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Blood alcohol concentration (“BAC”) is a measure of the amount of alcohol in a person’s bloodstream. A person operating a motor vehicle must not exceed a certain level of BAC; otherwise, he/she may be charged with DUI.

As a general rule, California DUI law imposes a legal BAC limit for drivers of .08% blood alcohol concentration. This rule, however, comes with certain exceptions prescribing lower limits for commercial, taxi, limo, rideshare, and underage drivers. To elucidate, limits are as follows

Type of Driver

BAC legal limit

Adult drivers (non-commercial vehicles)


Commercial drivers


Taxi, limo, and ride-sharing drivers


Underage drivers


In some instances, DUI charges can still be had regardless of BAC i.e. even if BAC is lower than the legal limit described above. A person can still be considered to be “under the influence” within the meaning of the penal statute (regardless of BAC) if the person is under the influence of alcohol and/or drugs and that the person can no longer drive as well as a cautious sober person under similar circumstances.[5]

Blood alcohol concentration (“BAC”)

Blood alcohol concentration (“BAC”) is a measure of the amount of alcohol in a person’s bloodstream. It is also referred to as “blood alcohol content” or “blood alcohol level.”

BAC is expressed as a percentage representing the grams of alcohol present in every 100 milliliters of someone’s blood. Needless to say, a higher number means more alcohol in his or her system and a lower number indicates less alcohol. Certain tools are used to measure BAC. In California jurisdiction, we make use of DUI blood tests or DUI breath tests.

Accuracy of BAC measurements

In the prosecution of DUI in California, the legally admissible and acceptable evidence to prove that a person has in fact exceeded the legal limit is the result of a DUI chemical tests. This is considered as a scientifically accurate means by which authorities measure blood alcohol in the defendant’s system.

Tools in Measuring BAC

Methods in measuring BAC are usually done by a DUI breath test or a DUI blood test.

1. DUI breath tests

The most commonly used test for BAC in California is a breath test. This has two categories namely “preliminary alcohol screening” (“PAS) tests for pre-arrest and DUI breath tests for post-arrest. Breath tests are minimally invasive and deliver instant results.

A breath test does not actually make direct measurements of the percentage of alcohol in a person’s blood; rather it measures the alcohol present in the deepest part of a person’s lungs, where it is closest to the individual’s blood supply.

Preliminary alcohol screening (PAS) tests

Preliminary alcohol screening (PAS) tests is administered upon the motorists when he is being suspected of being under the influence and prior to being arrested for DUI. This test is a roadside breathalyzer administered by the officer using a handheld device. This is used to measure the suspect’s breath alcohol concentration. Officers use the PAS test to determine whether to arrest the person for DUI. The use of this tool is usually done after a traffic stop or at a DUI sobriety checkpoint.

However, the administration of pre-arrest may be declined by the motorist, except when the driver is under 21 years of age,[6] or The driver is on probation for a prior DUI offense.[7]

Post-arrest DUI breath tests

Vehicle Code 23612(a)(1)(A) provides that after a driver has been lawfully arrested for DUI, the motorist may elect to be subjected to either breath or blood test. The driver must necessarily make a choice and take either test notwithstanding him/her previously taking a PAS breath test.[8]

If the motorist refuses to take the aforementioned chemical tests, he/she may be meted with serious consequences including automatic suspension of the driver’s license for at least one year.[9]

This post-arrest breath test, as compared to PAS tests, are administered on a desktop device at the police station. Aside from this, the most important distinction between a DUI breath test and PAS is that the post-arrest test is not optional, no matter where it is given.

2. DUI blood tests

The most accurate method to determine alcohol content within a person’s system probably is thru DUI blood test. This test is aimed at directly measuring the alcohol in a person’s blood. Another advantage offered by this method, as compared to breath tests, is that a part of the sample taken from the driver can be saved and have it independently tested later.[10] This allows the lawyer for the defendant to file a California DUI blood split motion that allows the defendant to challenge the charges against him.

In determining alcohol thru blood tests, the driver’s blood will be taken by a licensed professional and then a part of this sample will be sent to a laboratory for analysis. With this method, it normally takes weeks before obtaining test results.

3. Urine Tests

A urine test is another means by which blood alcohol concentration may be determined in a person’s system. Urine tests are least popular as compared to blood and breath tests since while it can accurately detect the presence of alcohol they are considered to be less reliable than blood tests of breath tests for determining the specific amount of alcohol present.

The only instances when a urine test is utilized in a California DUI case is when both a blood and breath test are unavailable, or the person arrested is incapable of taking one of the tests and the other one is not available.[11]

A medical condition (such as a breathing disorder or clotting disorder), or an extremely high level of inebriation or unconsciousness (making it difficult or impossible to complete a breath test) usually makes it difficult, if not impossible to administer blood or breath tests upon the motorist. Hence, a urine test is resorted to in such cases.

Legal Limit for BAC in California

California laws prescribe a limit in terms of alcohol content in the blood when operating a motor vehicle. If a motorist is found to have exceeded the legal limit, he is considered to be under the influence and may be proceeded against criminally. This is what is known as a “per se” DUI under the Vehicle Code sections.

“Per se” is a Latin phrase that literally means “in itself.” Driving while having a BAC at or above the California DUI legal limit is in itself considered a criminal act already. With this, the prosecutor no longer finds it necessary to prove that person’s driving was actually impaired.

California’s “per se” DUI code sections

Vehicle Code 21352(b)

driving with a BAC of .08% or higher

adults, non-commercial vehicles

Vehicle Code 23140,

underage DUI with a BAC of .05% or higher

drivers under 21

Vehicle Code 23152(d)

commercial DUI with a BAC of .04% or higher

drivers of commercial vehicles

Vehicle Code 23152(e)

“passenger for hire” DUI with a BAC of .04% or higher

taxi, limo, Uber and Lyft drivers, etc

“Zero Tolerance” laws in California

California’s zero-tolerance laws are made applicable to underage drivers and those who are under DUI probation. These laws are

Vehicle Code 23136 VC

California’s “Zero Tolerance” law for underage drivers

Vehicle Code 23154 VC

California’s zero-tolerance law for people on DUI probation

It should be noted that a different legal limit is prescribed for drivers under 21 or who are on DUI probation violate. For these drivers, legal limit for BAC ies set at .01%.

BAC in DUI traffic stop and arrest

Law enforcement make us of BAC in two different stages, namely: during the DUI investigation, and during the DUI arrest.

Blood alcohol measurements during a DUI investigation (“PAS” test)

If a driver is being suspected by the officer as being under the influence, the officer may commence a DUI investigation. In the course thereof, the officer may ask the driver to take a PAS test.[12] If the test results reveal that the alcohol level exceeded the legal limit, the officer proceeds with arresting the driver and performs a post-arrest breath or blood test on the motorist thereafter. On the other hand, if the tests yields a result wherein BAC is below the legal limit, then the officer will let the person go with just a traffic ticket or a warning.

Declining PAS test

An adult driver, as a general rule, can actually decline being subjected to PAS test. If he/she thinks that he/she may be over the legal limit, taking PAS test will only work against his/her favor. In such cases, he or she can (and probably should) politely decline to take one. Notwithstanding that, the office is not precluded from arresting the motorist anyway, for reasons other than DUI of course.

The above-stated rule admits two exceptions such that drivers who are under the age of 21 or on DUI probation cannot refuse a PAS without consequences.

Post-arrest breath or blood tests for blood alcohol

Unlike PAS test, a motorist cannot refuse to take a post-arrest chemical test without any consequence. Results from chemical tests are considered “evidentiary” which means that the prosecutor can use it in court to as evidence to prove that the driver drove while being under influence.

At the election of the driver, he may choose between going through a breath or blood test. However, there are instances when the officer requires testing of the driver’s blood instead of giving him a choice of taking a breath test. This is most especially true when the officer believes that the driver is under the influence of drugs.[13]

Title 17 procedures and chemical tests for alcohol

Title 17 of the California Code of Regulations prescribes the procedures for DUI chemical tests which must be strictly followed. Failure to conform with these standards to the letter can result in the dismissal of DUI charges.

BAC in California DUI Prosecution

In prosecuting a California DUI case, the prosecutor must prove that the driver was either over the legal limit and, therefore, guilty of a DUI per se, or was actually impaired and, therefore, guilty of driving under the influence. In doing so, the prosecutor will have to present BAC as evidence.

There is DUI per se BAC when the BAC level has exceeded the prescribed limit (such as .08 for an adult in a non-commercial vehicle); as such, the motorist is deemed legally drunk for DUI purposes. DUI per se relies on an “objective” measurement of impairment.

Driving under the influence, which is a “subjective” test, requires that the prosecutor prove that the driver was unable to drive safely due to alcohol.

Successful prosecution of DUI charges requires that additional evidence be presented since BAC is just one element of a “subjective” drunk driving prosecution. The rule of the thumb is that the higher a driver’s BAC level, the more likely that the person was impaired.

Additional evidence for the prosecution usually includes testimony from the arresting officer about driving pattern and physical symptoms of intoxication, proof of when the driver drove, and testimony from other witnesses (including passengers in the vehicle, if any).

Preferred Test: Blood Test or Breath Test

There is no easy way to answer which test should be preferred by the motorist when faced with DUI arrests. Each of these methods presents pros and cons, and both of them may be challenged by the defendant’s skilled attorney.

California DUI breath test

California DUI blood test


  1. It is less invasive and less stressful for most people, and
  2. The results are available right away.


  1. A blood sample allows a more accurate assessment of BAC than a breath same. This can be useful when someone is very close to the legal limit).
  2. Part of the blood sample can be saved and independently tested by a lab of the driver’s choosing. Breath samples are not preserved.

Choice given to motorist is not limited to breath or blood tests; he can also opt to refuse being subjected to a chemical test (when applicable), albeit with consequences. Advantages of a chemical test refusal are that there will be no way to prove the driver’s BAC. This can be useful to avoid criminal prosecution for drunk driving.

In such cases, in the absence of BAC results, the prosecution will be constrained to prove guilt by showing that the defendant’s driving was actually impaired by alcohol and/or drugs.

However, it should be emphasized that a refusal to take a chemical test is itself admissible as evidence of guilt.

Consequences for refusing to take chemical tests are as follows:

  1. Automatic suspension of the driver’s license by the California DMV for at least one year,
  2. Additional time in jail if the driver is ultimately convicted of driving under the influence, and
  3. Nine months of California DUI school (instead of the usual three months) if the driver is nevertheless convicted.[14]

Who might benefit from a chemical test refusal?

Despite the consequences of a refusal, it might be a good choice if:

  • The driver does not depend on driving to get around (and can, therefore, afford to suffer the automatic driver’s license suspension), and/or
  • The driver is very drunk and does want to suffer the additional consequences of driving with a very high BAC (.015% or higher).[15]

Legal Defenses

BAC results can be challenged by raising any of the following arguments, as applicable on a case-to-case basis:

  1. The officer did not properly advise the driver of the choices and consequences involved in taking DUI chemical tests;
  2. The officer did not observe proper procedures as set for in Title 17 of the California Code of Regulations during the administration of the DUI test;
  3. The testing equipment was not calibrated as required under California law;
  4. The breath or blood sample was mishandled and possibly contaminated;
  5. The driver suffered from a medical condition such as GERD or diabetes that resulted in a falsely elevated BAC reading;
  6. The driver was on a high-protein/low-carb diet;
  7. There was “residual mouth alcohol” in the driver’s mouth;
  8. The driver had “rising blood alcohol” at the time of the test (meaning his/her BAC was lower at the time of driving).

The list of legal defenses provided above is not exhaustive, there are a considerable number of ways to challenge a DUI charge. The legal defenses to be advanced depend largely on the facts of each case. Hence, you need to consult a highly skilled and experienced attorney to determine which would best serve your interest.

Please set up a free consultation to discuss your case with a knowledgeable California DUI defense lawyer. Give us a call today at (626) 827-7222.

[1] California Vehicle Code 23152(b).

[2] Vehicle Code 23152(d).

[3] Vehicle Code 23152(e).

[4] Vehicle Code 23140 VC

[5] Vehicle Code 23152(a).

[6] Vehicle Code 13388

[7] Vehicle Code 13389

[8] Vehicle Code 23612(a)(1)(A)

[9] Vehicle Code 23612(a)(1)(D)

[10] Vehicle Code 23158(b) VC.

[11] Vehicle Code 23612(a)(2)(A).

[12] Vehicle Code 23612(a)(2)(A)

[13] Vehicle Code 23612 (a)(2)(C) VC.

[14] Vehicle Code 13353

[15] Vehicle Code VC 23578

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