A preliminary alcohol screening (PAS) test result that is lower than a DUI breath test taken later can be a sign of rising blood alcohol. And rising blood alcohol can be a DUI defense in certain situations.
A PAS test is a breathalyzer test on a handheld device that is usually performed after a person has been stopped on suspicion of DUI--but before s/he has been arrested. Officers use these tests to decide whether or not to arrest someone for DUI.
A DUI breath test, on the other hand, is a desktop test usually administered at the police station, after the suspect has been arrested for DUI--and often a significant amount of time after the PAS test.
How can this form the basis for a DUI defense? Because of "rising blood alcohol" or "rising BAC." Rising blood alcohol is exactly what it sounds like: the fact that your blood alcohol rises steadily for a certain period of time (anywhere from half an hour to several hours) after you stop drinking. A DUI breath test is often administered several hours after you drove a vehicle. If your blood alcohol was rising in this period, it is possible that you were over the legal limit for the test--but not at the time you drove!
PAS test results that are lower than DUI breath test results are useful for proving that your blood alcohol was on the rise. To give an example, let's say a woman is pulled over after leaving a party for a short drive home. A PAS test administered by the roadside shows that she has a BAC of 0.08%. She is arrested for DUI, transported to the police station, and given a DUI breath test. The breath test shows that she has a BAC of 0.09%.
Clearly, this DUI defendant's blood alcohol was rising between her PAS test and her DUI test. This means that it may have been below 0.08% (the legal limit) at the time she drove.
California law presumes the validity of any DUI breath test administered within three (3) hours of an arrest. This means that the burden is on you to prove rising blood alcohol as a defense (something that usually requires testimony from an expert witness). But PAS test results that are lower than your DUI breath test results are very helpful for this purpose.