DUI Breath Tests & How to Challenge Them in Court

DUI breath tests are subject to errors that can call the blood alcohol concentration (“BAC”) results into question and get them excluded from evidence. Reasons the test might be inaccurate include:

• The officer administering the test did not follow the required testing procedures;¹
• A high-protein/low-carb diet or a medical condition such as GERD “tricked” the testing device; or
• A phenomenon such as residual mouth alcohol or rising blood alcohol wrongly inflated your BAC level.

The following chart compares evidentiary breath tests and blood tests in DUI cases.

	DUI Blood Tests	DUI Breath Tests
Accuracy	Generally more accurate	Less accurate, can be affected by acid reflux, GERD, mouth alcohol, and more
Invasiveness	Invasive, requires blood draw	Non-invasive, requires breathing into a device
Time to results	Longer, typically weeks	Immediate results
Administration	Requires medical professional	Can be administered by law enforcement
How it measures alcohol	Direct measurement of blood alcohol content (BAC)	Estimates BAC based on breath alcohol content
Detects drugs?	Yes	No
Retention of sample	Sample can be retained for future testing	No sample retention
Refusal consequences	Driver’s license suspension	Driver’s license suspension

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

• 1. When is a breath test given in a DUI case?
• 2. Can I refuse to take it?
• 3. Should I choose breath or blood?
• 4. How do Title 17 regulations apply?
• 5. What is the science behind breath tests?
• 6. Can medical conditions produce false readings?
• 7. Can “residual mouth alcohol” trick a Breathalyzer?
• 8. What other factors can contribute to falsely high readings?
• Additional Reading

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:

1. A roadside preliminary alcohol screening (“PAS”) test, given during a DUI traffic stop and investigation, and
2. A post-arrest “evidentiary” breath test (usually, but not always, given on a desktop device at the police station or, at a sobriety checkpoint, on a mobile police unit).

For more information on drunk driving laws and investigations, 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 tests.

Preliminary Alcohol Screening (“PAS”) Test

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

A PAS is typically given on a handheld instrument such as a Breathalyzer. You are not required to take this test and may decline to do so.² An exception is if you are:

• Currently on DUI probation,³or
• Under the age of 21 (and, therefore, being investigated for underage DUI).⁴

What happens if I refuse to take a PAS test?

If you are on DUI probation or under age 21, a refusal to take a PAS test is considered a chemical test refusal in California. It will result in a mandatory suspension of driving privileges regardless of whether you are convicted of driving under the influence.⁵

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

There is no penalty for refusing to take a PAS test (or any field sobriety test) if you are 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 tests on the Portable Evidential Breath-Alcohol Testing System (PEBT). This device is based on the Draeger Alcotest handheld 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 if you have been placed under arrest, do not take a test on one of these units. Rather, if an officer asks you to take a breath test on a handheld device in the field, we advise requesting a DUI blood test instead.⁸

Post-arrest Evidentiary DUI Breath Test

A chemical test is mandatory for all drivers who have lawfully been arrested for driving under the influence. You must take a post-arrest chemical test (known as an “evidentiary test”) even if you have already taken a preliminary breath test.⁹

Most drivers have a choice of a breath or blood test. Exceptions are when:

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

2. Can I refuse to take it?

Refusal to take a post-arrest chemical test has consequences, such as:

• A driver’s license suspension for at least one year (though you can usually still drive with an ignition interlock device (IID) in your car), and
• A mandatory jail time enhancement of 48 hours if you are ultimately convicted of drunk driving.¹³

You may, however, refuse to take a breath test if you have chosen the blood test.

3. Should I choose breath or blood?

The main advantage of a breath test is that it is quick and non-invasive. However, 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 you for independent retesting via a “blood split” motion.¹⁴

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

Ultimately, what is most important is not which test you choose, but whether the police officers comply with Title 17 procedures.

4. How do Title 17 regulations apply?

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 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, your test results will be ruled inadmissible. There will be no proof of your BAC. This will often lead to a plea bargain or even an outright dismissal of the charges.

What are the 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 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
• You must be observed continuously for 15 minutes before the breath test is given, and during this time you must not:
  • Smoke,
  • Eat, drink, or put anything in your mouth, or
  • Burp, regurgitate, or vomit (which could bring alcohol from your stomach into your mouth);¹⁹
• The breath test operator must collect air from deep in the lungs (deep “alveolar” air);²⁰
• The operator must obtain two 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”);²¹ and
• The laboratory performing the analysis must keep detailed records of equipment calibration, personnel and test results.²²

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.

We will obtain police and laboratory records to look for these errors. We will also be looking to see whether the officer who administered the test:

• Checked to make sure your mouth was empty;
• Observed you continuously for a full 15 minutes before beginning the test (to make sure you did not burp or regurgitate or put anything in your 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 breath tests?

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 breath testing samples be “essentially alveolar in composition.”²⁴

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 you exhale:

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, you are required to blow hard during a breath test.

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

Partition Ratios

Unlike blood tests, 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

Although California’s partition ratio is fixed at 2,100: 1, everyone’s lungs absorb alcohol from the blood at a different rate.²⁷ 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 does not matter. The BAC limit (as well as the 2,100: 1 partition ratio) has been set by the California legislature.

So, if your BAC is .08% or higher on a breath testing device that uses a 2,100:1 partition ratio, you are deemed legally too drunk to drive. It does not matter whether you are 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 your driving was actually impaired by alcohol.²⁸

This can be important in borderline cases, such as when your 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.

However, if your partition ratio was not 2,100:1, your 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 your driving was actually impaired by alcohol.

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 you reveal ALL your medical conditions and dietary issues to your DUI defense lawyer–even if it does not seem relevant. It might be the key to keeping you from being wrongfully convicted based on inaccurate test results.

Let’s take a brief look at some of the most common health conditions that lead to inaccurate BAC results.

Gastrointestinal Reflux Disease (“GERD”)

Chronic digestive conditions such as

• gastrointestinal reflux disease (“GERD”),
• heartburn, or
• acid reflux

can create falsely high BAC readings. Stomach contents sometimes flow back into the mouth.²⁹

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

High Protein / Low Carbohydrate Diets

High-protein / low-carb diets can “trick” breath test devices. Popular diets that can produce this result include:

• Atkins,
• South Beach,
• Zone, and
• Whole 30.

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 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.

Diabetes or Hypoglycemia

Diabetes can lead to falsely elevated BAC readings on a 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 breathalyzer.

7. Can “residual mouth alcohol” trick a Breathalyzer?

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

Breath tests require you to blow hard to produce deep lung air in the sample. However, as this breath exits through the mouth, it picks up any residual mouth alcohol.

Even a small amount of mouth alcohol is present, it will be picked up and deposited into your sample.

Causes of Residual Mouth Alcohol

Things that can cause residual mouth alcohol include:

• Recently consumed alcohol, even if not enough to make you legally drunk (“one for the road”),
• Cough syrup, or
• Mouthwashes and sprays that contain 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 you before beginning a DUI breath test.³⁶

If the test is done too soon—or if you consume any alcohol during the waiting period– it can create falsely high BAC readings.

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 the time a breath test is taken, blood alcohol levels might be higher than when you were driving.

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

Inhaled Chemicals (Such as Acetone)

Although rare, BAC can sometimes be falsely inflated if you 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 breath testing equipment.

Back in the 1980s, it was discovered that some 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,
• handheld police transmitters,
• teletypes, and
• police radar.

Newer breath testing units can generally handle this type of interference. However, we will still want to review the type of equipment and circumstances under which your test was given.

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

Additional Reading

For more in-depth information, refer to these scholarly articles:

• Alcohol Breath Testing: Is There Reasonable Doubt – Syracuse Journal of Science and Technology.
• Comparison of breath-alcohol screening test results with venous blood alcohol concentration in suspected drunken drivers – Forensic Science International.
• Breath and Blood Alcohol Concentration Measurement in DUI Cases – Forensic Metrology.
• Measuring alcohol in blood and breath for forensic purposes-a historical review – Forensic Science Review.
• The Accuracy of Handheld Pre-Arrest Breath Test Instruments as a Predictor of the Evidential Breath Alcohol Test Results – Journal of Forensic Sciences.

---

Legal references: