The short and complicated answer to this question is that it depends. Even though the results of a roadside breath test, or preliminary alcohol screening, are often not accurate enough to meet the stringent evidentiary threshold in criminal cases, they may provide an officer with the evidence necessary to establish probable cause for an arrest.
Please note that PAS devices are not the same as those used in police stations and jails to test your blood alcohol concentration level for official purposes. In fact, significant differences make the PAS devices used in roadside testing unreliable at best.
Breathalyzers are better than PAS devices
The breath testing machines used for your official BAC level, referred to as Breathalyzers (a brand name), are considered more accurate than the PAS devices used on the side of the road. These larger machines must meet stricter standards when it comes to calibration of the machine and the use of it by an officer. The courts consider these machines as more accurate as well. They are designed to limit the potential for false positives, so the results are more often admitted as evidence in court.
Then what's the point of a PAS device?
If the Breathalyzer gives more accurate results, why do police officers use PAS devices during DUI traffic stops? Because they must establish probable cause in order to arrest you on suspicion of driving drunk, and the PAS device is a tool used to that end. However, the test is not required in order to establish probable cause if other alleged evidence exists.
The officer may not tell you that you have the right to refuse to participate in roadside sobriety tests, including a breath test. The more evidence the officer believes he or she has, the "better" the arrest. You may refuse to submit to a roadside breath test, but you could end up suffering some consequences because of it. For instance, you may lose your license for a certain amount of time, and the prosecutor may use your refusal as evidence against you in court.
If you find yourself facing the choice of whether to submit to a roadside breath test, take a moment to consider the consequences of each choice. You may find further information about your rights and legal obligations regarding this and other roadside tests by consulting the legal resources in your area. It's better to be prepared that caught unaware.