When you are stopped for DUI, the officer might ask you to blow into a portable breathalyzer machine. Depending on that reading, you may be arrested and asked to blow into a breathalyzer at the police station.

Many factors can influence the accuracy of the breathalyzer reading including whether you are diabetic, used mouthwash or regurgitated. But even if none of those many factors are present, the machine’s reading could still be a little bit off.

First, the portable breathalyzer is not considered accurate enough to be used as evidence against you, although it can provide a basis for the police to make the initial arrest.

But even the breathalyzer at the police station is not completely foolproof. In Illinois, police must perform accuracy checks of their machines within every 62 days. The machine is given two tests using a reference sample. If the machine reads within 10% of the reference sample’s alcohol content, the machine can be certified as accurate.

For defendants who are just a tiny bit over the .08 legal limit, 10% can mean the difference between a conviction and a not guilty verdict. For a .08 legal limit, the machine can range from .072 to .088 and still be considered accurate. If your breathalyzer came in at .086, you might have a fightable case.

Portable breathalyzers are also checked for accuracy every 93 days, but again, they may not be used as evidence of guilt.

If you have questions about this or another related criminal or traffic matter, please contact Matt Keenan at 847-568-0160 or email

(Besides Skokie, Matt Keenan also serves the communities of Arlington Heights, Chicago, Deerfield, Des Plaines, Evanston, Glenview, Morton Grove, Mount Prospect, Niles, Northbrook, Park Ridge, Rolling Meadows, Wilmette and Winnetka.)

About mdkeenan

A criminal and school law attorney with over 17 years of experience, I have successfully represented clients all over the Chicago area. My practice includes DUI, felony, criminal, misdemeanor, homicide, internet crime, retail theft, traffic offenses, cyberstalking, drug crimes, weapons violations, domestic battery and juvenile crime. I also represent families involving school cases. My clients come from all over the Chicago area including Skokie, Wilmette, Niles, Northbrook, Glenview, Evanston, Winnetka, Highland park, Northfield, Park Ridge, Des Plaines and Mount Prospect. I am a member of the ACLU and the Illinois State Bar Association. I serve as a volunteer for First Defense Legal Aid. Se habla espanol.
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