How reliable is breath testing in determining a person's blood alcohol level?


The answer to this question is extremely long and the subject of great debate. Breath testing is used by law enforcement personnel for its ease and convenience, not its accuracy. Further discussion of this matter may be found at FAQ4.

Reliability depends upon the ability of the machine to accurately measure the presence of breath alcohol. Breath alcohol supposedly reflects the level of alcohol in a person's blood. Alcohol in the blood, of course, produces a depressive effect upon the central nervous system, leading to intoxication. However, even a perfectly functioning Intoxilyzer 5000 (the breath machine Texas uses) has problems producing reliable results as to intoxication.

A couple of simple facts illustrate some deficiencies in the machine:

1) The machine strictly assumes that your breath and blood have a direct ratio of 2100:1. That means an equal amount of alcohol will be found in 2100 ml of breath and 1 ml of blood from the same person. Human beings' true ratios range from under 1000:1 to over 3000:1. The 2100:1 assumption helps some people and hurts others. Any given person's result could be inaccurately skewed by as much as 50%, depending on the person's true breath to blood ratio. The problem of breath to blood ratios in and of itself is good reason to cast serious doubt upon the reliability and accuracy of breath testing, and no make or model of breath test machine corrects the problem.

2) The machine also strictly assumes your breath temperature to be 34 degrees Celsius, which is about 93 degrees Fahrenheit. If your breath temperature is higher than that, your test result will be higher than your true level. The converse is also true. Newer machines can adjust for this inaccuracy, but the machine Texas uses does not.

This website is not meant to be exhaustive, so consult a qualified attorney for further information to this broad question.

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*All answers are for people 21 years or older, do not involve enhancements, are not exclusive, and are limited to Texas.

**This page is for informational purposes ONLY and must not be relied upon as legal advice because it is NOT a substitute for the advice of a qualified attorney, nor does it establish an attorney-client relationship.

Not certified by the Texas Board of Legal Specialization.

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