BAC Breath Tests – How they Work


In Wisconsin, the BAC (Blood Alcohol Content) limit for driving is 0.08. If you are caught operating a motor vehicle with a BAC level at or above this limit you can be charged with an OWI. But how exactly do police test BAC levels? In this article we will explain the most common tools used to test blood alcohol levels and how they work.

PBT – Preliminary Breath Test

A PBT (short for Preliminary Breath Test) is a handheld device that police use at the scene to estimate a person’s BAC level. It is not an accurate tool as it measures a person’s mouth alcohol level rather than their blood alcohol level. Results from a PBT are generally not admissible as evidence of a person’s intoxication at the time of driving. Instead, officers use PBTs to establish probable cause that a person has in fact been drunk driving, in order to make an arrest for OWI. In order to prove a drunk driving offense in court, an officer has to get a more accurate result using a different test.

EBT – Evidentiary Breath Test

If an officer suspects you of drunk driving, and gathers enough preliminary evidence to arrest you for the offense, they will then take you to a testing facility (either a police station or hospital) to test your true BAC level at the time of driving. They do this by taking a blood, breath, or urine sample. Blood and urine samples are taken and chemically analyzed in a lab to measure the amount of alcohol in your system. The third method, the breath test, is measured by a breath-testing machine (such as the Intoxilyzer). Depending on the exact machine used, this method tests BAC levels by using infrared spectroscopy, or by chemical reaction in the breath. These tests are much more accurate than the PBT given at the scene, and are admissible in court.


Although the EBT is admissible in court, they are far from flawless. There are multiple factors that can skew your BAC result and make it seem like you are more intoxicated than you actually are. For example, if you suffer from acid reflux (or GERD) the test may detect an inaccurate amount of alcohol. Similarly, breathing patterns can impact the test negatively. If you are nervous and hyperventilating during the test, you may not get a perfect result. If you are charged with OWI and you believe your breath test was inaccurate, a defense lawyer can investigate your case and defend your rights.