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Are Breath Tests Accurate?

Posted by attorney James Nesci

Are Breath Tests Accurate?

Keep in mind that a police-administered breath test creates an estimate of the amount of alcohol that is in your breath. Like all estimates, it can be accurate or inaccurate, depending upon the circumstance. However, breath alcohol levels do not necessarily accurately reflect blood alcohol levels. In other words, a sample of breath with a certain amount of alcohol in it may be accurately analyzed for the amount of alcohol in the specimen of breath, but it may bear little relationship to the amount of alcohol in the blood. This is because of several factors:

The Intoxilyzer 8000, which is the machine used in Arizona, is acknowledged to have up to a 10% random error factor by the government.

How hard and long you blow into the machine can results in an overestimation or underestimation of your actual blood alcohol content by as much as 15% in either direction.

Your end-expired breath temperature and core-body temperature can make a difference of 8.6% for each degree centigrade that your temperature varies from what the machine expects. Studies show that the average human end-expired breath temperature is 35 C, but the machines are set to 34 C, which means that the average subject’s breath alcohol concentration is over-estimated by 8.6%.

Hematocrit, which is the ratio of solids to liquids in your blood can have an effect of up to 14% in either direction.

Partition Ratio is the ratio of alcohol in your blood to the alcohol in your breath. The machine is programmed to make a calculation, assuming that there are 2100 parts of alcohol in your blood for every one part in your breath. Unfortunately, humans vary from 900:1 all the way up to 3400:1, but the machine does not take that into account. This means that a person with a .034 breath alcohol content could have a police-administered test result that reads as high as .080 based on this factor, alone.

All of these factors are individual, which means that they can be additive, or cancel each other out. When added together, a police-reported breath test result could be as much as 105.6% too high or 109.6% too low.

If you have any questions about this, or any other legal issue, please feel free to contact Nesci & St. Louis PLLC at (520)622-1222, or visit us on the web at

Nesci & St. Louis PLLC is a criminal-defense law firm located at 216 North Main Avenue in Tucson and is solely responsible for the content of this newsletter. This newsletter is for informational purposes only and is not intended as legal advice. No attorney-client relationship is established by the use of this information. Laws and rules change rapidly and every effort to ensure the accuracy of this information has been made as of the date of this newsletter. © 2011

Additional resources provided by the author

Garriott's Medicolegal Aspects of Alcohol, 5th Edition, Arizona DUI Defense, 3rd Edition

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