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Understanding when breath tests are inaccurate

For many drivers, roadside sobriety tests can be an aggravating experience. Not only is it difficult to deal with false accusations about your behavior, but the demand that you submit to a test that has a history of problems with accuracy may tempt you to become defiant or to even attempt to refuse a test. This is generally a bad idea because implied consent laws provide for stiff penalties even if you later prove to be sober after refusing the test. The important thing to understand before participating in a test is why it might be inaccurate. That way, you can work with your attorney to mount a solid DUI defense.


One of the most common issues with breath tests is that they are not properly calibrated. When the test is expecting the temperature of the outside air to be drastically different, then the machine can provide inaccurate readings. Officers need to be sure they calibrate the test before administering it, otherwise the results are useless.

Acetone and other contaminants

The other reason why many breath test readings are inaccurate is because of contaminants. Handheld breath-testing units do not just measure the amount of ethanol in a breath sample before drawing conclusions. They measure what are called methyl group compounds, which include not only acetone but also 70 or 80 other compounds found in human breath.

Normally, properly calibrated breath tests will filter out a baseline amount of these contaminants since they are known to be present, but there are factors that can push this estimate out of range and cause faulty readings:

  • Medical issues that affect the chemicals in your breath, such as diabetes
  • Exposure to methyl group compounds without drinking, such as when you handle them at work

Other medical causes

Breath testing relies on assumed averages to be able to calculate the blood alcohol content in a test subject's blood. If the subject's blood volume and other vital signs diverge too far from the average, accurate readings become impossible. This is one of the ways experienced attorneys can build a DUI defense along with the other information above.

If you are arrested

Remember, the implied consent law makes refusing tests a bad idea. Go along with the officer's requests for testing, but ask for an attorney as soon as you know you are under arrest. Then, it is a good idea to wait until you have more solid legal advice to move forward.

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