How Breath and Blood Alcohol Testing Works, and Where It Fails
A breath-testing instrument does not measure alcohol in the blood. It measures alcohol in exhaled air and converts that value using an assumed ratio, which is the single most important thing to understand about blood alcohol testing and the reason two methods that appear to measure the same quantity can disagree.
That conversion sits at the center of a large body of forensic science and of criminal cases. In Arizona, the reported BAC level determines the tier of the offense: 0.08 for a standard DUI, 0.15 for Extreme DUI, and 0.20 for Super Extreme DUI, each carrying a mandatory jail term and an ignition interlock requirement. A measurement question becomes a sentencing question.
How Does an Infrared Breath Instrument Work?
Evidential breath instruments most often use infrared absorption spectroscopy. Ethanol absorbs infrared light at characteristic wavelengths, and the instrument compares the light entering the sample chamber with the light reaching the detector to determine concentration.
The physical principle is straightforward. Molecules absorb infrared energy at frequencies corresponding to the vibrational modes of their bonds, and the carbon-hydrogen stretching region gives ethanol a distinctive signature. Beer's law relates the amount of light absorbed to the number of molecules along the light path, so absorbance is proportional to concentration.
The complication is specificity. Other compounds contain carbon-to-hydrogen bonds and absorb in overlapping regions, so instruments monitor multiple wavelengths and apply filters to reduce interference. This works well for the substances designers anticipated. It works less well for unusual chemistry in a particular subject's breath.
What Do Fuel Cell Sensors Measure?
A fuel cell sensor oxidizes ethanol at a catalytic electrode and measures the resulting electrical current, which is proportional to the amount of alcohol consumed in the reaction.
Fuel cell devices are common in handheld roadside units. They are more specific to ethanol than simple semiconductor sensors, but they are sensitive to temperature, they drift with age, and their response depends on how much sample reaches the electrode. Handheld results are generally treated as preliminary screening rather than evidential values.
Both technologies share a deeper assumption: that the sampled air represents deep-lung air in equilibrium with pulmonary blood, and that the ratio of alcohol in that blood to that air is close to the population average used for the conversion. Individual ratios vary. When a person's actual ratio differs from the assumed ratio, the converted result shifts in a predictable direction.
What Makes a Breath Result Come Out Too High?
Several well-documented conditions push breath results upward, which is why observation periods, procedures, and instrument records exist in the first place.
Mouth alcohol. Residual alcohol in the oral cavity from belching, reflux, dental work,k or recent drinking can produce a reading far higher than the alveolar sample it is supposed to represent.
Elevated body temperature. Warmer breath carries more alcohol vapor, so a fever or exertion nudges the value up.
Breathing pattern. Prolonged exhalation and breath holding raise the measured value, while hyperventilation lowers it.
Interfering compounds. Certain volatile organic compounds, including those produced during certain metabolic states, can affect instruments that rely solely on absorption.
Radio frequency interference and maintenance lapses. Instruments require calibration checks with a known reference, and the records of those checks are part of the evidence.
None of these makes breath testing useless. They make it a measurement with an uncertainty range rather than a fixed truth, which is exactly how metrologists treat any instrument reading.
Where Does Blood Alcohol Testing Show Up Outside DUI Cases?
Test results influence far more than driving cases. Alcohol is a common feature of the reports behind assault, disorderly conduct, and domestic violence allegations, and a stated concentration shapes how police, prosecutors, and judges read the rest of the file.
Arizona has a mandatory arrest policy in domestic violence cases, which means officers responding to a qualifying call must make an arrest rather than exercise discretion at the scene. A number attached to that arrest report follows the case, and a Scottsdale domestic violence lawyer often has to separate what a test actually established from the inferences layered on top of it. A concentration measures a chemical quantity. It does not establish intent, memory, aggression, or credibility.
How Is Blood Alcohol Measured in a Laboratory?
Forensic laboratories typically use headspace gas chromatography with flame ionization detection. A sealed vial of blood is warmed, volatile compounds partition into the headspace above the liquid, and a sample of that headspace is injected into a column that separates compounds based on their interactions with the stationary phase.
Ethanol emerges at a characteristic retention time, and the detector response is compared against calibrators of known concentration. An internal standard added to every vial corrects for injection variability.
The vulnerabilities are mostly outside the instrument:
Collection. Sample tubes must contain an anticoagulant and a preservative in the correct amounts, and the skin preparation should not introduce alcohol.
Storage. Vials stored at warm temperatures or for too long can support microbial growth, and some organisms can generate ethanol in the tube, raising the apparent concentration.
Sample type. Hospital laboratories often report serum or plasma values, which are higher than whole-blood values due to differences in water content. Converting between them without accounting for that difference produces an inflated number.
Documentation. Chain of custody, analyst notes, calibration data, and quality control runs all factor into whether a number can be trusted.
Why Timing Matters More Than People Expect
Alcohol concentration is not static. It rises during absorption, peaks, then falls at a relatively steady rate during elimination, so the time between an incident and a test is a scientific variable rather than a procedural detail.
A sample collected while concentration is still rising will report a value higher than the concentration at the earlier moment of interest. Retrograde calculation attempts to work backward, but it depends on assumptions about absorption state, elimination rate, and stomach contents that are rarely known with confidence in a real case.
How Are These Results Challenged in Court?
Results are challenged by examining the record behind the number rather than by disputing the science of chromatography or spectroscopy.
In Arizona, cases from Scottsdale and the surrounding communities of Phoenix, Tempe, Mesa, Chandler, Gilbert, Paradise Valley, and Peoria are heard in municipal courts and the Maricopa County Superior Court, and the underlying laboratory and instrument records are discoverable. A DUI defense attorney in Scottsdale will typically request maintenance and calibration logs, documentation of the observation period, raw chromatographic data, quality control results, and the analyst's notes.
Frequently Asked Questions
Why can a breath test and a blood test give different numbers?
They measure different things. A breath instrument measures alcohol in exhaled air and converts it using an assumed blood-to-breath ratio, while a laboratory measures alcohol in the blood sample itself. Individual physiology, breathing patterns, body temperature, and the timing of each sample all contribute to differences, so two results collected minutes apart can legitimately differ.
Does mouthwash or hand sanitizer affect a breath test?
Products containing alcohol can raise a reading if residue remains in the mouth when the sample is given. This is why protocols require an observation period before an evidential test, during which the subject should not eat, drink, smoke, belch, or vomit. Effects from an external product fade quickly, which is one reason the documented timing of the observation period matters.
Can a blood sample change after it is drawn?
Yes. Blood in a tube is a biological sample. If the preservative is absent or insufficient, and the vial is stored at warm temperatures, microorganisms can ferment glucose into ethanol, thereby raising the measured concentration. Conversely, poorly sealed vials can lose ethanol to evaporation. Storage temperature, tube contents, and the interval before analysis are all legitimate subjects of examination.
How precise is a reported blood alcohol concentration?
No measurement is infinitely precise. A well-run laboratory can produce highly reproducible results, but the reported value still carries uncertainty arising from calibration, instrument performance, and sampling. Reporting a result with its uncertainty range is standard metrological practice, and it is particularly relevant when a legal threshold lies close to the reported value.
The Law Office of Joshua A. Lopez, LLC, known as The Attorney Josh, defends DUI and other criminal cases in Scottsdale and throughout Maricopa County. Joshua A. Lopez handles every case personally as a solo practitioner and offers free consultations for anyone trying to understand what a test result in their case really means.
