Rencher Law Group, P.C.
Areas of Practice

Rencher Law Group, P.C.
1438 Market Street
Dolan Building - 1st Floor
San Francisco, CA 94102
Phone: (415) 877-4486




Rencher Law Group, P.C., Attorneys, San Francisco, CA

DUI Evidence


There are two forms of evidence used to prove that a person drove under the influence of drugs or alcohol (DUI): clinic-administered chemical tests and field sobriety tests. Chemical testing at the police station is mandatory, since holding a drivers’ license is “implied consent” to the tests. However, field sobriety tests, including the preliminary alcohol screening breathalyzers, are optional. Since the tests are not perfectly reliable, they form the chief defenses for those accused of DUI.

Clinic-administered Chemical Tests

Blood Test: A person is guilty of DUI in California if he or she has more than a 0.08 blood alcohol content (BAC). Therefore, seemingly the best way to prove this is by directly testing the blood. The chief problem is that blood cannot be drawn by an unqualified police officer at the scene of the arrest and outside of a hygienic jail health clinic. If blood is drawn at the jail clinic, then there will be a lapse in time, during which the BAC could drop or rise depending on when and how much alcohol was consumed. Since it is impossible to measure blood at the actual time of driving, clinic-administered chemical tests are bound to be not fully reliable. Furthermore, although a blood test is accurate and can detect the presence of a wide range of drugs, it is possible to taint the sample through administrator error.

Urinalysis: Another clinic-administered chemical test is the urinalysis. The problem with urine testing is that it lags behind the actual BAC. The main confounding factor is the time elapsed since the subject last went to the bathroom. As a rule, the bladder contains the average alcohol content since the last voidance. If the driver hasn’t voided for a long time, then quick shots of alcohol just before driving will not show up in the urine. On the other hand, alcohol consumed hours before driving may show up in the urine even after the BAC has gone back down below 0.08. As a result, multiple urine tests should be taken up to several hours after the arrest to track changes and interpolate back to the driving period.

Evidentiary Breath Test: Breathalyzers come in many varieties. The more accurate models are big, heavy, and kept at the police station. They are called evidentiary breath tests, because their accuracy can help show proof beyond a reasonable doubt (the standard of proof required in court), as opposed to mere probable cause to arrest. However, they still only indirectly measure the actual alcohol content of the blood. After all, the law does not define a DUI by the amount of alcohol in the breath.

Breathalyzers are inaccurate if there is the presence of breath spray, mouthwash, stomach gases from digesting sugars, vomit, acid reflux, high body temperatures and fevers, hyperventilation, holding breath, degradation over time, diabetes, asthma inhalants, and so on. In addition, people vary in their “blood-breath ratio,” e.g., someone with a low blood-breath ratio who tests high could actually be below a 0.08 BAC. The temperature of the breathalyzer itself can affect results. The composition of the blood varies between males and females, which affects test results. These confounding factors can throw evidentiary breathalyzers into doubt.

Field Sobriety Tests

Whereas chemical testing aims to establish evidence of a BAC greater than 0.08, field sobriety tests provide some evidence of impairment, that is, the inability to drive a car with standard caution and prudence. Also, refusal to submit to a field sobriety test, which is a driver’s right in California, may be admissible to show a guilty consciousness. However, the main use of field sobriety tests is to provide the probable cause needed to arrest – the lowest legal standard of proof.

Field sobriety tests encompass a wide range. With the Nystagmus test, the officer looks for jerks in the eye as it follows a pen. Another test is standing on one leg with arms out to the side counting out loud. The walk and turn test tests the arrestee’s ability to walk heel to toe on a painted line. The officer may tell the suspect to bring the index finger to the nose with eyes closed. The Rhomberg balance test requires suspects to tilt their head back for 30 seconds. Most of these tests look for shaking, tremors, tension, swaying, and statements made during tests which show intoxication. They also give the officer a chance to observe other standard signs of intoxication such as breath odor, demeanor, complexion, clothing, and unusual behavior.

Preliminary alcohol screening (PAS) breathalyzers, which are light and handheld, are generally less accurate than station breath testers, but they can be used almost contemporaneously with driving. This gives them the advantage of reliability. Because little time has elapsed between operating a motor vehicle and testing, the test results are much more likely to reflect the BAC during driving. However, their low accuracy means they can only be used as a field sobriety test to establish probable cause. Experts recommend that breath tests be conducted twice, with several minutes between, that control subjects be used, and that other procedures be implemented. Since, as a practical matter, strict adherence to these procedures may not be possible at the scene of arrest, the PAS’s accuracy is further thrown into doubt. Also, these little gadgets are prone to misuse, like a quick heating by the officer just before administration.

RLG attorneys are experienced in all aspects of DUI law. If you have been arrested for DUI, a criminal defense attorney at RLG will fight to protect your rights. We serve the entire Bay Area.

For outstanding legal representation, please contact RLG for a free consultation: (415) 877-4486







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