California DUI Breath Tests

 The results of a California DUI breath test are part of most California DUI cases--especially those where the defendant is charged with Vehicle Code 23152(b) VC driving with a BAC of 0.08 or above.

Under California's "implied consent" law, everyone arrested for DUI is required to submit to a DUI breath test or DUI blood test. If you refuse, you may face additional penalties for chemical test refusal.1

But a breath test result showing you were over the legal limit does not mean you will necessarily be convicted of DUI. An invalid or inaccurate breathalyzer result can be a key defense to DUI charges.

There are numerous reasons why your breath test might be inaccurate or might not hold up in court. These include:

  • The officer administering the test did not follow the procedures set forth in Title 17 of the California Code of Regulations;2
  • A medical condition like gastroesophageal reflux disease or a special diet artificially inflated your breath test results; and
  • A phenomenon like residual mouth alcohol or rising blood alcohol led to inaccurate DUI breath test results.

Below, our California DUI defense attorneys address the following frequently asked questions about DUI breath testing in California:

1. Can I Refuse a DUI Breath Test?

In the course of a DUI investigation, police generally administer two types of breath tests:

  1. A preliminary alcohol screening (“PAS”) breath test given on a handheld, roadside breathalyzer. This is given before a driver is arrested; and
  2. An evidentiary test on a desktop breathalyzer. This is given after the driver is arrested, usually at the police station (or with a mobile police unit taken to the scene of a checkpoint).

Refusal to take these two DUI breath tests has greatly different legal consequences.

1.1. Pre-arrest Preliminary Alcohol Screening (PAS) test

California Highway Patrol officers and local law enforcement are equipped with hand-held breath testing instruments. These are used to conduct Preliminary Alcohol Screening (PAS) tests to measure the BAC of suspected drunk drivers.

A preliminary alcohol screening is a pre-arrest DUI breath test administered in the field.

You may be offered a hand-held PAS test when you are pulled over. You are not required to take this preliminary test unless you are:

  • under the age of 21,3or
  • on probation for DUI.4

Otherwise, the PAS test is treated as a field sobriety test (FST). It merely helps the officer decide whether to arrest you for DUI. There is no penalty for refusing to take a PAS test (or any of the field sobriety tests, for that matter).5

(Note that in some counties (including Ventura and Orange) police use the Portable Evidential Breath-Alcohol Testing System (PEBT) to conduct post-arrest DUI breath tests. This device is based on the same Draeger Alcotest device used by some police departments for preliminary screenings. However, the PEBT can be hooked up to a printer by hardline or blue tooth. If you have clearly been placed under arrest and a cop is requesting that you take a breath test on a handheld device in the field, we advise requesting a blood test instead.)

1.2. Post-arrest evidentiary DUI breath test 

If you actually get arrested for a DUI, then officers can require you to submit to a chemical test. You may not refuse this test without consequence (and the US Supreme Court has recently confirmed that this requirement is constitutional). Even if you have already taken a preliminary breath test, you must take a post-arrest chemical test.6

Unless drug use is also suspected, you should be offered the choice of whether to take a California DUI blood test or a breath test. If you are unable to complete your chosen test for any reason -- including a medical condition listed below -- the other test will be administered.7

Refusal to take a blood or breath test after you have been placed under arrest typically will result in:

  1. suspension of your driver's license for at least one year, and
  2. a mandatory jail enhancement of 48 hours if you are ultimately convicted of DUI.8

1.3. Is it better to choose a DUI breath test or blood test?

There is no widespread consensus among DUI lawyers as to whether a DUI breath test or blood test is more advisable.

On the one hand, DUI breath testing is generally considered less reliable and less accurate than blood testing. As we explain below, breath tests are prone to a host of errors that may -- and frequently do -- render falsely high readings.

There is no widespread consensus on whether a DUI breath test or blood test is preferable for a potential defendant.

Moreover, unlike blood samples, breath samples cannot be preserved. Thus there is no way to conduct later independent testing to verify the accuracy of the test.

On the other hand, these issues may be used by the defense attorney to attack the breath test evidence in court.

And certain medical conditions – such as asthma, emphysema, and unconsciousness – may prevent you from taking a breath test altogether.

2. What are the Required Procedures for a DUI Breath Test Under Title 17?

Title 17 of the California Code of Regulations sets forth strict procedural requirements for the collection of DUI breath samples. Failure to follow procedures can result in falsely high BAC results from a breath test--and/or the breath test results not being admissible.

There are dozens of rules set forth in Title 17. Five of the most important are:

  1. The DUI breath testing device must be regularly calibrated and tested;9
  2. The person giving the test must be trained on the specific device used;10
  3. You must be observed continuously for 15 minutes before the DUI breath test is given and during such time must not:
    • eat, drink, or put anything in your mouth,
    • burp, regurgitate, or vomit (which could bring alcohol from your stomach into your mouth), or
    • smoke;11
  4. The DUI breath test operator must collect air from deep in your lungs;12 and
  5. The operator must obtain two breath samples that do not differ from each other by more than 0.02 grams per 100 milliliters of blood alcohol.13

Note that it may take more than two “blows” in order to obtain the two legally required readings.

3. How Do DUI Breath Tests Work?

Unlike California DUI blood testing, DUI breath testing does not directly measure blood alcohol.

DUI breath testing devices have to mathematically convert the amount of alcohol present in breath into the amount of alcohol present in blood. They do this by measuring the alcohol present in the deepest part of your lungs, where it is closest to your blood supply.14

To understand this, consider that when we exhale, air emerges in the following order:

  1. from the mouth/nasal area, then
  2. from the throat and upper airway, and finally
  3. from the lungs.
The highest breath alcohol concentration for a DUI breath test comes from the air in the alveoli.

The highest breath alcohol concentration comes from the alveoli, located deep within the lungs. The alveoli are balloon-like sacs that inflate when you inhale and deflate when you exhale.

The alveoli sit just above your capillaries, slender blood vessels that are no more than one one-thousandth of a millimeter thick. They allow oxygen to pass from the lungs into the blood, and carbon dioxide and other wastes to pass from the blood into the lungs.

The capillaries also allow the passage of a fraction of the alcohol present in the blood into the lungs. This is why California law requires that samples of your breath given for DUI breath testing be “essentially alveolar in composition.”15

This “deep lung” air is the last to leave your lungs. As a result, in order for a DUI breath test to be accurate, you are asked to blow hard when you take a breath test.

Not everyone is capable of blowing hard enough for a reliable breath test reading. Older people -- and some women -- may have trouble generating the volume of air required.

4. Can Procedural Errors Cause Falsely High BAC Results?

In a word--yes. Operator and procedural mistakes lead to inaccurate and inflated DUI breath test results quite frequently.

DUI breath testing procedural errors leading to a falsely high BAC reading usually consist of either

  1. improperly calibrated or tested equipment, or
  2. operator error.

4.1. Improperly calibrated or tested equipment

If the DUI breath testing device is faulty, it follows that any readings from it will be faulty as well.

To prevent this, Title 17 imposes strict rules for DUI breath device calibration and testing, including:

  • the DUI breath test must be given on a device approved by the state of California,16
  • the device must be calibrated every 10 days or 150 uses (whichever occurs first),17 and
  • the laboratory must keep detailed records of instrument calibration and testing18.

4.2. Operator error

Many falsely high BAC readings from breathalyzer tests result from simple human error.

Both operator error and equipment failure can lead to inaccurate DUI breathalyzer results

Mistakes people administering DUI breath tests sometimes make include (without limitation):

  • not checking your mouth to ensure it is empty before administering the breath test,
  • failing to observe you continuously for a full 15 minutes before administering the breath test for DUI,
  • not recording the time at which the 15-minute period started,
  • not recording the time at which each “blow” was made, and
  • improperly attaching the mouthpiece to the machine.

5. Can My Medical Condition or Diet Affect My DUI Breath Test Results?

Certain medical conditions and diets can “trick” a breathalyzer into a falsely high BAC reading.

5.1. Gastrointestinal reflux disease (“GERD”)

Chronic GERD, heartburn, or acid reflux can create falsely high BAC readings. In people with GERD, stomach contents sometimes flow back into the mouth.19

Breathalyzer machines can read the stomach acid contained in this “backwash” as alcohol.

5.2. Hypoglycemia and high protein / low carbohydrate diets

Certain Atkins-style low-carbohydrate / high-protein diets “trick” DUI breath test devices. These diets cause the body to create “ketones,” which get converted to acetone that is then excreted in the breath. Some DUI breath testing devices are unable to distinguish between acetone and the ethyl alcohol found in alcoholic beverages.20

Moreover, diabetics have an automatic DUI defense because their condition causes them to emit ketones, which again can be mistaken by the breathalyzers for alcohol.

This is why it is critical to disclose ALL medical conditions and dietary issues to your DUI defense lawyer -- even if you don't think they are important. They may be the key to keeping you from being wrongfully convicted of DUI based on inaccurate DUI breath test results.

6. What is "Residual Mouth Alcohol"?

After you drink anything containing alcohol, some alcohol remains in the mucosal linings of the mouth. This is known as “residual mouth alcohol.” Small amounts of residual mouth alcohol can create falsely high BAC readings on a DUI breath test.

Residual mouth alcohol can produce inflated DUI breath test results.

Residual mouth alcohol remains in the mouth for approximately 15-20 minutes. When you exhale, residual alcohol is picked up by deep lung air as it passes out of your mouth. This is how residual mouth alcohol fools California DUI breath testing instruments.

Things that can cause residual mouth alcohol include:

  • recently consumed alcohol, even when it's not enough to make you legally drunk (“one for the road”),
  • taking cough syrup, or
  • gargling with mouthwash or using a breath spray that contains alcohol.

Most mouth alcohol dissipates after 15-20 minutes. This is why Title 17 regulations call for a continuous observation period of 15 minutes before a DUI breath test is administered.21

Example: Ralph has dinner at a friend's house to celebrate his promotion. Ralph drinks nothing but water during the meal. Just before he leaves his friend's house, the friend opens a bottle of expensive champagne and insists Ralph have a glass. Ralph drinks his glass of champagne and leaves.

A few minutes after he leaves his friend's house, Ralph thinks he sees an animal in the road. He swerves to miss it. A Ventura County Sheriff's officer pulls him over and arrests him for DUI. The officer asks Ralph to take a PEBT breath test and he agrees. The device indicates that he has a BAC of 0.10.

Unless Ralph is very small, the results of the PEBT are probably not reliable. Ralph only had one glass of champagne. Since he drank it less than 15 minutes before the PEBT test, residual mouth alcohol is most likely making the DUI breath test reading falsely high. In this case, a DUI blood test probably would have shown that Ralph was below the legal limit.

7. What Other Factors Can Contribute to Falsely High DUI Breath Test Results?

DUI breath testing equipment has become more sophisticated. Even so, there are other factors that could, under certain circumstances, produce falsely high BAC results on a breathalyzer test.

These include:

  • "rising blood alcohol" levels (when the alcohol in your system is not fully absorbed at the time of your DUI breath test),
  • the presence on your breath of volatile “interferents” (such as inhaled acetone from paint or other workplace substances),
  • environmental factors such as radio frequency interference, and
  • inherent error rates within DUI breath testing instruments.22

8. What is the "Partition Ratio" Defense to a VC 23152(a) Charge Based on a DUI Breath Test?

Because everyone's lungs absorb alcohol from the blood at a different rate, DUI breath tests only approximate BAC levels. As a result, even if all California DUI procedures are followed, your BAC may not be as high as the breathalyzer indicates.

The "partition ratio" means that DUI breath test results can show only approximate BAC levels.

This becomes important when you are charged with actual impaired driving under California Vehicle Code 23152(a) VC.23

A DUI breath sample must be mathematically converted to derive a blood-alcohol percentage. This conversion factor -- known as a “partition ratio” -- reflects the relationship between alcohol measured in the breath and alcohol in the blood.

Breath-testing machines in California incorporate a partition ratio of 2,100 to 1. This means the amount of alcohol in 2,100 milliliters of breath is considered equivalent to the amount of alcohol in 1 milliliter of blood.24

The 2,100-to-1 ratio roughly approximates the partition ratio of most people. In actuality, however, partition ratios can vary widely, both in the general population and for one individual at different times.

Because of this discrepancy, the California Supreme Court has held, in People v. McNeal, that evidence about partition ratio variability may be admitted to defend against a VC 23152(a) charge driving under the influence charge--but not a "per se" DUI charge under VC 23152(b).25

This is because a Vehicle Code 23152(a) charge requires proof that your ability to drive safely was impaired because you consumed alcohol.

But under VC 23152(b), you are deemed intoxicated as a matter of law if you have a BAC of .08 or greater. In these cases your actual partition ratio is irrelevant. This is because the California State Legislature included the 2,100-to-1 partition ratio in the definition of the offense--and therefore the DUI breath test results are taken at face value.26

Call us for help...


If you or a loved one wants to know more about the role of DUI breath test results in a DUI case, we invite you to contact us at Shouse Law Group. We can provide a free consultation in office or by phone. We have local offices in Los Angeles, the San Fernando Valley, Pasadena, Long Beach, Orange County, Ventura, San Bernardino, Rancho Cucamonga, Riverside, San Diego, Sacramento, Oakland, San Francisco, San Jose and throughout California.

Additionally, our Las Vegas Nevada DUI defense attorneys represent clients accused of DUI offenses in Nevada. For more information, please see our article on Nevada's DUI breath testing procedures, or contact our local attorneys at one of our Nevada law offices, located in Reno and Las Vegas.

Legal References:

  1. Vehicle Code 23612(a)(1)(A) VC – Implied consent to DUI breath or blood testing. (“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 [being under 21 and driving with a BAC of .05-.07], 23152 [DUI], or 23153 [DUI causing injury]...(C) The [breath or blood] testing shall be incidental to a lawful arrest and administered at the direction of a peace officer having reasonable cause to believe the person was driving a motor vehicle in violation of Section 23140, 23152, or 23153.”)
  2. Title 17 of the California Code of Regulations (“CCR”) commencing with section 1215 [setting forth procedures that law enforcement is required to follow in administering DUI breath tests].
  3. Vehicle Code 13388 VC [requiring PAS breath test for DUI suspects under 21].
  4. Vehicle Code 13389 VC [requiring PAS breath test for DUI suspects on probation for DUI].
  5. Vehicle Code 23612(h) VC – Right to refuse preliminary alcohol screening DUI breath test.
  6. Vehicle Code 23612(a)(1)(A) VC – Implied consent to DUI breath or blood testing, endnote 1 above.
  7. Vehicle Code 23612(a)(2)(A) VC – Choice between DUI blood test and DUI breath test. (“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.”)
  8. Vehicle Code 23612(a)(1)(D) VC – Consequences of refusing a post-arrest DUI breath or blood test. ("“The person shall be told that his or her failure to submit to, or the failure to complete, the required chemical testing will result in a fine, mandatory imprisonment if the person is convicted of a violation of Section 23152 or 23153, and (i) the suspension of the person's privilege to operate a motor vehicle for a period of one year, (ii) the revocation of the person's privilege to operate a motor vehicle for a period of two years if the refusal occurs within 10 years of a separate violation of Section 23103 as specified in Section 23103.5, or of Section 23140, 23152, or 23153 of this code, or of Section 191.5 or subdivision (a) of Section 192.5 of the Penal Code that resulted in a conviction, or if the person's privilege to operate a motor vehicle has been suspended or revoked pursuant to Section 13353, 13353.1, or 13353.2 for an offense that occurred on a separate occasion, or (iii) the revocation of the person's privilege to operate a motor vehicle for a period of three years if the refusal occurs within 10 years of two or more separate violations of Section 23103 as specified in Section 23103.5, or of Section 23140, 23152, or 23153 of this code, or of Section 191.5 or subdivision (a) of Section 192.5 of the Penal Code, or any combination thereof, that resulted in convictions, or if the person's privilege to operate a motor vehicle has been suspended or revoked two or more times pursuant to Section 13353, 13353.1, or 13353.2 for offenses that occurred on separate occasions, or if there is any combination of those convictions or administrative suspensions or revocations.”)
  9. 17 CCR 1220.2(a)(5). (“All instruments used for alcohol analysis [DUI breath tests] shall be in good working order and routinely checked for accuracy and precision.") See also 17 CCR 1221.4(a)(2). (“The accuracy of [DUI breath testing] instruments shall be determined.”) See also 17 CCR 1220.3 -- Quality Control Program; 17 CCR 1221.2 -- Standard of Performance; and 17 CCR 1221.3 --Approved Instruments.
  10. 17 CCR 1221.4(a)(3). (“Breath alcohol analysis [DUI breath tests] shall be performed only with instruments for which the operators have received training . . . .")
  11. 17 CCR 1219.3 -- Breath Collection. (“A breath sample shall be expired breath which is essentially alveolar in composition. The quantity of the breath sample shall be established by direct volumetric measurement. The breath sample shall be collected only after the subject has been under continuous observation for at least fifteen minutes prior to collection of the breath sample, during which time the subject must not have ingested alcoholic beverages or other fluids, regurgitated, vomited, eaten, or smoked.")
  12. Same.
  13. 17 CCR 1221.4(a). (“Procedures for breath alcohol analysis [DUI breath testing] shall meet the following standards: (1) For each person tested, breath alcohol analysis shall include analysis of 2 separate breath    samples which result in determinations of blood alcohol concentrations which do not differ from    each other by more than 0.02 grams per 100 milliliters -- .")
  14. For a detailed discussion of breath testing methodology and science, see People v. McNeal (2009) 46 Cal 4th 1183. See also Jeanne Swartz, Breath Testing for Targeting Hardcore Impaired Drivers, American Prosecutors Research Institute (December 2004).
  15. 17 CCR 1219.3 -- Breath Collection, endnote 11 above.
  16. 17 CCR 1221.2. (“(a) Instruments for breath alcohol analysis [DUI breath testing] shall meet the following standard: (1) The instrument and any related accessories shall be capable of conforming to the "Model    Specifications for Evidential Breath Testing Devices" of the National Highway Traffic Safety    Administration of the U.S. Department of Transportation, which were published in the Federal    Register, Vol. 49, No. 242, Pages 48854-48872, December 14, 1984, and are hereby adopted and incorporated.”)
  17. 17 CCR 1221.4.
  18. 17 CCR 1220. (“(a) All laboratory methods used for forensic alcohol analysis [DUI chemical testing] shall be subject to standards set forth in this Article. (b) Each licensed forensic alcohol laboratory shall have on file with the Department detailed, up to- date written descriptions of each method it uses for forensic alcohol analysis. (1) Such descriptions shall be immediately available to the person performing an analysis and shall be available for inspection by the Department on request. (2) Each such description shall include the calibration procedures and the quality control program for the method.”)
  19. See, e.g., Mayo Clinic, GERD [medical condition that can impact breathalyzer results].
  20. False Breathalyzer Results from Acetone in Breath, Alcohol Problems and Solutions.
  21. 17 CCR 1219.3 -- Breath Collection, endnote 11 above.
  22. See Thomas E. Workman Jr., Massachusetts Breath Testing for Alcohol: a Computer Science Perspective,  8 J. High Tech. L. 209 (2008).
  23. Vehicle Code 23152(a) VC – Driving under the influence. (“It is unlawful for any person who is under the influence of any alcoholic beverage or drug, or under the combined influence of any alcoholic beverage and drug, to drive a vehicle.”)
  24. 17 CCR 1220.4(f). (“A breath alcohol concentration shall be converted to an equivalent blood alcohol concentration by a calculation based on the relationship: the amount of alcohol in 2,100 milliliters of alveolar breath is equivalent to the amount of alcohol in 1 milliliter of blood.”)
  25. People v. McNeal (2009) 46 Cal 4th 1183.
  26. People v. Bransford (1994) 8 Cal.4th 885. See also Assem. Com. on Public Safety, com. on Assem. Bill No. 4318 (1989-1990 Reg. Sess.) as introduced May 15, 1990, p. 2. As cited by the California Supreme Court in People v. McNeal, endnote 6,“the express purpose of Assembly Bill No. 4318 was to ‘[e]liminate the need for conversion of a breath quantity to a blood concentration of alcohol by statutorily defining driving under the influence in terms of the concentration of alcohol found in the breath when breath analysis is used.' (Ibid.; see also Sen. Rules Com., Off. of Sen. Floor Analyses, Rep. on Assem. Bill No. 4318 (1989-1990 Reg. Sess.) Aug. 9, 1990, p. 1.)”


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