Challenging Breath and Blood Tests in a DUI

Challenging Breath and Blood Tests in a DUI

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Challenging Breath and Blood Tests in a DUI at trial

Robert L. Miller

Many people charged with a DUI, and even many lawyers that are not familiar with DUI defenses, don’t know that you can even challenge a breath or blood test in a case. Our Orange County DUI Lawyers do in every case, but often, part of a DUI Defense Lawyers job at a trial is to explain to the jury, and judge, what defenses there are to a breath test, or what defenses there are to a blood test, and how that helps show reasonable doubt in a DUI case at trial.

On this page are several challenges at trial that apply to DUI trials that are usually missed by other attorneys.

1. Failure to object to the admissibility of the preliminary alcohol screening (PAS) device.

A PAS is simply a hand held, portable breath test.  Under California law, a PAS device result is not admissible to show the blood alcohol level as evidence of intoxication.  It can only be used to show the presence of alcohol, not the amount or result.

The California Supreme Court has ruled that a PAS test can be excluded as a result or limited as to how it can be used.  Lawyers often in court will concede the admissibility of the PAS testing in a motion to challenge the probable cause for the stop (a motion to suppress evidence), or worse, filing to raise the issue of probable cause to arrest in a motion because their clients PAS result was above a .08% blood alcohol level.

California courts have not set forth an evidentiary exception to the basic rules of evidence = that the burden of proof for the admissibility of the scientific evidence was on the prosecution.  In our work examining the defenses available to a breath test in a DUI case, we have yet to see an officer who can testify to the manufacturer of their PAS device (or a machine run in PAS mode, instead of evidentiary mode), and what the California rules under Administrative Title 17 are regarding the maintenance of PAS devices, and whether or not they have been followed.

2.  Failure to obtain maintenance and calibration information and records for the Alcosensor IV or other breath testing device.

The breath machine used in Orange County DUI cases is just that – a machine.  Machines are subject to margins of error, like any measuring device, and can break, or malfunction, or otherwise not work properly.  It is essential for any DUI lawyer to obtain these records in every DUI case that involves a breath test.  Any problem with the breath machine can result in the suppression of the breath test results, which can in turn lead to dismissal of the entire case.  It is a big deal.

3.  Failure to argue that the blood alcohol level at the time of driving was below a .08%.

This argument is known as a retrograde extrapolation or backwards relation argument.  The breath test result is always rebuttable.  The law only makes the blood alcohol level at the time of driving relevant.  And, because the blood or breath is never taken while the person was driving, the timing, and margins of errors, are extremely important.  An expert can, in many cases, testify that based on the time the alcohol was consumed, that the actual Blood alcohol level while driving was below a .08%..


While the prosecution always tries to make the breath or blood test unassailable, there are several avenues to be used in challenging Breath and Blood Tests in a DUI.  A good DUI defense lawyer can pursue those to show a jury reasonable doubt.

If you have questions for a DUI Defense Attorney in Orange County, call our firm at (877) 568-2977 anytime.  We are here to help.


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