I am Joel Brand, and I defend DUI cases across California. If you were just arrested for DUI, you may have noticed that two different breath readings were recorded that night. The officer held a small handheld device up to your lips on the side of the road, and later a larger machine at the station or jail printed out a separate result. Those two numbers are sometimes different, and that gap can raise real questions about your case. This post explains what each test actually is, which one the prosecutor will rely on, and why the difference between the two readings is something worth discussing with a defense attorney.
The Two Machines Are Not the Same Thing
The handheld device the officer used at the roadside is called a Preliminary Alcohol Screening device, or PAS. The larger machine used later, usually at the police station, jail, or booking facility, is called the evidentiary breath testing instrument. California law treats these two devices very differently, and understanding that distinction is the first step in making sense of your night.
What the PAS Device Is Actually For
The PAS handheld breath test at the roadside is designed to help an officer decide whether there is probable cause to arrest you. It is a screening tool, not a final measurement. In most cases, if you are over 21 and not on DUI probation, you had the legal right to refuse the PAS test. Many people do not know that because officers are not always clear about it. If you blew into the PAS and the number was higher than what the station machine later recorded, or lower, that difference may have legal significance. You can read more about how that test fits into the stop itself in the library article on what happens in the first days after a DUI arrest.
What the Evidentiary Machine Is Actually For
The larger machine at the station is the one that produces the official result. This is the number that appears in the police report, the one the prosecutor will point to in court, and the one the DMV considers in an administrative per se suspension action against your license. California requires this test to be performed on a machine that is regularly calibrated and maintained according to specific regulations. The reading from that machine is the evidentiary breath test result, and it carries far more legal weight than anything the PAS device recorded.
Why the Two Numbers Sometimes Differ
There are several reasons why your PAS result and your evidentiary result might not match. First, your blood alcohol level changes over time. Alcohol is absorbed and then eliminated at a rate that varies by person, by what you ate, and by other factors. If there was a gap of 30, 45, or even 60 minutes between the roadside PAS and the station test, your BAC could have risen or fallen in that window. This is part of what attorneys sometimes call the rising BAC defense. Second, the PAS device may simply be less accurate than the station instrument. Third, conditions at the roadside, including temperature, residual mouth alcohol, or recent belching, can affect a PAS reading in ways they may not affect a properly administered evidentiary test. The 15-minute observation period required before the evidentiary breath test is meant to control for some of those variables. You can read more about that requirement in the library article on mouth alcohol as a defense.
Which Number Can the Prosecution Actually Use?
The PAS device is not Title 17 compliant, meaning it does not meet the California regulations that govern how evidentiary chemical testing must be performed and documented. Because of that, the roadside number generally cannot be introduced at trial as proof of your blood alcohol level. Its lawful role is narrower. It helps the officer develop probable cause to arrest you. That role still matters a great deal to your case, because if the basis for the arrest was weak, the arrest itself can be challenged, and that can put the evidence gathered afterward in question. Your attorney can examine the time stamps on both tests, the calibration logs for both machines, and the officer's observation notes. Bad calibration of the PAS device itself is sometimes a valid line of attack. See the library article on bad calibration as a DUI defense for more on that.
Can You Challenge Either Reading?
Yes, both readings can be challenged, though the legal standards differ. The evidentiary result is subject to Title 17 of the California Code of Regulations, which sets detailed rules for how blood and breath testing must be conducted, maintained, and documented. A failure to follow those rules may give your attorney grounds to question the reliability of the result. The PAS device is held to a somewhat different standard, but its calibration records are still discoverable. If your attorney files a motion to suppress, the goal may be to exclude one or both readings, or to undermine the weight the jury or judge gives them. Whether that is a realistic strategy in your case depends on the specific facts, including the time between tests, the recorded readings, and how carefully the officer documented the observation period.
What If You Only Have One Number?
Some people are surprised to learn that they only received one breath test, or that they were asked to choose between breath and blood at the station and never had a PAS at all. If the officer skipped the PAS entirely, that is actually not unusual, and it does not hurt your case. If you refused the evidentiary test at the station, that is a separate and serious matter under California's implied consent law, and it carries its own consequences. If you were taken to a hospital and a blood draw was done there, the analysis of that result follows a different chain of custody. In any of those scenarios, the absence or presence of a PAS reading changes the picture.
What the Gap Between Readings Means for Your Defense
If your PAS reading was noticeably lower than your evidentiary result, a rising BAC argument may help explain that your BAC was still climbing when you were driving and only peaked after you were already off the road. If your PAS reading was higher, it may be worth questioning whether the PAS device was properly calibrated or whether mouth alcohol contaminated that result. Either way, the gap is a data point your attorney should look at carefully. The specific facts in your case, including what you drank, when you drank it, whether you ate, and exactly how much time passed between the tests, all feed into that analysis. These are not arguments you can realistically develop on your own, which is why early involvement of a defense attorney matters. You can read more about what the overall defense process looks like in the California DUI defenses guide.
What You Should Do Right Now
Write down everything you remember about the timeline of your arrest. Note when the officer first asked you to blow into the handheld device, when you arrived at the station, and when the station test was administered. If you were told any numbers at the scene, write those down too. That information can be valuable to your attorney and may not appear clearly in the police report. Also remember that the 10-day window to request a DMV hearing is running right now, and missing it results in an automatic suspension of your license without any opportunity to contest it.
If you have questions about your breath test results or anything else from your arrest, I want to hear from you. You can get a free written case analysis right here on this page. Call me directly at (888) 271-6644. I answer my own phone, 24/7. You can also find more articles about what happens after a DUI arrest at more from the DUI blog.