A PBT test is a preliminary breath test, a quick roadside screening that police use to measure the alcohol level in your breath during a traffic stop. It’s a handheld device, small enough for an officer to carry on a belt, and it gives a blood alcohol concentration (BAC) reading within seconds. The key thing to understand: a PBT is a screening tool, not the final word. Its primary purpose is to help an officer decide whether there’s enough evidence to arrest you for impaired driving.
How a PBT Works
Most PBT devices use a fuel cell sensor made of platinum. When you blow into the mouthpiece, the sensor reacts with any ethanol in your breath and generates a small electrical current. The more alcohol present, the stronger the current, and the device translates that into a BAC number on its display.
Your breath doesn’t contain blood, so the device has to estimate what your blood alcohol level would be. It does this using a fixed conversion ratio. In the United States, Canada, and Australia, the standard ratio is 2,100:1, meaning the device assumes that 2,100 liters of exhaled air contains the same amount of alcohol as one liter of blood. The actual ratio varies from person to person. Research measuring it directly in 100 subjects found an average of about 2,382:1, with individual results ranging from 2,125 to 2,765. That built-in variability is one reason PBT results are treated as estimates rather than precise measurements.
How Accurate PBTs Are
Police-grade PBT devices are reasonably accurate, but they’re less precise than the larger machines used at a police station. A professional-grade handheld unit like the LifeLoc FC10 reports accuracy within plus or minus 0.005 BAC at readings up to 0.100, and within 5% at higher levels. That means if your true BAC is 0.080, the device might read anywhere from 0.075 to 0.085.
A study comparing nearly 1,000 paired roadside PBT readings with later evidential breath tests found that the PBT results had noticeably more variability. The standard deviation of the difference between PBT and evidential readings was 0.025, compared to just 0.010 between two evidential tests taken back to back. In roughly 85.5% of cases, the PBT reading was equal to or higher than the evidential result. The remaining 14.5% showed differences that couldn’t be explained by normal sampling variation alone. In practical terms, a PBT is more likely to read slightly high than slightly low.
What Can Throw Off the Results
Several things besides drinking can produce a positive PBT reading. Mouthwash and breath sprays often contain significant amounts of alcohol, and using them shortly before a test can spike the reading. Over-the-counter cold medicines like NyQuil and certain cough drops also contain alcohol. Asthma inhalers have been flagged as potential sources of false positives. Oral pain relievers containing topical anesthetics, such as Anbesol, may interfere with readings as well.
People with diabetes face a specific risk. When blood sugar runs high, the body can produce ketones, which are chemically similar enough to ethanol that some fuel cell sensors register them as alcohol. Exposure to fumes from paints, adhesives, or cleaning chemicals in poorly ventilated spaces is another known cause of false readings. Even certain fermented foods, including kimchi, sauerkraut, and foods made with active yeast, can temporarily affect results.
This is exactly why officers are generally instructed to observe you for at least 15 minutes before a formal breath test, ensuring you haven’t belched, vomited, or put anything in your mouth that could contaminate the reading. For a roadside PBT, though, that observation period isn’t always followed as strictly, which adds to the device’s limitations.
PBT vs. the Station Breath Test
There are two different breath tests you might encounter during a DUI stop, and they serve very different purposes. The PBT happens at the roadside and is a screening tool. The evidential breath test (EBT) happens later, typically at a police station or booking facility, using a larger, more sophisticated machine that undergoes stricter calibration and quality assurance.
The EBT is the test that produces a result accurate and reliable enough to be used as evidence against you. It’s calibrated according to a formal quality assurance plan that accounts for how often the device is used, environmental conditions like temperature and humidity, and manufacturer specifications. PBT devices also require calibration, typically every 6 to 12 months, but they aren’t held to the same forensic standard.
Can PBT Results Be Used in Court?
In most states, PBT results cannot be presented to a jury as proof of your BAC. They exist to help establish probable cause for an arrest, not to convict you. Colorado’s Supreme Court addressed this directly, ruling that PBT results are only admissible in hearings held outside the presence of a jury to determine whether the officer had probable cause. The court found that PBT evidence could not be used at trial for any purpose, including challenging a defendant’s testimony about whether they had been drinking.
This is the general pattern across the country. The PBT gives the officer a number to support the decision to arrest, and the evidential test at the station provides the number that matters in court. Some states allow limited exceptions, but the principle holds: PBTs are investigative tools, not forensic evidence.
What Happens If You Refuse
The consequences of refusing a PBT vary significantly by state and are typically less severe than refusing the post-arrest evidential test. Some states treat PBT refusal as a civil infraction with a modest fine. Others impose no penalty at all for declining the roadside screening.
Refusing the evidential test after arrest is a different matter entirely. Every state except Wyoming imposes administrative penalties for refusing a post-arrest BAC test, usually an automatic license suspension. In at least 12 states, refusal is a criminal offense. The U.S. Supreme Court’s 2016 decision in Birchfield v. North Dakota confirmed that states can criminalize refusal of breath tests, though not warrantless blood tests. Many states deliberately set refusal penalties higher than the penalties for failing the test itself, specifically to discourage people from refusing.
The distinction matters. Refusing a roadside PBT might carry a small fine or no penalty, but it won’t necessarily prevent an arrest if the officer has other evidence of impairment, such as your driving pattern, field sobriety test performance, or the smell of alcohol. Refusing the post-arrest evidential test, on the other hand, can trigger an automatic license suspension that kicks in regardless of whether you’re ultimately convicted of impaired driving.
Why PBTs Still Matter
Despite their limitations, PBTs serve a practical function that benefits both law enforcement and drivers. Before PBT laws existed, officers had to rely entirely on subjective observations to decide whether to arrest someone. That meant more people who weren’t legally impaired were arrested, processed, and eventually released, wasting time and resources while putting sober drivers through an invasive experience. The PBT gives officers an objective data point at the roadside that helps filter out people who are below the legal limit, speeding up the process and reducing unnecessary arrests.
Breath tests conducted within roughly two hours of driving reflect BAC at the time of driving within normal experimental uncertainty. That time window is important because alcohol levels rise and fall, and a test taken hours later at a station might not reflect what was happening behind the wheel. The PBT captures a snapshot closer to the moment that actually matters.