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Preliminary Breath Testing in a Michigan DUI Case

Typically, if you are found guilty of drunk driving after an arrest at a DUI stop in Michigan, your sentence will include some type of probationary period. During this time, you may be ordered to refrain from alcohol and drug use, which can be monitored through a variety of different means. For example, some probation officers may require you to take a preliminary breath test, also called a PBT, to check for alcohol use.

What is the PBT?

The preliminary breath test is taken on a small, hand-held device that is designed to be easily portable. Police officers will often use the PBT during roadside drunk driving investigations to gather the probable cause needed to initiate an arrest.

Much like the evidentiary breath test, the portable breath test device requires a breath sample. When you blow into the machine, a fuel cell oxidizes any alcohol that is present on your breath, which alters the electrical current in the device. Through this process, the machine is supposed to calculate your blood alcohol content (BAC).


The main problem with the preliminary breath test machine is that it can produce highly inaccurate BAC readings. In fact, the results of a PBT that was given during a roadside OWI investigation are not permissible as evidence in trial because the machines are known to produce unreliable results.

Additionally, other molecules besides beverage alcohol can cause the fuel cell to oxidize. This means that a PBT can create false results indicating you’ve consumed alcohol, when in reality you have not.

What does this mean if you are on probation and are required to submit to a PBT? A flawed test result could lead to a probation violation, which in turn can trigger a number of penalties including potential jail time.