Causes of False Positives on a Breathalyzer Test

If you were arrested for DUI and you have been told that a breathalyzer test is "proof" that you have been drinking and over the limit, that might not be true at all. False positive breathalyzer results are more common than people think, and if not challenged in court, it could lead to serious legal consequences.

Keep reading to learn more about the causes of false positives on a breathalyzer test:

How a Breathalyzer Actually Works

Breathalyzer devices are designed to estimate blood alcohol concentration, sometimes called blood alcohol content (BAC), by measuring the alcohol molecules in your breath. The machine that is used by law enforcement officers is regulated by the California Department of Public Health.

Breathalyzer test results are estimates of alcohol content; they do not measure blood directly. It is assumed by the law that they are fairly accurate, but the truth is, there are many ways to get false positive breathalyzer readings, including the following:

1. Residual Mouth Alcohol

One of the most common causes of a false positive breath test is due to residual alcohol in the mouth. If there is still alcohol in the mouth at the time of the testing, the device might measure a higher concentration than than is actually in your bloodstream. This could happen if you took a sip of alcohol in the past few minutes, if you used a mouthwash containing alcohol, if you used breath spray, of it you vomited or burped right before the test. Even fermented foods, prescription medications, food with sugar alcohol, energy drinks, or bread could cause a false positive reading.

Breathalyzer test devices are sensitive, and even a small amount of alcohol in the mouth can produce a false positive result.

In California, an officer is required to wait 15 minutes for observation before administering a breath test. If the waiting period wasn't established, or there is residual alcohol, there could be false readings.

2. Improper Calibration or Maintenance

For these tools to work correctly, they must be properly maintained and calibrated. If they are not, BAC readings can be inaccurate.

Defense strategies often center around breathalyzer machines and attorneys will want to see maintenance records and calibration records from law enforcement agencies to determine if the machine was properly functioning at the time of the test.

3. Medical Conditions That Mimic Alcohol

Certain medical conditions can cause the body to produce substances that a Breathalyzer interprets as alcohol.

Acid Reflux or GERD (Gastroesophageal Reflux Disease)

Individuals with gastroesophageal reflux disease (GERD) may experience regurgitation of stomach contents into the mouth. This can introduce alcohol vapor into the breath sample, even if blood alcohol levels are lower.

Diabetes and Ketones

People with diabetes may produce ketones during episodes of low insulin or ketoacidosis. Some breath testing devices may mistake acetone (a type of ketone) for ethyl alcohol, leading to inaccurate readings.

Low-Carbohydrate Diets

Ketogenic diets can also increase acetone levels in the body, potentially interfering with breath test sensors.

These physiological variables are not always accounted for during roadside testing, as most of the time, officers fail to get a medical history on site.

4. Radio Frequency Interference

Although less common today, certain breath testing devices can be affected by radio frequency interference (RFI). Police radios, cell phones, and other electronic devices may disrupt internal components of the machine if not properly shielded.

While modern equipment includes safeguards against RFI, technical malfunctions still occur — especially in older devices.

5. Environmental Alcohol Exposure

Breath testing devices can detect alcohol vapors in the surrounding environment.

This may be relevant if you were:

• Working around solvents or cleaning agents

• In a bar or restaurant environment

• Using products containing alcohol vapors

Improper air blank testing (which checks the surrounding air before the sample is taken) can lead to contaminated readings, too, and affect breathalyzer results.

6. Rising Blood Alcohol Levels

Timing matters.

Alcohol absorption is not instantaneous. After drinking, BAC continues to rise as alcohol is absorbed into the bloodstream. If you were tested some time after driving — particularly if you had recently consumed alcohol — your BAC at the time of testing may have been higher than it was while driving.

This phenomenon is often referred to as “rising BAC.”

Prosecutors must prove your BAC was over the legal limit at the time you were operating the vehicle — not merely at the time of testing. Expert testimony is sometimes used to analyze absorption rates and challenge assumptions made by the prosecution. A horizontal gaze nystagmus defense lawyer in CA can help guide you through the process.

7. Partition Ratio Variability

Breath testing devices rely on a standardized blood-to-breath partition ratio (typically 2100:1). This assumes that 2100 milliliters of breath contain the same amount of alcohol as 1 milliliter of blood.

However, individual partition ratios vary based on:

• Body temperature

• Breathing pattern

• Hematocrit levels

• Lung function

If your natural partition ratio differs from the assumed standard, your reported BAC may be artificially high.

This is a scientific issue that experienced DUI defense attorneys understand and may explore in appropriate cases. They can also the use of dashcam and body camera footage in DWI cases in CA.

8. Improper Testing Procedure

Human error is always a factor.

Breath test operators must be properly trained and certified. Errors can include:

• Failing to observe the subject properly

• Improperly instructing the subject

• Allowing insufficient breath sample volume

• Contaminating the mouthpiece

• Misreporting results

Even minor procedural mistakes can affect admissibility or weight of the evidence.

Why False Positives Matter in a DUI Case

Breath test results are often presented to juries as objective and scientific. But as outlined above, they are subject to both mechanical and human error.

In Sacramento DUI cases, breath test evidence is typically introduced in proceedings before the Sacramento County Superior Court, as well as during administrative license suspension hearings through the DMV.

Challenging the reliability of a breath test can:

• Reduce charges

• Create reasonable doubt

• Support a motion to suppress evidence

• Strengthen negotiations for a lesser offense

A high BAC number is not the end of your case — particularly if the underlying data is flawed.

What You Should Do After a Breath Test DUI Arrest to Limit Legal Implications

If you believe your Breathalyzer result was inaccurate:

1. Write down what you ate, drank, or used prior to the stop.

2. Note any medical conditions or medications.

3. Document the timeline of your alcohol consumption.

4. Contact a DUI defense attorney promptly.

In California, you have a limited window to request a DMV hearing to contest automatic license suspension. Acting quickly preserves important rights. For more information, a great criminal defense attorney in Sacramento may be able to help.

Work with Goss Law If You Suspect a False Positive Breath Test and Need a DUI Defense

Breath testing devices are useful investigative tools — but they are not infallible. Mechanical issues, physiological variables, environmental factors, and human error can all contribute to false positives.

If you are facing DUI charges based primarily on a breath test result, do not assume the number tells the whole story. A thorough review of the testing process, machine records, and your individual circumstances may reveal significant weaknesses in the prosecution’s case.

Understanding the science — and its limitations — is the first step toward building an effective legal defense. Reach out to Goss Law to discuss your options.