False high BAC readings usually come from one of a handful of causes: alcohol lingering in the mouth at the moment of the test, a medical condition or diet that changes what you exhale, a device that hasn’t been calibrated, an officer who cut the observation period short, or a blood sample that was contaminated or fermented after collection. Some of these shift a reading by hundredths of a point. Others can push a sober person above the legal limit. Knowing which is which matters if you’re deciding how to defend a DUI charge built on a number you don’t trust.
Mouth Alcohol and the Observation Period
Breathalyzers are built to measure air from deep in the lungs, which tracks the alcohol actually circulating in your blood. When alcohol is sitting in your mouth or throat instead, the device reads a concentrated dose that has nothing to do with what’s in your bloodstream. This is the single most common reason breath tests come back too high.
Alcohol-based mouthwash or breath spray leaves residue. So do burping and vomiting, which bring stomach contents back up into the mouth. Dentures, bridges, and braces can trap small amounts of alcohol in crevices. Even food cooked with alcohol or fermented food can contribute.
This is why testing protocols require an observation period. The officer is supposed to watch you for at least 15 minutes and confirm that you haven’t put anything in your mouth, burped, or vomited. That window lets residual mouth alcohol dissipate so the sample captures deep lung air. When officers skip that period, do it while filling out paperwork, or watch multiple subjects at once, contamination becomes a real risk and one of the more successful bases for suppressing a test.
Medical Conditions That Push Readings Up
Acid Reflux
Gastroesophageal reflux disease pushes stomach contents back into the esophagus and could, in theory, carry alcohol vapor up during a breath test. Defense attorneys raise this often, but the science is thinner than the argument suggests. A study of 10 subjects with confirmed reflux found that even when episodes occurred during testing, the breath readings did not deviate widely from actual blood alcohol at five-minute intervals; the researchers called a falsely elevated evidential result from reflux “highly improbable.”1PubMed. Reliability of Breath-Alcohol Analysis in Individuals With Gastroesophageal Reflux Disease The sample was small, and a severe episode timed to the mouthpiece could still introduce error. Worth telling your attorney about; not a defense by itself.
Diabetes and Ketone Production
People with diabetes sometimes carry elevated acetone on their breath. Some breath sensors can’t distinguish acetone from ethanol. A Department of Transportation study found the practical effect is small: diabetics healthy enough to drive don’t produce enough breath acetone to raise a BAC reading by more than about 0.01% to 0.02%, and most modern evidential devices are built to tell the two apart. The same study estimated fewer than 1,000 devices nationwide lacked that capability.2National Transportation Library. The Likelihood of Acetone Interference in Breath Alcohol Measurement Real interference, but rarely enough on its own to put a sober person over the limit.
Auto-Brewery Syndrome
Auto-brewery syndrome is a rare condition in which gut yeast or bacteria ferment carbohydrates into ethanol inside the body. Documented cases have shown blood ethanol as high as 0.40% produced entirely by internal fermentation, with no drinking involved.3PubMed Central. Autobrewery Syndrome and Endogenous Ethanol Production It’s usually tied to severe fungal infections, prior gastric surgery, or intestinal disease like Crohn’s. Fewer than 20 correctly identified cases had appeared in the medical literature as of 2021.4PubMed Central. Blood Alcohol Concentration in the Clinical Laboratory: A Narrative Review Dramatic when it happens. Very uncommon.
Fever and Elevated Body Temperature
Breathalyzers assume exhaled breath is at roughly 34°C. When your core temperature is elevated by fever, hard exertion, or a hot patrol car, alcohol in your breath vaporizes more readily and the device reads high. The measured effect is about an 8.6% increase in the breath alcohol reading for each degree Celsius above normal, with extreme cases showing overestimates as high as 23%.5PubMed. Effect of Hyperthermia on Breath-Alcohol Analysis Someone running a 101°F fever can see a meaningful bump from temperature alone.
Diet, Medications, and Inhalers
Keto and Fasting Diets
Ketogenic diets and prolonged fasting drive the body into ketosis, and acetone accumulates in the blood and breath. There’s a further wrinkle: under certain conditions the liver converts acetone into isopropanol using the same enzyme that handles drinking alcohol. Ignition interlock devices and some older breath instruments react to isopropanol, producing a false positive with no drinking involved.6PubMed. False-Positive Breath-Alcohol Test After a Ketogenic Diet This matters most for people on very low-calorie diets who are already subject to interlock testing from a prior conviction.
Asthma Inhalers
A study of 60 asthma patients found that common inhalers containing salbutamol, salmeterol, and formoterol produced positive breath alcohol readings in the minutes right after use. At one minute post-inhalation, mean readings hit 0.45 mg/L, above the legal limit. The cause was the propellant gases, not ethanol. Readings dropped to zero within 10 minutes.7The BMJ. Using Asthma Inhalers Can Give False Positive Results in Breath Tests If you use an inhaler and are asked to blow, say so before the test. Standard protocol is a 20-minute wait when inhaler use is known.
Rising BAC and Test Timing
After your last drink, your body is still absorbing alcohol into your bloodstream for anywhere from 30 minutes to over two hours, depending on what’s in your stomach. During that absorption phase your BAC is climbing. If you’re stopped shortly after your last drink and tested 20 or 30 minutes later at the station, the reading may be noticeably higher than your BAC was when you were actually driving.
In most states, the legal question is your BAC at the time of driving, not at the time of testing. Prosecutors sometimes use retrograde extrapolation to estimate what your BAC was earlier, but the math assumes you had already peaked and were eliminating alcohol. If you were still absorbing, the estimate runs the wrong direction and overstates your driving-time BAC. This is one of the more scientifically credible lines of DUI defense.
Device Calibration and Interference
Breath testing devices drift and require regular calibration checks. Federal Department of Transportation regulations require quality assurance plans that set calibration intervals based on how often the device is used and the environment it operates in. A device that fails a calibration check must be taken out of service until it passes again.8U.S. Department of Transportation. DOT Rule 49 CFR Part 40 Section 40.233 Agencies that fall behind on calibration create an opening to challenge every result produced during the gap. Those records are discoverable.
Radio frequency interference from nearby electronics has been raised as a source of error. A Department of Justice survey found most states reported few real-world problems, and modern devices include RFI detection that flags or aborts a test when interference is present. High humidity and temperature swings can also affect sensor performance, but well-maintained equipment in a controlled setting keeps the risk low.
Officer Errors During the Breath Test
The person running the test is a common source of error. The observation period is the biggest failure point: an officer who is distracted or juggling multiple subjects may miss a belch or swallow that reintroduces mouth alcohol. Departments generally require 15 to 20 minutes of continuous observation and confirmation that the subject hasn’t ingested anything or experienced anything that could contaminate the sample.
Technique matters too. The subject has to provide a sustained exhalation of deep lung air. A weak or short breath may not give the instrument enough sample, and some testing errors capture upper airway air more concentrated with mouth alcohol. Some devices flag inadequate samples automatically; not all do. When the officer skips the manufacturer’s instructions, the result becomes hard to defend as reliable.
Blood Test Errors
Blood draws are generally more accurate than breath tests, but they’re not immune. The largest risk is microbial contamination. If a sample is improperly stored, exposed to the wrong temperature, or left too long, microorganisms can ferment glucose in the tube and produce alcohol that wasn’t there when the blood was drawn.4PubMed Central. Blood Alcohol Concentration in the Clinical Laboratory: A Narrative Review Proper preservatives and anticoagulants prevent this, but a broken chain of custody or an expired collection kit compromises the result.
Swabbing the draw site with an alcohol-based disinfectant is another classic error. Forensic blood draws generally require a non-alcohol antiseptic, but mistakes happen, particularly when the draw is done in an emergency room rather than a dedicated forensic facility. Healthy, sober people do produce trace amounts of endogenous ethanol through normal metabolism, but at the 99th percentile the level is only about 0.002%.4PubMed Central. Blood Alcohol Concentration in the Clinical Laboratory: A Narrative Review
What to Do If You Think Your Reading Was Wrong
The most useful practical step is requesting an independent blood test as soon as possible. Most states give you the right to obtain your own chemical test after submitting to the officer’s. The closer in time it is to the original test, the more useful the comparison. A large discrepancy between a breath result and a blood draw taken shortly after is strong evidence of instrument error.
On the legal side, defense attorneys attack BAC evidence on several fronts: calibration and maintenance records, gaps in the required observation period, the officer’s training records, and the device’s own error logs. Under Birchfield v. North Dakota, breath tests can be administered without a warrant incident to a lawful DUI arrest, but blood tests require either a warrant or valid consent.9Justia U.S. Supreme Court. Birchfield v. North Dakota A blood draw without either can be suppressed outright.
One boundary worth knowing: refusing the test isn’t a workaround. Every state except Wyoming has implied consent laws that impose administrative penalties, typically a license suspension, for refusal, and in at least a dozen states refusal is a separate criminal offense.10NHTSA. BAC Test Refusal Penalties In most situations the better move is to take the test, note anything unusual about how it was done, and challenge the result afterward with a DUI defense attorney.