and help you assert any defense you may have. Contact us for a free consultation.