Yes, a breathalyzer can be wrong. These devices don’t measure your blood alcohol directly; they estimate it from your breath using a fixed mathematical assumption about how the two relate, and that assumption doesn’t fit every body. On top of that structural limitation, mouth contamination, certain medical conditions, diet, body temperature, calibration lapses, and operator mistakes can all push a reading above or below your actual blood alcohol concentration. Whether breathalyzers can be wrong is not really in dispute among people who study them. The useful questions are how, by how much, and what you can do about it.
The Built-In Assumption Every Breathalyzer Makes
Every breath-testing device in the United States converts breath alcohol into an estimated BAC using a fixed conversion factor called the partition ratio, set at 2100:1. The device assumes that 2,100 milliliters of your exhaled breath contain the same amount of alcohol as one milliliter of your blood. That single number is where the biggest structural error lives, because the real ratio isn’t fixed. It varies from person to person and even within the same person over time.
Studies measuring actual blood-to-breath ratios in test subjects have found mean values ranging from roughly 1,550:1 to over 2,500:1.1Institute of Forensic Research. Inter- and Intra-Individual Variations of Blood/Breath Alcohol Ratio If your personal ratio is below 2,100:1, the machine overestimates your BAC. Someone whose true ratio sits near 1,600:1 could blow well over the legal limit while their actual blood alcohol is under 0.08%. This is not a malfunction and not something an officer did wrong. It’s baked into how the technology works. The 2100:1 figure was chosen as a conservative average, but “average” still means a large share of people fall on the side where the device reads high.
Your ratio at any given moment is influenced by your breathing pattern, lung capacity, body temperature, and whether your BAC is still rising or already falling.
Mouth Alcohol and Recent Contamination
Breathalyzers are meant to sample air from deep in your lungs, but they can’t tell lung air apart from alcohol vapor sitting in your mouth. That makes recent oral contamination one of the most commonly raised sources of a false high reading.
The obvious cause is recent drinking. A few minutes after your last sip, residual alcohol can still cling to your mouth, tongue, and throat. Alcohol-based mouthwash and breath sprays do the same thing, and some medications carry meaningful amounts of ethanol, including certain cough syrups and oral pain relievers. Any of these can spike a reading above your actual BAC.
Gastroesophageal reflux disease (GERD) and burping are also raised as possible sources of mouth alcohol because they can push stomach contents up into the throat. One small study of subjects with confirmed gastric reflux found that the reflux did not produce “widely deviant” breath readings when samples were taken at five-minute intervals.2PubMed. Reliability of Breath-Alcohol Analysis in Individuals With Gastroesophageal Reflux Disease That study involved only ten subjects and used proper waiting periods. In a real traffic stop where a reflux episode happens to coincide with the sample, the contamination risk is not the same, and defense attorneys regularly raise GERD as a challenge.
Medical Conditions and Diets That Skew Readings
Diabetes is the medical condition most often cited as a source of false readings, but the picture is more nuanced than the internet suggests. Diabetic ketoacidosis raises blood and breath acetone, which older devices could misread as alcohol. Evidentiary breath testers approved by NHTSA are required to distinguish acetone from ethanol at the 0.02 BAC level.3U.S. Department of Transportation. Approved Evidential Breath Measurement Devices A 1989 study testing fuel cell and chemical oxidation methods found no cross-reaction with ketone bodies, even in grossly ketotic diabetic subjects.4PubMed. Ketone Bodies Do Not Give Falsely Positive Alcohol Tests
Ketogenic and very low-calorie diets are a different story. These diets produce high concentrations of acetone in the blood, and the liver’s alcohol dehydrogenase enzyme can convert some of that acetone into isopropanol. Devices using electrochemical oxidation won’t react to the acetone itself, but they do respond to isopropanol and other non-ethanol alcohols, which can produce a false positive.5PubMed. False-Positive Breath-Alcohol Test After a Ketogenic Diet The device isn’t broken in that scenario. It’s accurately detecting an alcohol, just not the one you drank.
Breath Temperature
Breathalyzers assume breath exits at 34°C (93.2°F). Research suggests the actual average is closer to 35°C, and individual variation runs well beyond that. A fever, vigorous physical activity, sitting in a hot car, or being held in a warm room can all raise breath temperature.
Studies have estimated that each 1°C rise above the 34°C baseline inflates the reading by roughly 6.5% to 8.6%. For someone with a true BAC of 0.07%, a one-degree elevation could push the reading to 0.075% or higher. Two degrees can cross the 0.08% line without a drop of additional alcohol. No widely used breathalyzer in the U.S. measures or corrects for the actual temperature of the breath sample.
Rising BAC After Your Last Drink
Alcohol takes time to absorb. Depending on what you ate and how fast you drank, your BAC may still be climbing 30 to 90 minutes after your last drink. During this absorptive phase, breath alcohol runs disproportionately high compared to blood alcohol, because the lungs see freshly absorbed alcohol before it has equilibrated throughout the body. The blood-to-breath ratio during absorption is significantly lower than during elimination, so breath tests taken during that window systematically overestimate true blood alcohol.1Institute of Forensic Research. Inter- and Intra-Individual Variations of Blood/Breath Alcohol Ratio
This matters because a traffic stop usually happens shortly after you leave a bar or restaurant, which is often the peak absorption window. Your BAC at the moment of the test can be higher than it was when you were actually driving.
Calibration, Maintenance, and Operator Problems
A breathalyzer is only as accurate as its last calibration. These devices drift with time and use, and without regular checks against a known alcohol standard, readings become unreliable. Federal Department of Transportation rules require each evidentiary breath tester to follow a manufacturer-developed quality assurance plan with minimum calibration intervals, tolerances, and inspection requirements. If an external check produces a reading outside the tolerance, the device must be pulled from service.6Federal Motor Carrier Safety Administration. Implementation Guidelines for Alcohol and Drug Regulations – Chapter 7
Those rules are clear on paper; enforcement varies by agency. Calibration logs, maintenance records, and service histories are discoverable in court, and gaps in those records are one of the most effective grounds for challenging a result.
Operator mistakes are the other side of this. Failing to collect a deep lung sample is the most common problem. A shallow or short breath captures upper-airway air rather than the alveolar air that correlates with blood alcohol. Some subjects hyperventilate before blowing, which lowers readings; others are told to blow too long or too hard, which can raise them. Officers are required to complete training on the device and on procedures like recognizing mouth alcohol and confirming the subject hasn’t eaten or vomited recently. When those steps get rushed, the number on the printout may not reflect the subject’s true BAC.
Roadside Screening vs. Station Test
Not all breathalyzers carry the same weight, and the distinction matters if you’re facing a DUI charge. The small handheld device an officer uses during a traffic stop is a preliminary breath test (PBT), also called a portable breath test. Its purpose is to give the officer enough information to decide whether to arrest. PBT results are generally not admissible as evidence of BAC in court because the devices are significantly less accurate than their stationary counterparts.
The evidentiary breath test happens later, typically at the police station, using a large device that meets NHTSA’s model specifications. These instruments must be capable of printing results, assigning unique test numbers, running air blank tests, performing calibration checks, and distinguishing acetone from ethanol.3U.S. Department of Transportation. Approved Evidential Breath Measurement Devices The number that carries legal weight is the one from the evidentiary device, not the roadside screen.
Safeguards That Are Supposed to Catch Errors
Testing protocols include several safeguards, and it helps to know both what they do and where they fall short.
Observation Period
Before an evidentiary test, the officer is supposed to observe you to make sure you don’t burp, vomit, eat, drink, or put anything in your mouth. Federal DOT regulations set this waiting period at a minimum of 15 minutes after a screening test, with the confirmation test starting no more than 30 minutes later.7eCFR. 49 CFR 40.251 The purpose is to let any mouth alcohol dissipate. In practice, officers sometimes conduct this observation while completing paperwork, and the quality of that observation varies.
Duplicate Samples
Most protocols require two separate breath samples. If they don’t agree within a specified tolerance, the test is invalid and must be repeated or abandoned. This catches random errors but not systematic ones. If your breath temperature is elevated or you’re still in the absorptive phase, both samples will be equally inflated, agree perfectly with each other, and both overestimate your true BAC.
Acetone Detection
Approved evidentiary devices must distinguish acetone from alcohol at the 0.02 BAC level.3U.S. Department of Transportation. Approved Evidential Breath Measurement Devices That handles the diabetes concern for most practical purposes, but it doesn’t catch every endogenous alcohol, including the isopropanol that ketogenic dieters can produce.
How to Challenge a Breathalyzer Result
If you believe a reading was inaccurate, several avenues exist. The strongest challenges usually focus on the device’s calibration and maintenance records. If the agency can’t produce documentation showing the machine was calibrated on schedule and within tolerance, a court may exclude or discount the result.
Procedural challenges are also common. Whether the officer conducted a proper observation period, collected a deep lung sample, and followed the manufacturer’s operating instructions is fair game. Operator certification records can be subpoenaed to confirm the administering officer completed the required training.
Medical evidence matters when a condition like GERD, diabetes, or a ketogenic diet could explain an elevated reading. An independent blood test taken reasonably close in time to the breath test is the most direct rebuttal available, because a blood draw measures actual blood alcohol rather than estimating it. Many states give you the right to request an independent blood test after arrest, and using that right creates a second data point that either confirms or contradicts the breath reading.
The partition ratio itself can be challenged through expert testimony. A toxicologist who can show that your individual blood-to-breath ratio likely differs from 2100:1 based on your physiology or the timing of the test can undermine the result even if the device worked correctly.
Refusing the Test Has Its Own Costs
Knowing that breathalyzers can be wrong is not the same as being able to refuse one without consequence. All states have implied consent laws, meaning that by driving on public roads you’ve already agreed to submit to chemical testing if lawfully arrested for impaired driving.8National Highway Traffic Safety Administration. Countermeasures That Work – BAC Test Refusal Penalties Nearly every state imposes automatic administrative penalties for refusal, most commonly a license suspension that often exceeds what you’d face for failing the test. Some states have also made refusal a separate criminal offense.
Penalties vary considerably by state, and NHTSA itself has noted that they “cannot be categorized in a straightforward manner.”8National Highway Traffic Safety Administration. Countermeasures That Work – BAC Test Refusal Penalties Refusing because you distrust the device is understandable, but the consequences of refusal are usually more immediate and more certain than the odds of successfully challenging a breath test after the fact.