Understanding your Blood Alcohol Concentration (BAC) as it Relates to a California DUI

“BAC” stands for “blood alcohol concentration” or “blood alcohol content.” It is a measure of the amount of alcohol in someone's bloodstream. BAC is commonly used to try to establish whether someone is driving under the influence.

Blood alcohol concentration is expressed as a percentage – for instance .08%. The higher the number, the more alcohol there is a person's bloodstream.

Does BAC indicate whether someone is drunk?

Blood alcohol content does not, by itself, determine whether someone's driving will be impaired by alcohol. Some people may be impaired at a relatively low percentage, while others may be able to drive safely with a relatively high BAC.

But regardless of actual impairment, drivers may not lawfully drive in California once their BAC exceeds a certain level. This amount is sometimes referred to as California's “legal limit” for alcohol--also known as the “per se” DUI limit.

What is California's per se “legal limit” for blood alcohol?

California's “legal limit” for BAC depends on the driver's age and the type of vehicle driven. Common legal limits that lead to “DUI per se “charges in California include:

  • Adult drivers (non-commercial vehicles): .08%.1
  • Commercial drivers: .04%.2
  • Taxi, limo, and ride-sharing drivers: .04%.3
  • Underage drivers: .05%.4

Can someone be under the influence with a BAC below California's “legal limit”?

Someone can be found guilty of driving under the influence even if his or her BAC is under the “legal limit.” A person is considered to be under the influence (regardless of BAC) anytime that:

  1. Due to the influence of alcohol and/or drugs,
  2. That person can no longer drive as well as a cautious sober person under similar circumstances.5

To help you better understand how BAC is used in a California DUI case, our California DUI defense lawyers discuss the following, below:

california dui bac law legal defense blood alcohol test vial
Blood alcohol concentration (“BAC”) is used as a measure of possible impairment in a California DUI case

1. What is 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 – for instance .08% (California's “legal limit” for most adult drivers). This percentage represents the grams of alcohol present in every 100 milliliters of someone's blood.

A higher number indicates that a person has more alcohol in his or her system. A lower number indicates less alcohol.

BAC can be measured directly by a DUI blood test. Or it can be approximated by a DUI breath test. We discuss these in Section 3, below.

2. How accurate are BAC measurements?

DUI chemical tests for BAC are considered a scientifically accurate way to measure blood alcohol. More importantly, they are legally admissible as evidence in a California driving under the influence prosecution.

3. How is BAC measured?

BAC is measured by either a DUI breath test or a DUI blood test. Both are considered reliable when properly administered. Both are admissible in court.

Let's take a closer look at each of them.

3.1. DUI breath tests

The most commonly used test for BAC in California is a breath test. Breath tests are minimally invasive and deliver instant results.

Breath tests in California fall into two basic categories (discussed below):

How DUI breath tests measure blood alcohol

A breath test does not directly measure the percentage of alcohol in someone's blood. Instead, it measures the alcohol present in the deepest part of someone's lungs, where it is closest to the individual's blood supply.

A “Breathalyzer”-style machine then mathematically converts this amount into an approximately equivalent blood alcohol percentage. The formula it uses to do this is known as a “partition ratio.”

In actual fact, partition ratios can differ from person to person and situation to situation. But in California, by law, the partition ratio is deemed to be 2,100 to 1.6 This means that the amount of alcohol present in 2,100 milliliters of deep lung breath is considered legally equal to the amount of alcohol in 1 milliliter of blood.

california dui breathalyzer legal defense
Breath tests are legally equivalent to blood tests in California DUI cases.

3.1.1 Preliminary alcohol screening (PAS) tests

A preliminary alcohol screens (PAS) test is usually given before someone is arrested for DUI. Typically, this will occur after a traffic stop or at a DUI sobriety checkpoint.

A pre-arrest PAS test may lawfully be declined by a driver unless:

  • The driver is under 21 years of age,7 or
  • The driver is on probation for a prior DUI offense.8

3.1.2. Post-arrest DUI breath tests

After a driver has been lawfully arrested for DUI, he or she will be given the choice of a breath or blood test. The driver must take this test even if he/she previously took a PAS breath test. 9

Consequences of refusing to take a chemical test include the automatic suspension of the driver's license for at least one year.10

Most people elect to take the breath test as it is less invasive. This post-arrest breath test is usually given on a desktop device at the police station. Otherwise, it is similar to a PAS test.

In some counties, the police may give this post-arrest breath test roadside. In this case, it will appear similar to a PAS test. The important difference, however, is that unlike a PAS test, a post-arrest test is not optional, no matter where it is given.

3.2. DUI blood tests

Blood tests directly measure the alcohol in a person's blood. This makes them the most the most accurate test for determining a person's blood alcohol concentration—at least, when everything is done strictly according to the procedure.

The driver's blood will be taken by someone licensed by the state.11 Part of this sample will be sent to a lab for analysis. It can take several weeks for a driver's lawyer to get these test results.

The main advantage of a DUI blood test over a DUI breath test is that part of the sample can be saved. This means the driver's lawyer can get ahold of it and have it independently tested.12

As Riverside DUI defense attorney Michael Scaffidi13 explains,

"When a client chooses a blood test, we can file a California DUI ‘blood split' motion. This lets us obtain part of the same blood sample the police used and have it sent to a lab of our choice for independent testing. If the results are different, we can often use them to challenge the prosecution's case.”

3.3. Can BAC be measured by a urine test in California?

Blood alcohol concentration is not normally measured by a urine test in California DUI cases. Urine tests can accurately detect the presence of alcohol. But they are less reliable than blood tests of breath tests for determining the specific amount of alcohol present.

So a urine test will only be used in a California drunk driving case if:

  • 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.14

Reasons an individual might be incapable of taking a blood or breath test can include:

  • 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).
scientist holding urine sample container BAC DUI california attorneys
Urine tests are usually not admissible in California DUI cases.

4. What is the legal limit for BAC in California?

By law, anyone who drives with a more than a certain amount of alcohol in his or her system is considered to be under the influence. This is known as a “per se” DUI under the Vehicle Code sections set forth below.

“Per se” is a Latin phrase meaning “in itself.”15 Some DUIs are “per se” because having a BAC at or above the legal limit is considered a violation of law in and of itself. In such a case, the prosecutor does not need to prove that the person's driving was actually impaired.

California's “per se” DUI code sections

California's “per se” DUI code sections are:

California also has two “zero tolerance” laws. Violation is technically not a DUI and compliance can be measured by a PAS test. These two laws are:

Drivers under 21 or who are on DUI probation violate these laws by driving with a BAC of .01% or higher. That person can also be charged with DUI if BAC is measured at any higher, applicable level (which will be confirmed with a post-arrest blood or breath test).

5. How is BAC used during a DUI traffic stop and arrest?

BAC is used by law enforcement at two different stages:

  1. The DUI investigation, and
  2. The DUI arrest.

Learn more about these stages in our article, “The Court Process in California DUI Cases.”

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

During a traffic stop, if the officer suspects a driver of being under the influence, he/she may start a DUI investigation. As part of the investigation, the officer may ask the driver to take a PAS test.16

If the driver “blows” a BAC below the legal limit, the officer will usually let the person go with just a traffic ticket or a warning.

If the test is positive, the officer will probably arrest the driver and require a post-arrest breath or blood test.

Should a driver agree to a PAS test?

An adult driver who thinks he/she may be over the legal limit is not usually well-served by taking a PAS test. He or she can (and probably should) politely decline to take one. The officer may arrest the person anyway.

Someone who is under the legal limit also has the right to decline. Some drivers prefer, however, to take the test in order to show the officer that they are not drunk.

Note that drivers who are under the age of 21 or on DUI probation may not refuse a PAS without consequences.17

teens drinking alcohol in car california legal defense
California law prohibits drivers under 21 from driving with any measurable alcohol in their bloodstream.

5.2. Post-arrest breath or blood tests for blood alcohol

A post-arrest chemical test is considered “evidentiary.” This means a prosecutor can use it in court to as evidence that a driver drove drunk.

This type of chemical test may not be refused by a driver without consequences.18

The driver must usually be offered the choice of a breath or blood test. 19 But in some cases, the officer can require testing of the driver's blood. This can be instead of (or in addition to a breath reading) if the officer reasonably believes the driver is under the influence of drugs.20

5.3. Title 17 procedures and chemical tests for alcohol

Title 17 of the California Code of Regulations sets forth strict procedures for DUI chemical tests.

If these procedures are not followed to the letter, an experienced California DUI defense attorney can challenge the results.

We discuss such challenges in Section 8, below.

6. How is BAC used during a California DUI prosecution?

During a California DUI case, the prosecutor will use BAC and other evidence to prove that the driver either:

  • Was over the legal limit and, therefore, guilty of a DUI per se, or
  • Was actually impaired and, therefore, guilty of driving under the influence.

DUI per se relies on an “objective” measurement of impairment. If BAC is at or above a specified level (such as .08 for an adult in a non-commercial vehicle), a driver is legally considered too drunk to drive.

Driving under the influence is a “subjective” test. The prosecutor must show that the driver was unable to drive safely due to alcohol.

BAC is just one element of a “subjective” drunk driving prosecution. The higher a driver's BAC, the more likely that the person was impaired. But additional evidence of impairment is usually required.

Such other evidence might include:

  • 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).
girl looks sad after breathalyzer test BAC DUI california attorneys can help
Drivers in California can be charged with drunk driving even if their BAC is below the “legal limit.

7. Should a driver choose a blood or a breath test if arrested for a California DUI?

Unfortunately, there is no easy answer as to the best chemical test to pick after a DUI arrest. Each has its advantages, as discussed below. Each can be challenged by an experienced California drunk driving lawyer.

The important thing is to be as polite as possible to the officer and to say as little as possible. A chemical test is not the only evidence that can be used to convict someone of driving under the influence. The officer may also testify about what the driver says and how he or she says it.

7.1. Advantages of a California DUI breath test

Most people arrested for DUI choose to take a breath test. The main advantages of a breath BAC measurement are that:

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

7.2. Advantages of a California DUI blood test

The main advantages to a DUI blood test are that:

  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.

7.3. Advantages of a chemical test refusal

The one advantage to refusing a chemical test is that there will be no way to prove the driver's BAC. This can be useful to avoid a criminal prosecution for drunk driving.

The prosecution will have to prove guilt by showing that the defendant's driving was actually impaired by alcohol and/or drugs.

But… a refusal to take a chemical test is itself admissible as evidence of guilt.21 (Note that this does not apply to a PAS test unless the driver is under 21 or on DUI probation).22

And the driver faces consequences for a test refusal, including:

  • Automatic suspension of the driver's license by the California DMV for at least one year,
  • Additional time in jail if the driver is ultimately convicted of driving under the influence, and
  • Nine months of California DUI school (instead of the usual three months) if the driver is nevertheless convicted.23

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).24

But keep in mind that a driver with a very high BAC might not be capable of making the best decision in the moment.

Regardless of the decision a driver in this position makes, it is a good idea to consult an experienced California drunk driving attorney as soon as possible afterward.

friend holding back the arm of another holding a beer going for his car keys california legal defense dui
Drivers with a very high BAC might be better off accepting the consequences of a chemical test refusal after a California drunk driving arrest.

8. How can a driver fight DUI test results in court?

Just because a breath or blood test showed a certain BAC, it doesn't mean the results were accurate.

We discuss potential errors at length in our articles on:

But in general, BAC results can be challenged by taking one or more of the following positions:

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

For more information, please see our article, “20 Legal Defenses for Fighting California DUI Charges.”

Arrested for DUI in California? Call us for help…

california dui legal defense blood alcohol content receptionists
Call us for help

If you or a loved one has been charged with driving under the influence, we invite you to contact our California DUI lawyers for a free consultation.

Call us at 855-LAWFIRM to discuss the best defenses to your California DUI case, including a possible challenge of your BAC results.

We have local offices throughout the state, including in Los Angeles, Orange, San Bernardino, Riverside, Ventura and San Diego counties, as well as central and northern California.

We also have offices in Las Vegas and Reno for those that need to challenge BAC in a Nevada DUI case.

Legal references:

  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 23152(b): “For purposes of this article and Section 34501.16, percent, by weight, of alcohol in a person's blood is based upon grams of alcohol per 100 milliliters of blood or grams of alcohol per 210 liters of breath.”

    See also 17 California Code of Regulations 1220.4(f): “A breath alcohol concentration shall be expressed as the number of grams of alcohol per 210 liters of breath.”

    See also People v. Bransford (1994) (“[E] vidence of the variability of partition ratios was not relevant evidence because… section 23152(b) defined the offense without regard to such ratios.”)

  7. Vehicle Code 13388 VC [requiring PAS breath test for DUI suspects under 21].

  8. Vehicle Code 13389 VC [requiring PAS breath test for suspects on probation for DUI]. See also Vehicle Code 23154(a).

  9. Vehicle Code 23612(a)(1)(A): “A person who drives a motor vehicle is deemed to have given his or her consent to chemical testing of his or her blood or breath for the purpose of determining the alcoholic content of his or her blood, if lawfully arrested for an offense allegedly committed in violation of Section 23140 [under 21 DUI], 23152 [DUI], or 23153 [DUI causing injury]…”

    See also Vehicle Code 23612 (g)(2)(B)(i): “If the officer decides to use a preliminary alcohol screening test, the officer shall advise the person that he or she is requesting that person to take a preliminary alcohol screening test to assist the officer in determining if that person is under the influence of alcohol or drugs, or a combination of alcohol and drugs. The person's obligation to submit to a blood, breath, or urine test, as required by this section, for the purpose of determining the alcohol or drug content of that person's blood, is not satisfied by the person submitting to a preliminary alcohol screening test. The officer shall advise the person of that fact and of the person's right to refuse to take the preliminary alcohol screening test.”

  10. Vehicle Code 23612(a)(1)(D).

  11. Vehicle Code 23158(a).

    See also 7 California Code of Regulations § 1219.1(a): "Blood samples shall be collected by venipuncture from living individuals as soon as feasible after an alleged offense and only by persons authorized by Section 23158 of the Vehicle Code."

  12. Vehicle Code 23158(b) VC.

  13. Riverside DUI defense lawyer Michael Scaffidi is a former police officer and police sergeant who now uses his inside knowledge to defends clients accused of DUI and other criminal offenses throughout San Bernardino and Riverside Counties.

  14. Vehicle Code 23612(a)(2)(A), endnote 9.

  15. See, e.g., Dictionary.com, definition of “per se.”

  16. Vehicle Code 23612(a)(2)(A), endnote 9.

  17. Vehicle Code sections 13388 and 13389.

  18. Vehicle Code 23612(a)(1)(A), endnote 9.

  19. Vehicle Code 23612(a)(2)(A): “If the person is lawfully arrested for driving under the influence of an alcoholic beverage, the person has the choice of whether the test shall be of his or her blood or breath and the officer shall advise the person that he or she has that choice. If the person arrested either is incapable, or states that he or she is incapable, of completing the chosen test, the person shall submit to the remaining test. If a blood or breath test, or both, are unavailable, then paragraph (2) of subdivision (d) [urine test] applies.”

  20. Vehicle Code 23612 (a)(2)(C) VC.

  21. See, e.g., California Criminal Jury Instructions (CALCRIM) 2130. Refusal - Consciousness of Guilt.

  22. People v. Jackson (2010) 189 Cal.App.4th 1461, 117 Cal.Rptr.3d 775.

  23. Vehicle Code 13353 VC.

    See also Vehicle Code 23538(b)(2) The court shall refer a first offender whose blood-alcohol concentration was 0.20 percent or more, by weight, or who refused to take a chemical test, to participate for at least nine months or longer, as ordered by the court, in a licensed program that consists of at least 60 hours of program activities, including those education, group counseling, and individual interview sessions described in Chapter 9 (commencing with Section 11836) of Part 2 of Division 10.5 of the Health and Safety Code.

  24. See, e.g., Vehicle Code 23578 VC: “In addition to any other provision of this code, if a person is convicted of a violation of Section 23152 or 23153, the court shall consider a concentration of alcohol in the person's blood of 0.15 percent or more, by weight, or the refusal of the person to take a chemical test, as a special factor that may justify enhancing the penalties in sentencing, in determining whether to grant probation, and, if probation is granted, in determining additional or enhanced terms and conditions of probation.”

    See also Vehicle Code 23538(b)(2): “The court shall refer a first offender whose blood-alcohol concentration was 0.20 percent or more, by weight, or who refused to take a chemical test, to participate for at least nine months or longer, as ordered by the court, in a licensed program that consists of at least 60 hours of program activities, including those education, group counseling, and individual interview sessions described in Chapter 9 (commencing with Section 11836) of Part 2 of Division 10.5 of the Health and Safety Code.”

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