Its part of the reasonable suspicions that they look for before giving the breathalyzer test. Slurred speech, bloodshot eyes, etc. That will then lead to the field sobriety tests. And based on his observations of how you perform them, he could find that he has probable cause for an arrest. He doesn't even have to give you the roadside breathalyzer. In fact, the numerical results of the roadside breathalyzer isn't even admissible in court other than to give probable cause for an arrest. While reasonable suspicion allows an officer to temporarily stop and detain a motorist in order to investigate further if the officer thinks the motorist may have committed a crime, an officer must meet the higher standard of probable cause before making an arrest. Probable cause simply means that an officer has enough evidence to believe a motorist has probably committed a crime, thus justifying his or her arrest. In the context of a DUI stop, an officer could have probable cause for an arrest after administering a field sobriety test and/or a breath test if the results point to probable intoxication. https://dui.findlaw.com/dui-arrests/what-is-reasonable-suspicion-for-a-dui-stop-.html In court, challenging an arrest based on lack of probable cause can be an uphill battle. Most officers are good at piecing together enough evidence to establish probable cause. And courts are often reluctant to second-guess an officer’s probable cause determination. https://www.nolo.com/legal-encyclopedia/reasonable-suspicion-and-dui-arrests.html It's all available online. And I guaran-damn-tee you that if the conversation you and the officer has gives him reasonable suspicion, it very well will not be excluded from court. What's going to get a person in trouble or not is when they take them down to the station and administer the official breathalyzer test from the calibrated machine. Everything before that is just the reasons the officer had that lead him to believe you had probably committed a crime and needed to be breathalyzed.