DUI Breath Tests & How to Challenge Them in Court


DUI breath tests are subject to a variety of errors that can call the results into questions. Reasons that a breath test might not accurately reflect someone's blood alcohol concentration (“BAC”) include:

To help you better understand breath tests in drunk driving cases, our California DUI defense lawyers discuss, below:

california breathalyzer fst legal defense attorneys
Most people arrested for drunk driving in California choose to take a DUI breath test to measure their blood alcohol content (BAC).

1. When is a breath test given in a DUI case?

Law enforcement uses two types of breath tests in DUI cases. These are given (if at all) during two stages in the process as follows:

For more information on DUI investigations, please see our article on “The stages of a California DUI case.”

And in the meantime, let's take a closer look at each of these two types of breath tests.

1.1. Preliminary alcohol screening (“PAS”) tests

A preliminary alcohol screening (PAS) test is given during a DUI investigation. This occurs after someone has been pulled over or stopped at a DUI checkpoint, but before an arrest has been made.

A PAS is typically given on a hand-held breath testing instrument, such as a Breathalyzer. Most drivers are not required to take this preliminary test and may freely decline to do so.2 An exception is drivers who are:

What happens if I refuse to take a PAS test?

For people on DUI probation and drivers under age 21, refusal to take a PAS test is considered a chemical test refusal in California. It will result in a mandatory driver's license suspension regardless of whether the driver is convicted for DUI.5

Otherwise, the PAS test is treated as a field sobriety test (FST).6 It merely helps the officer decide whether to arrest someone for driving under the influence.

There is no penalty for refusing to take a PAS test (or any field sobriety test) if the driver is 21 or older and not on DUI probation.7

Warning: portable evidentiary breath test units

In some California counties (including Ventura and Orange) police conduct post-arrest breath tests on the Portable Evidential Breath-Alcohol Testing System (PEBT). This device is based on the Draeger Alcotest handheld breath testing device used by some police departments for preliminary screenings.

The difference is that the PEBT can be hooked up to a printer by hardline or blue tooth. This turns it into an “evidentiary” unit. We recommend that a driver who has clearly been placed under arrest not take a breath test on one of these units.

Rather, if an officer asks a driver to take a breath test on a handheld device in the field, we advise requesting a DUI blood test instead.8

1.2. Post-arrest evidentiary DUI breath test

A chemical test is mandatory for all drivers who have lawfully been arrested for DUI. A driver must take a post-arrest chemical test (known as an “evidentiary test”) even if he or she has already taken a preliminary breath test.9

Most drivers may take their choice of a breath or blood test. Exceptions are when:

  • The officer reasonably believes the driver was driving under the influence of drugs (“DUID”) (in which case a blood test can be required);10
  • The driver is unable to blow hard enough to take a DUI breath test,
  • The driver is unconscious or has died;11 or
  • Due to the need for medical care, the driver has been taken to a medical facility where breath testing equipment is not available.12

2. Can a driver refuse to take a DUI breath test?

Refusal to take a post-arrest chemical test has consequences. Such consequences typically include:

  • Suspension of the driver's license for at least one year, and
  • A mandatory jail enhancement of 48 hours if the driver is ultimately convicted of DUI.13

A driver may, however, always refuse to take a breath test if he/she has chosen a blood test.

(If the defendant is convicted in criminal court of a chemical test refusal allegation enhancement, then the court may permit the defendant to drive without restrictions if he/she gets an ignition interlock device (IID) installed.)

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 breath test or blood test is best.

The main advantage of a breath test is that it is quick and non-invasive. But, as we explain below, breath tests are prone to a variety of errors. These can--and often do—result in falsely high BAC readings.

The main advantage of a blood test is that a portion of the blood drawn is saved. This blood must then be made available to the defendant for independent retesting via a “blood split” motion.14

Breath samples are not saved

Unlike blood samples, breath samples are not preserved.15 Thus there is no way to conduct later independent testing to verify the accuracy of the test.

But ultimately, what is most important is not which test a driver chooses, but whether the police comply with Title 17 procedures.

blood test vial california dui breath test legal experts attorneys
One advantage a California DUI blood test has over a breath test is that part of the sample can be saved and independently retested later.

4. How do Title 17 regulations apply to DUI breath tests?

4.1. What is Title 17 of the California Code of Regulations?

Title 17 of the California Code of Regulations sets forth DUI chemical testing procedures. Law enforcement and laboratories must follow these procedures when they collect and process DUI breath samples.

If Title 17 procedures are not followed, BAC measurements may be compromised. An experienced lawyer can use these errors to help fight California drunk driving charges.

The first step is a motion to exclude evidence under Penal Code 1538.5 PC. If the judge grants the motion, the breath test results will be ruled inadmissible. There will be no proof of the defendant's BAC. This will often lead to a DUI plea bargain or even an outright dismissal of the charges.

4.2. What are some important Title 17 breath testing regulations?

There are dozens of rules for DUI chemical tests set forth in Title 17. Some of the most important are:

  • The DUI breath testing device must be kept in good working order;16
  • The device must be calibrated every 10 days or 150 uses (whichever occurs first),17
  • The person giving the test must be trained on the specific device used;18
  • The driver must be observed continuously for 15 minutes before the DUI breath test is given and during such time must not:
    • Smoke,
    • Eat, drink, or put anything in his/her mouth, or
    • Burp, regurgitate, or vomit (which could bring alcohol from the stomach into the mouth).19
  • The DUI breath test operator must collect air from deep in the lungs (deep “alveolar” air)20;
  • 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 (note that this may require more than two “blows”); and21
  • The laboratory performing the analysis must keep detailed records of equipment calibration, personnel and test results.22

4.3. Can Title 17 violations lead to falsely elevated BAC results?

Title 17 violations that can lead to falsely elevated BAC readings fall into three broad categories:

  • Equipment that was not tested or was improperly calibrated,
  • Operator error, or
  • Failure to keep records as required by California law.

An experienced California drunk driving lawyer will obtain police and laboratory records to look for these errors. He or she will also be looking to see whether the officer who administered the test:

  • Checked to make sure the driver's mouth was empty;
  • Observed the driver continuously for a full 15 minutes before beginning the test (to make sure the driver did not burp or regurgitate or put anything in his/her mouth);
  • Properly attached the mouthpiece to the machine;
  • Recorded the time at which the 15-minute period started; and
  • Recorded the time at which each “blow” was made.

5. What is the science behind testing breath for BAC?

5.1. Deep lung air

DUI breath tests measure the amount of alcohol in “alveolar” (deep lung) air. The alveoli are balloon-like sacs located deep within the lungs and surrounded by capillaries.

Capillaries are slender blood vessels no more than one one-thousandth of a millimeter thick. They are thin enough to let oxygen pass from the lungs into the blood.

They also let carbon dioxide and other wastes, including alcohol, pass from the blood into the alveoli.23 This is why California law requires that DUI breath testing samples be “essentially alveolar in composition.”24

scientific diagram of human lung california legal defense attorneys dui
California breath tests measure BAC because capillaries are thin enough to let a small amount of alcohol pass from the blood into the lungs.

How breath testing devices measure deep lung air

To understand how a breath test works, consider the order in which air exits from the lungs when someone exhales:

  1. From the mouth/nasal area,
  2. From the throat and upper airway, and finally
  3. From the lungs.25

Because it is the deepest, alveolar air is the last to leave your lungs. It is also where alcohol concentration is the highest. As a result, in order to obtain an accurate reading, a test subject is required to blow hard during a California DUI breath test.

Not everyone is capable of blowing hard enough for a reliable breath test reading. Older people and those with medical conditions affecting the lungs may have trouble generating the volume of air required.

alveolus gas exchange diagram california dui expert legal defense attorneys
Alcohol passes from the capillaries into the alveoli—balloon-like sacs located deep within the lungs.

5.2. Partition ratios

Unlike blood tests, DUI breath tests do not directly measure blood alcohol concentration. The testing device measures the alcohol in deep lung air and mathematically converts the amount to a roughly equivalent BAC.

The conversion factor used is known as a “partition ratio.” The partition ratio is meant to reflect the relationship between alcohol measured in the breath and alcohol in the blood.

In California, the partition ratio for breath-testing machines is set by law at 2,100 to 1. This means the amount of alcohol in 2,100 milliliters (210 liters) of breath is considered equivalent to the amount of alcohol present in 1 milliliter of blood.26

How California's “partition ratio” can lead to inaccurate BAC results

By law, California's partition ratio is fixed at 2,100: 1.27 But, in fact, everyone's lungs absorb alcohol from the blood at a different rate. This means that actual partition ratios can vary widely--both in the general population and for one individual at different times.

In cases of Vehicle Code 23152(b), driving with a BAC of .08% or higher this doesn't matter. This number (as well as the 2,100: 1 partition ratio) has been set by the California legislature.

So, a driver whose BAC is .08% or higher on a DUI breath testing device that uses a 2,100:1 partition ratio is deemed legally too drunk to drive. It does not matter whether his or her driver is in fact actually impaired.

How the science of partition ratios can help someone fight VC 23152(a) charges

The science of partition ratios can help fight charges of driving under the influence under California Vehicle Code 23152(a). This section of the vehicle code requires the prosecutor to prove that the defendant's driving was actually impaired by alcohol.28

This can be important in borderline cases, such as when a defendant's BAC tests at .06% or .07%. When combined with other evidence (such as traffic violations, poor performance on field sobriety tests, or physical symptoms of intoxication), it could be enough to get a conviction.

But if the defendant's partition ratio was not 2,100:1, his/her BAC might not really be borderline. A DUI defense expert witness can explain this to the jury. This is sometimes enough to create reasonable doubt that the defendant's driving was actually impaired by alcohol.

california fst field sobriety test california dui legal defense attorneys
Certain medical conditions can mimic alcohol impairment—including producing an elevated BAC on a California drunk driving breath test.

6. Can medical conditions produce false readings?

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

  • Gastrointestinal reflux disease (“GERD”),
  • High protein/low carb diets, and
  • Diabetes.

It is critical for someone arrested for driving under the influence to disclose ALL medical conditions and dietary issues to his or her DUI defense lawyer--even if it doesn't seem relevant.

An experienced DUI lawyer will know whether a medical condition might lead to a DUI defense. It might be the key to keeping a driver from being wrongfully convicted based on inaccurate DUI breath test results.

Let's take a brief look at some of the most common health conditions that lead to inaccurate BAC results on DUI breath tests.

6.1. Gastrointestinal reflux disease (“GERD”)

Chronic digestive conditions such as gastrointestinal reflux disease (“GERD”), heartburn, or acid reflux can create falsely high BAC readings. In people with these conditions, stomach contents sometimes flow back into the mouth.29

Breathalyzer machines can read the stomach acid contained in this “backwash” as alcohol.30

gerd acid reflux diagram california dui attorneys
Medical conditions such as GERD let alcohol move from the stomach into the mouth, potentially making DUI breath test results unreliable.

6.2. High protein / low carbohydrate diets

High-protein / low-carb diets can “trick” DUI breath test devices. Popular diets that can produce this result include the Atkins, South Beach, Zone and Whole 30 diets.

These diets force the body to use stored fat instead of glucose as an energy source. This creates a by-product known as “ketones.”31 Ketones are chemically similar to acetone.

Many DUI breath testing devices cannot reliably distinguish between acetone and ethyl alcohol (the type of alcohol found in alcoholic beverages).32 So these diets fool the device into reading ketones as blood alcohol. 

6.3. Diabetes or hypoglycemia

Diabetes can lead to falsely elevated BAC readings on a DUI breath test. This is because diabetics' livers often produce “ketones,” which are chemically similar to a type of alcohol.33

Diabetics have trouble producing insulin. Insulin is a hormone that helps the body burn glucose for fuel. So diabetics must burn stored fat for energy.34

As discussed above, ketones are a by-product of the body burning fat stores for energy. Some of these ketones are excreted in the breath and can "fool" a DUI breath test.

7. Can "residual mouth alcohol" trick a Breathalyzer?

After sometime drinks an alcoholic beverage, some alcohol remains in the mucosal linings of the mouth. This is known as “residual mouth alcohol.”

DUI breath tests require the driver to blow hard so as to produce deep lung air in the sample. But as this breath exits through the mouth, it picks up any residual mouth alcohol.

Residual mouth alcohol remains in the mouth for approximately 15-20 minutes. So if even a small amount of mouth alcohol is present, it will be picked up and deposited into the breath sample.

Causes of residual mouth alcohol

Things that can cause residual mouth alcohol include:

  • Recently consumed alcohol, even if not enough to make the driver legally drunk (“one for the road”),
  • Cough syrup, or
  • Mouthwashes and breath sprays that contains alcohol.35

Most mouth alcohol dissipates after 15-20 minutes. This is why Title 17 regulations require a 15-minute waiting period with continuous observation of the subject before beginning a DUI breath test.36If the test is done too soon—or if the person consumes any alcohol during the waiting period-- it can create falsely high BAC readings.

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, the friend opens a bottle of champagne and insists Ralph have a glass. Ralph drinks his glass of champagne and leaves.

A few minutes after driving off, 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.

Ralph chooses a DUI breath test, which the officer immediately administers on a portable PEBT. It shows a BAC of 0.10%. This puts Ralph over the “legal limit” as set forth in California Vehicle Code 23152(b), driving with a BAC of .08% or higher.

But the results of the PEBT are probably not reliable. Ralph only had one glass of champagne and he drank it less than 15 minutes before the PEBT was administered.

Ralph's DUI defense, therefore, will likely include taking the position that residual mouth alcohol falsely elevated Ralph's BAC results.

california dui breath test breath spray legal defense attorneys
Many breath sprays contain alcohol that can falsely elevate BAC results on a California DUI breath test.

8. What other factors can contribute to falsely high readings?

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

"Rising blood alcohol" levels

After drinking, blood alcohol levels continue to rise for 30 to 45 minutes.37 In some cases, it can take as long as two hours for alcohol to be fully absorbed by the bloodstream.38

A DUI investigation and arrest can occur within this period. This means that by a DUI breath test is taken blood alcohol levels might be higher than when the person was driving.

An expert witness on drunk driving can often “back out” a driver's BAC. This may show that the arrest occurred when the defendant's BAC was still “on the rise.”

Inhaled chemicals (such as acetone)

Although rare, BAC can sometimes be falsely inflated in people who work around volatile chemicals (such as the acetone found in paint). If these volatile “interferents” are inhaled, they can be read by breath testing devices as alcohol.

Radio frequency interference

There is a scholarly debate about whether radio frequency interference (RFI) can compromise DUI breath testing equipment.

Back in the 1980s, it was discovered that some DUI breath testing devices produced unreliable results due to RFI. RFI resulted from the presence of other devices that emit radio frequencies. Common RF-emitting devices include AM and FM radios, police station dispatchers, hand-held police transmitters, teletypes, and police radar.

Newer breath testing units can generally handle this type of interference. But an experienced California drunk driving attorney will still want to review the type of equipment and circumstances under which a test was given.

These may be enough to raise a reasonable doubt about whether the BAC results were elevated because of radio interference from other devices.

Charged with drunk driving in California? Call us for help…

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Call us at 855-LAWFIRM

If you were charged with drunk driving after a DUI breath test, we invite you to contact us for a free consultation.

Call us at 855-LAWFIRM to speak confidentially to an experienced California drunk driving attorney in your area.

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.

We also have offices in Las Vegas and Reno that may be able to help people challenge DUI breath tests in Nevada.

Legal references:

  1. Title 17 of the California Code of Regulations (“CCR”) section 1215 and subsequent sections set forth the procedures law enforcement is required to follow in administering and processing DUI chemical tests.

  2. Vehicle Code 23612(i) – Right to refuse preliminary alcohol screening DUI breath test.

  3. Vehicle Code 13389 (a): If a peace officer lawfully detains a person previously convicted of Section 23152 [DUI] or 23153 [DUI causing injury] who is driving a motor vehicle, while the person is on probation for a violation of Section 23152 or 23153, and the officer has reasonable cause to believe that the person is in violation of Section 23154 [driving with BAC of .01% or higher while on DUI probation], the officer shall request that the person take a preliminary alcohol screening test to determine the presence of alcohol in the person, if a preliminary alcohol screening test device is immediately available. If a preliminary alcohol screening test device is not immediately available, the officer may request the person to submit to chemical testing of his or her blood, breath, or urine, conducted pursuant to Section 23612.

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

  5. Vehicle Code 13353.1 VC.

  6. Vehicle Code 23612(h): “A preliminary alcohol screening test that indicates the presence or concentration of alcohol based on a breath sample in order to establish reasonable cause to believe the person was driving a vehicle in violation of Section 23140, 23152, or 23153 is a field sobriety test and may be used by an officer as a further investigative tool.”

  7. Same.

  8. See 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.”

  9. Vehicle Code 23612(a)(1)(A).

  10. Vehicle Code 23612(a)(2)(C).

  11. Vehicle Code 23612(a)(5).

  12. Vehicle Code 23612(a)(3).

  13. Vehicle Code 23612(a)(1)(D): “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, administrative suspensions, or revocations.”

  14. 17 CCR 1219.1(f).

  15. See California v. Trombetta (1984) 467 U.S. 479 (holding that failure to retain breath samples after a DUI breath test does not violate a defendant's constitutional due process rights - see our article on Trombetta motions in California criminal cases).

  16. 17 CCR 1220.2(a)(5) and 17 CCR 1221.2(b).

  17. 17 CCR 1221.2(a)(2)(B).

  18. 17 CCR 1216.1(b).

  19. 17 CCR 1221.1(b)(1): “The breath sample shall be collected only after fifteen continuous minutes during which time the subject must not have ingested alcoholic beverages or other fluids, regurgitated, vomited, eaten, or smoked.”

  20. 17 CCR 1215 (r): “'Sample' or ‘Specimen' means a representative portion of blood, urine, tissue, an artificially constituted material, or a portion of expired breath which is essentially alveolar in composition obtained for the purpose of measuring its alcohol concentration.”

  21. 17 CCR 1221.2(a) (1).

  22. 17 CCR 1222 and 1222.1.

  23. For a detailed discussion of breath testing methodology and science, see People v. McNeal (2009) 46 Cal 4th 1183.

  24. 17 CCR 1215 (r), endnote 20.

  25. National Heart, Blood and Lung Institute, “How the Lungs Work.”

  26. 17 CCR 1220.4(f): “A breath alcohol concentration shall be expressed as the number of grams of alcohol per 210 liters of breath.”

  27. See same.

  28. Vehicle Code 23152(a): “It is unlawful for a person who is under the influence of any alcoholic beverage to drive a vehicle.”

    See also California Criminal Jury Instructions (CALCRIM) 2110. Driving Under the Influence : “A person is under the influence if, as a result of (drinking [or consuming] an alcoholic beverage/ [and/or] taking a drug), his or her mental or physical abilities are so impaired that he or she is no longer able to drive a vehicle with the caution of a sober person, using ordinary care, under similar circumstances.”

  29. See, e.g., Mayo Clinic, “Gastroesophageal reflux disease (GERD).”

  30. See, e.g., J.L. Booker and K. Renfroe, “The Effects of Gastroesophageal Reflux Disease on Forensic Breath Alcohol Testing,” J Forensic Sci. 2015 Nov;60(6):1516-22..

  31. See, e.g., WebMD, “What are ketones?”

  32. See, e.g., A.W. Jones and S. Rössner, “False-positive breath-alcohol test after a ketogenic diet,” Int J Obes (Lond). 2007 Mar; 31(3):559-61.

  33. WebMD, endnote 31.

  34. See, e.g., American Diabetes Association, “Insulin Basics.”

  35. See, e.g., R.L. Moore and J. Guillen, “The effect of breath freshener strips on two types of breath alcohol testing instruments,” J Forensic Sci. 2004 Jul;49(4):829-31.

  36. 17 CCR 1221.1(b)(1), endnote 19.

  37. See National Institute on Alcohol Abuse and Alcoholism, Alcohol Alert No. 35.

  38. Same.

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