What Can Make a Breathalyzer Test Inaccurate?
A Breathalyzer is a machine that identifies the amount of alcohol in air from your lungs and then calculates your blood alcohol level based upon a standard generalized formula. Many environmental and personal factors can alter the results both up and down. While most changes are slight, a combination of factors or a factor under extreme conditions can yield a result that is unreliable.
Breath testing machines have been around for a long time. The first known use was in 1931 when the drunkometer developed by Rolla Neil Harger of the Indiana University School of Medicine was used. Prohibition didn't end until December of 1933 so the drunkometer didn't have to be incredibly accurate to catch a law breaker.
Now we have machines that are very precise to the point where a few percentage points of error can put you above or below the line where there is a presumption of intoxication and impairment.
A DUI attorney considers these factors when a client is arrested for a DUI with a high breath test level.. The list below are most but not all of the factors that can skew an alcohol breath test unfairly against a client. Our practice often focuses on defending doctors and other professionals whose careers will be ruined if they are convicted of a drunk driving. This means that we necessarily look at every factor that can skew an alcohol test against the defendant. It is important to realize that more than factor can interplay to skew the results. The science on each individual variation is well established but there is often little science on a combination of factors. There is also a growing awareness that the GLP-1 drugs may affect alcohol absorption and blood alcohol levels. This is a new area and we are exploring it.
GLP-1's
Again,this is a brand new area but a recent YALE medical school article suggests that GLP-1's may protect the liver against harm from alcohol and also increase its effects. See YALE article You are still responsible for how much you drink but if the effect is 100% unanticipated you might have an involuntary intoxication defense. More generally, here is an article on how GLP-1's can fool you by slowing alcohol absorption so that you may drink more shots and feel fine - fine until they finally reach your brain. So the normal mechanism of "how do I feel" changes when you have started these new prescription medicines.
Radio frequency interference (RFI): Breathalyzers (also called breathalyzers) use sensitive electronic sensors to detect alcohol in breath. RFI from nearby devices like police radios, cell phones, computers, dispatch equipment, or even fluorescent lights can disrupt the machine's circuitry or internal electronics. This interference often causes falsely elevated readings (sometimes drastically higher than actual levels), though it rarely lowers them. Many models have built-in RFI detectors meant to flag or abort the test, but experts note these detectors aren't always reliable and may miss subtle interference. This is a well-documented issue in law enforcement and has led to erroneous DUI results in the past.
Whether the officer knows how to properly use the machine: Proper operation is critical. Officers must follow strict protocols, including a 15–20 minute observation period (to ensure no mouth alcohol from recent drinking, burping, vomiting, or residue remains), correct mouthpiece use, proper subject blowing technique (steady, full breath), and pre-test calibration checks. If the officer skips steps, uses the wrong procedure, or lacks certification/training, the result can be contaminated by mouth alcohol or other errors, leading to falsely high readings. Human error in administration is one of the most common reasons breathalyzer results are successfully challenged in court.
Mechanical defect: Breathalyzers are machines that require regular calibration, maintenance, and software updates. A mechanical defect—such as a faulty sensor, outdated calibration solution, temperature sensitivity, software glitch, or poor upkeep—can produce inaccurate results (either too high or too low). Even small calibration errors or residue from prior tests can skew readings. Law enforcement agencies are supposed to log maintenance, but lapses happen and can invalidate the test.
Your gender and weight: These do not affect the machine itself but strongly influence how much alcohol ends up in your blood and breath for the same amount consumed. Women typically have a higher percentage of body fat and less total body water than men, so the same drinks result in higher blood alcohol concentration (BAC) and thus a higher breath reading. Lower body weight means less dilution of alcohol in the bloodstream, leading to a higher concentration (and higher test result) compared to a heavier person. The breathalyzer assumes a standard conversion ratio and does not adjust for individual physiology, so lighter people and women often register higher than a heavier male who drank the exact same amount.
The amount and type of alcohol you consumed: This has the most direct and obvious effect. More alcohol (higher volume or higher percentage ABV) means more ethanol reaches your bloodstream and breath, producing a higher reading. The type matters too: stronger drinks (e.g., liquor vs. beer) raise BAC faster, and rapid consumption (shots vs. sipping) leads to a higher peak level before your body can metabolize it. Some beverages with carbohydrates or other ingredients can slightly slow or speed absorption, but the primary driver is the total ethanol ingested. The breathalyzer simply measures whatever alcohol is present in your exhaled breath at the moment of testing.
If you ate any food prior to taking the test: Food (especially if eaten close to drinking or right before the test) generally lowers the breathalyzer reading compared to drinking on an empty stomach. It slows gastric emptying, delaying alcohol absorption into the bloodstream, so your BAC (and breath alcohol) peaks later and lower at the time of testing. However, certain foods can occasionally cause a temporary false positive or elevated reading if they contain trace fermented alcohol (e.g., yeast in bread/pizza, spicy foods causing burps with stomach alcohol vapor, or very recent consumption of fermented items like kombucha). The standard 15–20 minute observation period is meant to minimize mouth-alcohol effects from food or drink. Overall, eating tends to reduce the test result rather than inflate it.
Mouth Alcohol: Recent use of mouthwash, breath sprays, or medicines containing alcohol can leave "residual alcohol" in the mouth, which the machine misinterprets as deep lung air.
Medical Conditions: Conditions such as GERD (Acid Reflux), heartburn, or hiatal hernias can cause alcohol vapors to travel from the stomach back into the mouth during the test.
Dietary States: People on high-protein, low-carb diets (like Keto) may produce isopropanol as a byproduct of ketosis, which some older Breathalyzers mistakenly flag as ethanol.
Temperature: Both the temperature of the device and the breath temperature of the individual can affect the results. A higher body temperature (such as a fever) can lead to a falsely elevated BAC.
Individualized Variations
Henry's law underpins the breath test. It is a core principle of physics, applying it to a living, breathing human body involves several assumptions that ignore inconsistencies that are too small to matter. If the differences in a static system vs the human body were large, this can lead to inaccuracies. In a laboratory, Henry’s Law assumes a closed system at a constant temperature; the human body is not but usually this does not matter. Hence we have the partition ratio where you multiply air measurement and assume using a 2100 to 1 ratio that this reflects the blood concentratino.
But there are cases where partition ratios can be less than 1300 to 1 or more than 3000 to 1. Certain medical conditions, factors we may not know about and normal human variations can skew the results.
Many of the above defenses are raised in DUI defenses because breathalyzers, while useful, are not infallible and rely on both the machine and proper procedure. Actual court outcomes depend on jurisdiction, the specific device model, and supporting evidence (like calibration logs or officer testimony). If this relates to a real situation, consulting a DUI attorney or toxicologist is the best next step for case-specific advice.
In terms of the partition ratio it is worth considering doing a test where a fixed amount of alcohol is given to the defendant and a breath test is administered. If in the lab the results are skewed (compared to a direct blood test) this can be powerful evidence of innocence. The other failures are not as directly proven. You need to obtain maintenance records or hire an expert to talk about inherent weaknesses. A lot of these defenses can run up the costs and trial in DUI cases is often a popularity contest between the defendant and the arresting officer - the reasonable doubt in attacking the tests is often a thermometer for the jury's attitude toward the participants. If they like you they mistrust the machine. If they like the officer the machine worked just fine.
If you are arrested for a Ca. DUI our expert attorneys can represent you. We defend Driving Under the Influence cases that are highly contested and those that are high stakes. DUI's involving physicians, judges, lawyers and people involved in contentious divorce proceedings require an extra level of effort by the attorney. If your case is not an off the shelf DUI, call Daniel Horowitz at 925-283-1863.
