Your breathalyzer result may not tell the whole story — and a single number should not define your future.
This article is for general information and is not legal advice. Reading it does not create an attorney-client relationship.
You passed the field sobriety tests. You felt fine to drive. Then a police officer pulled you over, handed you a small handheld device, and told you to blow.
Seconds later, a number appeared on that screen. That number — a blood alcohol content reading — may now be the centerpiece of a DUI case against you.
But here is something most people never hear: breathalyzer accuracy is not absolute. Breath test machines can produce incorrect readings. Calibration errors, improper procedures, medical conditions, and environmental factors can all push that number higher than your actual BAC.
In California DUI cases, a breath test result can feel like a slam dunk for the prosecution. But experienced DUI attorneys challenge these results regularly. And those challenges sometimes succeed.
If you are facing DUI charges in San Diego, understanding how breath tests work — and where they fail — could be one of the most important things you do.
What Breath Testing Devices Are Actually Measuring
Before we get into breathalyzer accuracy problems, it helps to understand what these machines are doing in the first place.
Breath test devices do not directly measure the alcohol in your blood. They estimate it.
When you exhale, the machine captures a sample of deep lung air. It then uses a chemical or infrared process to detect alcohol molecules in the air and calculate an estimated blood alcohol content. That estimated figure becomes your BAC reading.
The calculation relies on something called a partition ratio — the assumed relationship between alcohol in your breath and alcohol in your blood. California DUI law uses a standardized ratio of 2,100:1, meaning the device assumes 2,100 milliliters of breath contains the same amount of alcohol as 1 milliliter of blood.
The problem? That ratio varies from person to person. Studies in forensic science show the actual ratio can range from roughly 1,700:1 to 2,400:1 in real human beings. If your personal ratio is lower than the assumed standard, the machine will overestimate your BAC.
That is a built-in source of error — before we even consider equipment problems.
Two Types of Breath Test Devices Used in San Diego DUI Cases
In California, law enforcement uses two different types of breath testing devices, and they play different roles in a DUI arrest.
The Preliminary Alcohol Screening (PAS) Device
The PAS device is a small, handheld breathalyzer that a police officer uses on the side of the road during a traffic stop. It is a portable, field-use device.
For most drivers who are not on DUI probation, the PAS test is technically voluntary before arrest. However, many people do not know this, and police are not always required to make that crystal clear.
PAS results are generally used to establish probable cause for arrest. They are less precise than station machines, and DUI defense attorneys often scrutinize them closely.
Evidential Breath Testing Machines at the Station
After a DUI arrest, you will typically be taken to a police station or jail to take a second, more formal breath test on an evidential breath testing machine.
In San Diego and across Southern California, the most commonly used device is the Draeger Alcotest 9510. The result from this machine is the official BAC reading that goes into your criminal case and is used in DMV hearings.
These machines are supposed to be more accurate than PAS devices — but they are still machines, and they still fail.
Common Breathalyzer Accuracy Problems That Can Affect Your DUI Case
Breathalyzer accuracy depends on many factors that are not always within your control. Here are the most common problems DUI defense attorneys investigate in San Diego cases.
1. Improper Calibration and Maintenance
Breath test machines must be regularly calibrated and maintained according to strict protocols. California regulations require calibration checks and records to be kept.
If a machine has not been properly calibrated, its readings can drift. An officer may not know the machine is producing inflated results. Without accurate calibration records, the reliability of any reading from that device becomes genuinely questionable.
A skilled DUI attorney can subpoena maintenance and calibration logs for the specific device used in your case. If those records show missed checks, failed calibrations, or procedural gaps, that becomes a powerful tool in your legal defense.
2. The 15-Minute Observation Period Was Not Followed
Before administering an evidential breath test, the officer is required to observe you for a continuous 15-minute period. The purpose is to make sure you do not belch, burp, vomit, or put anything in your mouth — all of which can cause mouth alcohol to contaminate the sample.
If that observation period is not properly documented or was interrupted, the breath test result may be challenged. This is one of the most commonly overlooked procedural requirements in DUI cases.
3. Mouth Alcohol Contamination
Breathalyzer accuracy can be seriously undermined by mouth alcohol — residual alcohol in your mouth, throat, or esophagus that was not in your lungs.
Several things can cause elevated mouth alcohol readings:
- Burping or acid reflux shortly before the test
- Using mouthwash or breath spray that contains alcohol
- Dental work, gum disease, or oral bleeding
- Eating certain foods that produce fermentation in the mouth
Modern machines have some safeguards against mouth alcohol, but they are not foolproof.
4. Medical Conditions That Skew Results
Certain medical conditions can cause falsely elevated BAC readings, even when a person has consumed little or no alcohol. This is one of the most significant breathalyzer accuracy issues in DUI cases.
Conditions that may affect results include:
- Gastroesophageal reflux disease (GERD) or acid reflux — Can push stomach alcohol into the esophagus and mouth.
- Diabetes or low-carb diets — Can cause the body to produce acetone, which some machines misread as ethanol.
- Asthma or other respiratory conditions — Can affect how deep lung air is captured and measured.
- Hypoglycemia — May produce compounds that interfere with breath test chemistry.
If you have any of these conditions, your DUI attorney should know. This information can be central to your defense strategies.
5. Radio Frequency Interference (RFI)
Breath test machines can be affected by radio frequency interference from nearby electronic devices, police radios, cell phones, and other equipment. While newer devices have RFI detection features, interference issues have been documented in court cases across the country.
If the test was conducted near active radio equipment and the machine did not flag an RFI error, that does not automatically mean interference did not occur.
6. Environmental Contaminants
Breathalyzer accuracy can also be compromised by certain chemicals in the surrounding environment. People who work with paint, varnish, cleaning solvents, or other industrial chemicals may have traces of these substances in their breath.
Some of these compounds contain chemical structures similar enough to ethanol that older infrared devices may register a false positive. Forensic science experts who testify in DUI cases often raise this issue when a defendant has a documented occupational exposure.
7. Rising Blood Alcohol at the Time of the Test
This is called the “rising BAC” defense. Alcohol takes time to absorb into your bloodstream after you drink. If you consumed alcohol shortly before driving, your BAC may have been legal at the time you were behind the wheel, but continued to rise by the time you took the breath test.
For example, if your BAC was 0.07 when you were driving and 0.09 by the time you blew at the station, the result does not necessarily prove you were over the legal limit while you were actually driving.
Timing, the amount consumed, and when you ate last all factor into this analysis.
California DUI Law and What the Prosecution Needs to Prove
Under California Vehicle Code Section 23152, it is illegal to drive:
- With a blood alcohol content of 0.08% or higher (standard drivers)
- With a BAC of 0.04% or higher if driving a commercial vehicle
- While impaired by alcohol or drugs, regardless of BAC
The prosecution does not simply hand a jury your BAC number and rest. They must show that the number was obtained through a valid, reliable process. That is where breathalyzer accuracy arguments become relevant in your criminal case.
California DUI laws also require that breath testing follow specific procedures established by the California Code of Regulations, Title 17. Violations of Title 17 requirements — like the observation period, approved device use, and operator certification — can affect the admissibility or weight given to breath test evidence.
What Happens After a DUI Arrest in San Diego
Understanding the process helps you understand why time matters and why calling an experienced DUI attorney quickly is so important.
At the Scene After a DUI arrest, a police officer will typically confiscate your driver’s license and issue a pink temporary license valid for 30 days.
The DMV Hearing You have 10 days from the date of your arrest to request a DMV administrative hearing to contest your license suspension. If you miss that window, your driving privileges are automatically suspended. This is a separate process from your criminal case and has its own deadlines and procedures.
The Criminal Case Your DUI case will proceed through criminal court. Depending on the facts, you may be charged with a misdemeanor or, in cases involving injury or prior convictions, a felony DUI. The criminal process includes arraignment, pre-trial hearings, potential plea negotiations, and possibly a trial.
The serious consequences of a DUI conviction can include jail time, fines, mandatory DUI programs, license suspensions, and a permanent criminal record. For a commercial vehicle driver, a DUI conviction can end a career.
Why Blood Testing Is Sometimes a Better Option
When you are arrested for DUI, you have the right to choose either a breath test or a blood test (a urine test is available in limited circumstances, generally only when neither breath nor blood is feasible).
Blood testing has some advantages over breath testing when it comes to accuracy. Blood samples can be preserved and independently retested by a defense expert. They measure actual alcohol concentration in your blood rather than estimating it from breath.
However, blood testing also has its own chain-of-custody and lab procedure issues that a DUI defense attorney can investigate.
If you took a breath test, your attorney may be able to request an independent blood draw after the fact in some circumstances, though this option has limitations.
The Role of an Experienced DUI Attorney in Challenging Breath Test Results
Breathalyzer accuracy problems do not automatically win your case. They must be raised strategically, with the right evidence, in the right proceedings.
An experienced DUI attorney will typically:
- Review the arrest report for procedural errors and observation period documentation
- Subpoena calibration and maintenance records for the specific machine used
- Investigate your medical history for conditions that could affect the result
- Consult forensic science experts who can testify about testing limitations
- Challenge chain of custody and officer certification, if applicable
- Fight for your driving privileges at the DMV hearing
- Evaluate plea bargain options versus taking your case to trial
As a former Orange County Deputy Public Defender, Susan Hartman has extensive trial experience and understands how the criminal justice system works from the inside.
About The Law Offices of Susan L. Hartman
The Law Offices of Susan L. Hartman is a San Diego DUI defense firm with extensive experience in California DUI law, DMV hearings, and criminal defense. The firm handles DUI matters ranging from first-offense misdemeanor cases to felony DUI charges, helping clients across Southern California navigate a process that can feel overwhelming and unfair.
The firm’s approach starts with a deep understanding of the science behind breath testing — and an equally thorough knowledge of the criminal justice system, local DUI laws, and court procedures in San Diego County.
“When someone comes to us after a DUI arrest, they are often scared and confused. Our job is to make sure they understand exactly what is happening, what options they have, and what we are going to do to fight for them. A breath test number is not the end of the story.”
The firm has successfully defended clients charged with a wide range of DUI offenses, including cases ranging from first-time arrests to cases involving prior convictions. The law offices offer a free consultation, flexible payment plans, and a fee arrangement designed to make quality legal defense accessible.
Whether you are facing your first DUI charge or dealing with a more complex criminal case, the firm brings proven strategies and a genuine commitment to protecting your rights, your driving privileges, and your future.
Do Not Plead Guilty Before You Talk to a DUI Attorney
One of the biggest mistakes people make after a DUI arrest is thinking the breath test result means there is no point in fighting. That is simply not true.
Breathalyzer accuracy problems are real, documented, and regularly raised in California DUI cases. Courts and juries understand that machines are not perfect. A skilled legal defense can make the difference between a DUI conviction on your criminal record and a much better outcome.
Before you plead guilty, speak with a DUI attorney who knows the science, knows the local courts, and knows how to build a defense that works.
The Law Offices of Susan L. Hartman offers a free consultation. Call (619) 260-1122 today.
Frequently Asked Questions About Breathalyzer Accuracy and DUI Breath Tests in San Diego
How accurate are breathalyzer tests in California DUI cases? Breathalyzer accuracy varies depending on the device, calibration, operator technique, and the individual being tested. California DUI law uses a partition ratio of 2,100:1 to convert breath alcohol to blood alcohol, but real human ratios can vary significantly. This built-in variability, combined with calibration issues and individual factors, means breath test results can be higher than a person’s actual BAC.
Can I refuse a breath test during a DUI stop in California? In California, you can refuse the preliminary PAS roadside test if you are not on DUI probation and have not been arrested yet. However, once you are arrested, California’s implied consent law requires you to submit to a chemical test — either breath or blood. Refusing after arrest leads to automatic license suspension and can be used against you in court.
What is the difference between a PAS device and the station breath test machine? The PAS (Preliminary Alcohol Screening) device is a handheld, portable unit used roadside before arrest to establish probable cause for a DUI arrest. The station machine — such as the Draeger Alcotest 9510 used in San Diego — is a larger, evidential device used after arrest to produce the official BAC reading used in your criminal case and DMV hearings.
Can medical conditions affect a DUI breath test result? Yes. Conditions like GERD, diabetes, low-carb diets, and respiratory conditions can cause false or inflated BAC readings on a breath test. Acetone produced by diabetic ketosis, for example, can be misread by some breath test machines as ethanol. An experienced DUI attorney will explore these factors as part of your defense strategies.
What is Title 17, and why does it matter in a DUI case? Title 17 of the California Code of Regulations establishes the rules for chemical testing in DUI cases, including device approval, calibration requirements, operator certification, and the 15-minute observation period. Violations of Title 17 requirements can be used to challenge the reliability of breath test evidence in your criminal case.
How long do I have to request a DMV hearing after a DUI arrest in California? You have 10 days from the date of your DUI arrest to contact the DMV and request an administrative hearing to contest your license suspension. Missing this deadline typically results in an automatic suspension of your driving privileges. This DMV process is separate from your criminal court case.
What is a rising BAC defense in a California DUI case? A rising BAC defense argues that your blood alcohol content was at or below the legal limit while you were actually driving, but continued to rise by the time you were tested. Because alcohol takes time to absorb, someone who recently consumed alcohol may test higher at the station than they were on the road. Timing, the amount consumed, and eating habits are all relevant to this analysis.
Can an attorney get a DUI breath test result thrown out? In some cases, yes. If there were procedural violations — such as a failure to complete the 15-minute observation period, improper calibration, or device malfunctions — a DUI defense attorney may be able to challenge the admissibility or reliability of the breath test result. Even if the result is not excluded entirely, raising legitimate breathalyzer accuracy questions can create reasonable doubt.
What are the penalties for a first DUI conviction in California? Penalties for a first DUI conviction in California can include jail time (up to 6 months for a misdemeanor, depending on the facts), fines and fees, a DUI education program, license suspension, and a criminal record. Specific penalties depend on the facts of your case. An experienced DUI attorney can explain the range of outcomes and work to minimize the consequences.
Should I use a public defender for a DUI case involving breathalyzer accuracy issues? A public defender can provide competent representation, but they typically carry heavy caseloads. DUI cases involving breathalyzer accuracy challenges, medical conditions, or forensic science issues often benefit from the focused attention a private DUI attorney can provide. The Law Offices of Susan L. Hartman offers a free consultation and flexible payment plans to make private representation accessible.
What should I do immediately after a DUI arrest in San Diego? Contact a DUI attorney as soon as possible — ideally within the first 24 to 48 hours. You have only 10 days to request a DMV hearing. An attorney can begin preserving evidence, reviewing the police report, and building your defense before critical deadlines pass.
This article is for general information and is not legal advice. Reading it does not create an attorney-client relationship. DUI laws and procedures are complex and fact-specific. Always consult a licensed attorney about your individual situation.
Contact The Law Offices of Susan L. Hartman
If you are facing DUI charges in San Diego and have questions about breathalyzer accuracy or your legal options, contact the firm today for a free consultation.
The Law Offices of Susan L. Hartman 8880 Rio San Diego Dr, Suite 800, PMB 846 San Diego, CA 92108 (619) 260-1122
Serving clients in San Diego and throughout Southern California.