A PBT, or preliminary breath test, is a handheld device an officer uses at the roadside to estimate your breath alcohol level during a traffic stop. It gives a quick number the officer uses to decide whether to arrest you for impaired driving. It is not the same machine that produces the BAC evidence used against you in court, and that difference changes how accurate the result is, whether it can be used at trial, and what happens if you refuse.
How the Device Works
A PBT is about the size of a TV remote and runs on a battery. You blow into a disposable mouthpiece, and a fuel cell sensor inside reacts with any alcohol vapor in your breath. That chemical reaction generates a small electrical current proportional to the amount of alcohol present. The device converts the current into an estimated blood alcohol concentration and shows the number within seconds.
Larger station-based breath machines use the same basic fuel cell principle. The portable version trades some precision for speed and portability. It is built to screen, not to produce laboratory-grade evidence.
When an Officer Can Ask You to Blow
An officer cannot request a PBT at random. They need reasonable suspicion that you have been drinking, which is a lower bar than probable cause but still requires specific, observable facts. Typical triggers:
- Driving behavior like swerving, straddling lane lines, driving unusually slowly, or running a stop sign.
- Physical cues during the stop: the smell of alcohol, bloodshot or glassy eyes, slurred speech, or fumbling with your license and registration.
- Admissions. Telling the officer you had “just a couple of beers” is enough to justify a screening request.
The PBT usually comes near the end of a broader field sobriety evaluation. The officer may ask you to perform physical tests first, like walking heel-to-toe or standing on one leg. The PBT reading adds an objective number to the overall picture. If the result suggests a BAC at or above 0.08, or if it lines up with the impairment cues the officer already noted, it supplies the probable cause needed to arrest you and transport you for a more reliable test.
PBT Versus the Station Breath Test
This is where most drivers get confused. The device you blow into at the side of the road and the one you blow into at the station are two different instruments with two different legal roles.
- The roadside PBT is a portable screening device. Its job is to help the officer decide whether to arrest you. Results generally are not admissible to prove your BAC at trial.
- The evidentiary breath test at the station is a larger, more sophisticated instrument kept in a controlled environment. It receives documented calibration on a regular schedule, and its results carry enough reliability to be used as evidence in court.
Evidentiary machines are maintained under strict protocols; under federal DOT regulations, for example, manufacturers must submit a quality assurance plan setting calibration intervals, and a machine that fails a calibration check must be pulled from service until it is repaired.1US Department of Transportation. DOT Rule 49 CFR Part 40 Section 40.233 – What Are the Requirements for Proper Use and Care of EBTs? Roadside PBTs are calibrated too, but the standards are less rigorous and the roadside environment is less controlled.
Why the Number Can Be Off
A PBT is a useful screening tool, not a precise measurement. Several things can push the reading above or below your actual BAC.
Mouth Alcohol
If you recently took a drink, used mouthwash, or burped, residual alcohol in your mouth can produce a reading well above your true BAC. Evidentiary breath tests typically require an observation period of about 15 minutes before you blow, specifically to let mouth alcohol dissipate. That safeguard is rarely applied as carefully with a roadside PBT.
Medical Conditions
People with uncontrolled diabetes can generate acetone through ketoacidosis. The acetone converts to isopropanol, which is chemically similar enough to ethanol that a breath sensor can read it as alcohol. A case study in a military medical journal documented this effect in a sailor whose symptoms were initially mistaken for intoxication.2Oxford Academic. Early Detection of Diabetic Ketoacidosis by Breathalyzer in a Sailor Presenting With Symptoms of Alcohol Intoxication Acid reflux and GERD can also push stomach vapors into the mouth and inflate a reading.
Temperature and Humidity
Fuel cell sensors are sensitive to environmental conditions. Cold slows the chemical reaction inside the device; heat speeds it up. NHTSA requires approved screening devices to meet model specifications across varying conditions, but roadside weather is less predictable than a climate-controlled testing room.3US Department of Transportation. Approved Screening Devices to Measure Alcohol in Bodily Fluids
None of this means the PBT result is meaningless. Someone who blows a 0.15 at the roadside is not sober, even if the real number is a few hundredths off. It does mean the roadside figure should be read as a screening estimate, not a definitive measurement.
Can the Result Be Used Against You in Court?
In most jurisdictions, the PBT result is admissible for one narrow purpose: showing that the officer had probable cause to arrest you. If a defense attorney challenges the arrest itself, the prosecution can bring the PBT number in to show the officer had a reasonable basis for believing you were impaired.
Prosecutors usually cannot use the PBT reading at trial to prove your BAC exceeded the legal limit. That job falls to the evidentiary breath test or a blood draw. Some states carve out exceptions for specific situations, such as underage drinking cases or administrative license suspension hearings, where the PBT number may come in. As a general rule, though, the roadside number is an investigative tool, not trial evidence.
Can You Refuse the PBT?
Refusing the roadside PBT and refusing the post-arrest test are governed by different rules, and the consequences are very different.
Refusing the Roadside PBT
In most states, you can refuse the roadside PBT, and the consequences are mild. Some states treat a refusal as a civil infraction with a modest fine. Others impose no formal penalty for declining the screening. The officer can still arrest you based on your driving, your appearance, your performance on field sobriety tests, and the smell of alcohol, and many officers note the refusal itself as one more indicator supporting probable cause.
Refusing the Post-Arrest Test
The evidentiary test at the station is governed by implied consent. By driving on public roads, you have already agreed in advance to submit to chemical testing after a lawful DUI arrest. Implied consent generally applies to the evidentiary test, not the roadside screening.
Refusing the post-arrest test carries real consequences. Nearly every state imposes automatic license suspension for refusal, and as of recent data, refusal was itself a separate criminal offense in at least a dozen states.4NHTSA. BAC Test Refusal Penalties Some jurisdictions respond by obtaining a warrant for a blood draw.
The U.S. Supreme Court set the constitutional limits in 2016. In Birchfield v. North Dakota, the Court held that police may conduct a warrantless breath test after a lawful DUI arrest because a breath test is minimally intrusive, and states may criminalize refusal of that test. Blood draws are more invasive, require a warrant, and cannot carry criminal penalties for refusal without one.5Justia US Supreme Court. Birchfield v. North Dakota, 579 US (2016)
The practical takeaway: refusing the roadside PBT might cost you a small fine depending on your state. Refusing the evidentiary test after arrest can cost you your license for months, whether or not you are ultimately convicted.
What Happens After You Blow
A PBT reading is never the end of the process. If the result suggests impairment and the officer decides to arrest you, you will be transported to a station or hospital for an evidentiary breath test or blood draw. That second number is the one prosecutors use. If the PBT reads clean and the officer’s other observations do not point to impairment, you are generally free to go.
If you are arrested and later convicted, the evidentiary test drives the severity of your penalties. The PBT played a gatekeeping role: without it, the officer might not have had enough to justify the arrest. Knowing the difference between the screening device at the roadside and the test that actually counts in court is the clearest way to understand your rights at each step of a DUI stop.