Preliminary Alcohol Screening devices, commonly referred to as PAS tests, are one of the types of field evidence police officers use in determining probable cause to make an arrest. Although officers don’t often reveal this, a person stopped for a DUI is not obligated to take the PAS test, as it is a voluntary test and is not subject to the “implied consent" laws.



The most widely used PAS device in California is the Alco-Sensor III or IV. The Alco-Sensor utilizes fuel cell technology to detect the presence of alcohol in a breath sample. This handheld device is relatively small and is powered by a 9-volt battery. It has a mouthpiece tube that the DUI suspect blows into, and a sample chamber where the air goes. It determines alcohol concentration by measuring the electrical reaction caused by alcohol oxidation.

However, PAS devices may react to alcohols other than ethyl alcohol, which is the type of alcohol that is fit for human consumption. PAS devices react to rubbing alcohol, wood alcohol and acetone. This would skew the results for some diabetics, people who are fasting, and people whose professions expose them to chemicals, like painters. The body produces acetone as a byproduct of incomplete digestion in some individuals such as diabetics whose insulin levels are not controlled or those people who are fasting.

Another major problem with PAS tests is that they do not have mouth alcohol detectors, also known as “slope detectors," which determine if mouth alcohol is present. Mouth alcohol is the undetected, raw, unabsorbed alcohol in the mouth that falsely elevates the results of the breath test. The sources of mouth alcohol may include a substance ingested prior to the breath test, a substance regurgitated or burped from the stomach, or a case of gastroesophogeal reflux, also known as GERD. Mouth alcohol creates a different pattern than a normal breath sample. If a subject has no mouth alcohol the PAS device will read a continuous, though not linear, rise in breath alcohol until it reaches a plateau. If mouth alcohol is present, there may be a significant and sudden drop. A slope detector identifies and reports this drop as mouth alcohol.

Further, there is no strict time requirement that the police officer must follow in observing the DUI suspect before administering the PAS test. The breath tests given at the police station require a 15-minute observation period where the DUI suspect has to be “continuously" observed in order to determine that mouth alcohol does not skew the BAC result. Science has proven that alcohol evaporates from the mouth in 15 minutes, so that if a person refrains from eating anything or regurgitating any fluids for 15 minutes, there will be no residual alcohol in the mouth. However, PAS tests do not have this time requirement. Thus, with no time requirement and no slope detector, BAC readings from PAS tests often report an inaccurate BAC reading, which results in the police officer making an arrest for DUI.

PAS devices must be properly calibrated and maintained to assure the accuracy of its readings. Police agencies must keep detailed records of the maintenance schedule. For example, PAS devices often have to be checked every 150 tests, or 10 calendar days. Therefore, a defense attorney should get the past records of the PAS device to check for any issues with the device and to check for a pattern of suspicious test results.