top of page

Chemical Tests in DUI Cases

In most DUI cases the evidence used against the defendant will include the results of a Blood Alcohol Concentration test (BAC). BAC tests include breath, blood, and urine tests that measure the amount of alcohol a person has in his her blood at the time the test is administered. The following is a review of chemical tests in relation to DUI crimes.

Note: If the driver refuses to take a BAC test when requested to do so by a police officer then the driver will automatically lose his or her privilege to drive for at least a year.


Breathalyzers: Breathalyzers are the most common type of devices used to measure BAC in DUI cases. There are two different types of breathalyzers:

  • The Preliminary Alcohol Screening (PAS) device, which is a small portable device used by officers at the scene of a DUI stop that measures the amount of alcohol on the driver’s breath. The PAS device is a fairly inaccurate machine that is not used as evidence against the driver in a court of law, but rather, it is used to assist the officer in establishing probable cause to believe the driver is under the influence of alcohol. If that probable cause exists, then the officer will arrest the driver for suspected DUI and the driver will be taken to a police station to take a more accurate breathalyzer test. PAS device are subject to human error by way of improper administration, lack of proper machine maintenance, and many other factors.

  • The breathalyzer test, which is a non-portable device located at the police station. This is not a PAS device and it is considerably larger in size and more accurate than a PAS device. The results of a breathalyzer may usually be used as evidence against the driver in most, but not all, circumstances. There are many defenses to either the admissibility of the breathalyzer evidence, or to the reliability of the evidence, including, the the fact that breathalyzer is not an indicator of the driver's breath at the time of driving, human error in administration of the breathalyzer, lack of proper maintenance or procedure of the machine which leads to unreliable test results, and more.

Note: Before evidence results from a breathalyzer test can be used against a driver in a DUI case the district attorney must show that the driver was lawfully arrested, upon probably cause to believe that the driver was DUI, that the breathalyzer was properly maintained and calibrated, and that the test was administered shortly after the driver was witnessed driving. 

Blood Samples: A blood test can also be used to measure the driver's BAC in a DUI case. The blood is usually drawn from the driver at the jail or hospital. The DUI blood test is generally the most accurate of the different tests; however, this does not mean that blood tests are full-proof or that they are 100% accurate. Common errors do occur. For example, the vile that stores the driver's blood is partially filled with preservatives and anti-coagulants that are intended to preserve the blood evidence; however, these chemicals  can cause false fermentation (the blood produces its own alcohol) If the lab-tech does not mix together the chemical property. Improper refrigeration of the blood sample can cause the same effect.

Note: Blood draws, sampling, and analysis can only be performed by a certified lab-tech. It might come as a surprise that an increasing number of lab-techs are not properly certified to handle blood. Where this is the case, the credibility of the blood results is diminished substantially. Lab-techs are supposed to be trained on how to take samples of blood so as not to create false positives of the presence of alcohol. For example, if the phlebotomist uses an alcohol-based antibiotic to clean the skin at the area where he or she draws the blood then the blood results could be flawed due to the presence of the alcohol-based antibiotic cleanser mixing with the blood sample.

It is also important to remember that a DUI blood test only shows what the BAC was at the time the test was performed. If the blood test is performed long after a person is arrested for DUI, then it is possible for the BAC levels to have risen from the time of arrest to the time that the blood is tested. It is also important to remember that a person can be forced to take a blood test on an arrest for an accident DUI with injury.

DUI Defense: Scientific evidence of a high BAC after a DUI arrest is generally strong evidence against the driver, but it is not fatal to a DUI case. Sometimes the the BAC evidence is subject to wild inaccuracies due to human error, mechanical error, procedural error, or factual circumstance. A DUI attorney will review the evidence to look for these errors in an effort to defend the driver at both the DMV hearing and at the DUI criminal court.

Lastly, remember that it does not matter what the driver’s BAC is if the officer did not have a valid reason for stopping the driver in the first place. Officer must have probable cause to believe the driver is DUI before any arrest is made. Besides the PAS test, breathalyzers and blood tests are taken only after a driver is arrested. Therefore, if the arresting officer did not arrest with probable cause, all of the BAC evidence must be excluded. Also remember that chemical tests are only one part of the DUI defense analysis, a comprehensive defense will include a review of the field sobriety tests (FST), witness statements, dash-camera evidence, injury reports (in accident DUI cases), and more.

For more information on chemical tests used in DUI cases, including PAS devices, breathalyzers, blood tests, and urine tests, contact our DUI and criminal defense lawyers today for a free consultation. Our DUI Lawyers have successfully handled hundreds of drunk driving cases in San Bernardino County and our consultations are free are free. We are available every day of the week to assist you. Call today!


Criminal Defense Lawyers
San Bernardino County
Free Consultations
Se Habla Español


All Misdemeanor & Felonies

  • ​Carjacking

  • Welfare Fraud

  • Criminal Threats

  • DUI

  • Evading Police

  • Embezzlement

  • Theft

  • Voluntary Manslaughter

  • Murder

  • Robbery

  • Unlawful Taking of Vehicle

  • Prostitution

  • Loitering for Prostitution

  • Pandering

  • Pimping

  • Sexual Battery

  • Battery (Simple)

  • Assault

  • Assault with a Deadly Weapon

  • Branding a Firearm

  • Felon in Possession of a Firearm

  • Driving without a License

  • Interlock Ignition Device Tampering

  • Loitering for Drug Activity

  • DUI Boating

  • DUI with Injury

  • DUI Commercial Driver

  • Involuntary Manslaughter

  • Looting

  • Vandalism

  • Burglary of a House

  • Burglary of a Business

  • Shoplifting

  • Arson

  • Reckless Burning

  • Price Gouging

  • Underage DUI

  • Domestic Battery

  • Inflict Corporal Injury to Spouse

Criminal Defense Lawyers PC Misdemenor Felony Jail Prison Sentence Probation San Bernrdino Rialto Colton, Redlands, Victorville, Highland Hesperia, Yucaipa, Ontario, Rancho Cucamnga, Fontana, Colton, Loma Linda, Upland, Adelanto

San Bernardino County

Redlands, Yucaipa, Highland, Hesperia, Victorville, Upland, Big Bear, Ontario, Loma Linda, Grand Terrace, Yucaipa, Rancho Cucamonga

bottom of page