Breathalyzer tests are admissible in court in every U.S. jurisdiction, but admissibility is conditional: prosecutors must show the test was run on an approved, properly maintained device by a certified operator who followed the required procedures. When breath evidence gets excluded, it is almost never because a judge rejected the underlying science. It is because something in the paperwork, the machine’s history, or the officer’s conduct broke the chain of reliability the law demands.
The science itself has cleared every evidentiary standard courts use. Breath alcohol testing has been accepted under both the Daubert reliability framework and the older Frye general-acceptance test.1Library of Congress. Daubert v. Merrell Dow Pharmaceuticals, 509 US 579 (1993) So the fight in a DUI case is rarely about whether breathalyzers work in general. It is about whether this test, on this machine, run by this officer, followed the rules.
What Prosecutors Must Prove to Get the Result In
Before a jury ever hears a BAC number, the prosecution has to lay a foundation. Miss any single piece and the result can be kept out.
- Approved device. The breathalyzer must be a model approved for evidentiary use in the jurisdiction. Most states rely on the National Highway Traffic Safety Administration’s Conforming Products List, sometimes with a state-specific registry on top.2NHTSA. Alcohol Measurement Devices
- Current calibration and maintenance. The device must have been calibrated at the required intervals using a simulator solution held at a precise temperature, typically 34°C with a tolerance of ±0.2°C. The maintenance log has to be complete and current.3Intoximeters. Wet Bath Standard
- Certified operator. The officer administering the test must hold a current certification for that specific breathalyzer model. Expired certifications, or training on a different device, can disqualify the result.
- Completed observation period. The operator must continuously observe the subject for a set period before the test, commonly 15 to 20 minutes, to confirm the person did not eat, drink, burp, or vomit. Any of those can introduce mouth alcohol and inflate the reading.
Roadside Screening Is Not the Same Test
Two breath tests come up in a DUI stop, and they carry very different weight in court. The preliminary breath test (PBT) is the handheld device an officer uses at the roadside. In most jurisdictions the specific number a PBT produces is not admissible at trial to prove guilt, because portable units are more vulnerable to temperature and environmental interference. The fact that the PBT registered alcohol is usually admissible to explain why the officer moved forward with an arrest, but the reading itself is not treated as proof of BAC.
The test that matters at trial is the evidentiary breath test given at the station after arrest. These are the larger, stationary machines subject to the calibration, maintenance, and procedural rules above. When prosecutors talk about “the breathalyzer result,” this is the one they mean, and this is the one the admissibility fight is about.
How the Defense Attacks an Evidentiary Result
Observation and Operator Errors
The observation period is where most challenges start. If the officer looked away, stepped out, took a call, or began the test before the required minutes had elapsed, the defense can argue that mouth alcohol may have contaminated the sample. Where the observation is on video, defense attorneys will go through the footage minute by minute. Similar challenges arise when the officer skipped a step in the device’s protocol, such as starting the test before the machine finished its self-check.
Calibration and Maintenance Records
Defense counsel routinely subpoenas the device’s full maintenance and calibration history. They are looking for overdue calibration checks, unresolved error codes, failed accuracy tests, or repairs that were not documented properly. A machine even a day past its scheduled calibration date gives the defense a real argument. Courts have suppressed breath evidence when the logs showed a known malfunction that went unrepaired, or when the agency simply could not produce the records at all.
Medical Conditions and Mouth Alcohol
Gastroesophageal reflux disease (GERD) is one of the more effective medical defenses. The condition pushes stomach contents back up the esophagus, letting alcohol from the stomach reach the mouth and throat. Research has shown that GERD can produce artificially elevated readings even after a full observation period, because the contamination does not require a visible burp or regurgitation.4PubMed Central. The Limitations of Mouth Alcohol Detection Systems in Breath Alcohol Testing
Diabetes creates a different problem. When blood sugar is poorly controlled, the body breaks down fat and produces ketones, including acetone. Some breathalyzer models can misidentify acetone as alcohol, so a person in diabetic ketosis can register a positive reading without having consumed any alcohol. Alcohol-based mouthwash and dental appliances that trap residual alcohol can produce similar false elevations.
Individual Variation in the Blood-Breath Ratio
Breathalyzers convert breath alcohol to an estimated BAC using a fixed 2,100:1 ratio. That number is an average, not a biological constant. A controlled study of 100 subjects found actual ratios ranging from roughly 2,125:1 to 2,765:1.5National Center for Biotechnology Information. Reflections on Variability in the Blood-Breath Ratio of Ethanol For people at the lower end of that range, the machine can overstate BAC. When the reading sits close to the legal limit, this variability becomes a serious defense point.
Rising BAC
Alcohol does not hit the bloodstream instantly. After the last drink, BAC keeps climbing for up to an hour or more as alcohol absorbs from the stomach and small intestine. If there is a meaningful gap between the traffic stop and the station test, the BAC at testing can be higher than it was at the time of driving. Per se laws prohibit driving at or above the limit, not testing at or above it later. The defense builds this argument with a careful timeline and, often, a forensic toxicologist to work backward to an estimated driving BAC.
Software and Source Code
Modern evidentiary breathalyzers run on proprietary software, and defense attorneys have increasingly demanded access to the source code. The argument is direct: if the software has a bug in how it processes samples or calculates results, the output cannot be trusted. Manufacturers have resisted, calling the code a trade secret. Some courts have ordered manufacturers to make the code available to defense experts under controlled conditions. Where manufacturers have been unable to produce older versions, some courts have been receptive to the argument that the results cannot be independently verified.
What Refusing the Test Does
Every state has an implied consent law: by driving on public roads you have already agreed to submit to a chemical test if lawfully arrested for impaired driving.6NHTSA. Traffic Safety Facts – Implied Consent Laws Refusing does not make the case go away. It triggers its own consequences, often harsher than a failed test.
The most immediate is automatic license suspension. First-time refusals generally trigger at least 90 days, and many states impose a full year. These administrative penalties usually take effect within 30 days and apply regardless of whether you are ever convicted of DUI.6NHTSA. Traffic Safety Facts – Implied Consent Laws
The refusal itself is admissible at trial. In South Dakota v. Neville (1983), the Supreme Court held that admitting a defendant’s refusal does not violate the Fifth Amendment, because a refusal after a lawful police request is not a coerced act.7Justia US Supreme Court. South Dakota v. Neville, 459 US 553 (1983) Prosecutors routinely tell the jury about a refusal and argue it shows consciousness of guilt.
In Birchfield v. North Dakota (2016), the Court drew a line between breath and blood. States can criminalize the refusal of a warrantless breath test incident to a lawful DUI arrest, because breath testing involves minimal physical intrusion. States cannot impose criminal penalties for refusing a warrantless blood test, because a blood draw is significantly more invasive.8Justia US Supreme Court. Birchfield v. North Dakota, 579 US ___ (2016) The practical point: refusing a breath test after a lawful arrest exposes you to administrative penalties, potential criminal consequences, and a jury that will be told you refused.
When Expert Testimony Matters
When a DUI case goes to trial, both sides may call forensic toxicologists to explain or attack the breath evidence. A defense expert can testify to the variability in the blood-to-breath ratio, to how elevated body temperature can inflate a reading, or to how the timing of the last drink supports a rising BAC defense.5National Center for Biotechnology Information. Reflections on Variability in the Blood-Breath Ratio of Ethanol
Expert testimony is most valuable when the BAC reading sits close to the legal limit. A 0.09% looks damning at first glance, but a credible explanation of margin of error or absorption timing can plant real doubt about whether the driver was actually over the limit at the wheel. In a case where the breathalyzer is the centerpiece of the prosecution’s evidence, an expert is often the defense’s most effective investment.