A DUI per se charge rests entirely on your blood alcohol concentration: if a chemical test shows you were at 0.08% or above while driving, that number alone is the offense. A standard, impairment-based DUI requires the prosecution to prove something different, that alcohol or drugs actually compromised your ability to operate the vehicle safely. Both are commonly called “DUI,” and in most states you can be charged with both from a single traffic stop, but they rely on different evidence and are defended in different ways. That is the core of the DUI per se vs. DUI distinction, and it shapes almost everything that follows an arrest.
DUI Per Se: The Number Is the Case
“Per se” is Latin for “by itself.” In a per se DUI prosecution, the BAC reading is the entire offense. The state does not need to show you were weaving between lanes, slurring your words, or failing a field sobriety test. If a properly administered breath or blood test puts you at 0.08% or higher while you were behind the wheel, the legal standard is met.
Every state uses the 0.08% threshold for typical adult drivers. That uniformity is not an accident. Federal law withholds a portion of highway funding, 6% of certain apportionments, from states that don’t enforce a 0.08% per se law.1Office of the Law Revision Counsel. 23 USC 163 – Safety Incentives to Prevent Operation of Motor Vehicles by Intoxicated Persons Utah went further in 2018 and lowered its per se limit to 0.05%.2National Highway Traffic Safety Administration. Utah’s .05% Law Shows Promise to Save Lives, Improve Safety
The medical rationale behind the number is that at 0.08% most people experience reduced muscle coordination, difficulty detecting danger, and impaired judgment.3Cleveland Clinic. Blood Alcohol Content But in a per se case none of that has to be proven about you individually. Whether you felt fine and drove without incident is legally beside the point. The reading is the offense.
Impairment DUI: Proving You Couldn’t Drive Safely
An impairment-based DUI works from the opposite direction. Instead of pointing to a number, the prosecution has to build a picture: alcohol, drugs, or a combination of the two actually diminished your ability to drive. That picture is assembled from observable evidence. How were you driving before the stop? How did you behave when the officer approached? How did you perform on field sobriety tests?
The arresting officer’s observations do most of the work. Prosecutors rely on testimony about erratic lane changes, delayed reactions, bloodshot eyes, the smell of alcohol, fumbling with a wallet or license, and slurred speech. Dashboard and body-worn camera footage often becomes the single most important piece of evidence, because it lets the jury see for themselves rather than take an officer’s word for it.
Impairment charges become the primary theory in two situations. The first is when the driver’s BAC is below 0.08% but the officer believes impairment is obvious from behavior. The second is when drugs are involved. There is no universally accepted chemical threshold for drug impairment equivalent to the 0.08% standard for alcohol. Only a handful of states have set per se limits for specific substances like THC.4Governors Highway Safety Association. Drug-Impaired Driving Most drug-related DUIs are prosecuted as impairment cases.
You Can Be Charged With Both From One Arrest
This is the point most people miss. Prosecutors routinely file both a per se count and an impairment count from a single stop. They are not treated as the same offense. One says your BAC was over the limit. The other says your driving was compromised. Filing both gives the prosecution two independent paths to a conviction on one arrest.
The strategic effect is significant. If the defense knocks out the BAC evidence on procedural grounds, the impairment count can still survive on the officer’s testimony and the video. If the defense undermines the officer’s account of impairment, the per se count can still hold on a properly administered chemical test. Dual charges are a built-in backup plan.
Lower Thresholds for Commercial and Underage Drivers
The 0.08% figure applies to adult drivers in personal vehicles. Two groups face stricter per se limits, and it’s worth knowing which category you fall into before assuming the standard number is yours.
Commercial motor vehicle drivers can be disqualified at a BAC of just 0.04%, whether or not they show any signs of impairment. The rule applies on or off duty. A single violation triggers a one-year disqualification from operating commercial vehicles, and a second violation results in a lifetime disqualification.5Federal Motor Carrier Safety Administration. Driver Disqualified for Driving a CMV While Off-Duty With a Blood Alcohol Concentration
Drivers under 21 face zero-tolerance laws in every state, with per se limits ranging from 0.00% to 0.02% depending on the jurisdiction. Since it is illegal for anyone under 21 to purchase alcohol in the first place, the rationale is that there is no acceptable amount for an underage driver.
How the Evidence Differs
Because the two charges rest on different things, the evidence that decides each of them is different too.
Chemical Testing Drives Per Se Cases
Breath tests give an immediate BAC reading and are the most common tool in per se prosecutions. The devices require regular calibration and maintenance, and defense attorneys routinely question whether the machine was working properly, whether the operator was certified, and whether enough time passed since the driver’s last drink to rule out inflated mouth-alcohol readings. Blood tests are more precise and harder to challenge on accuracy, but they require consent or a warrant.
The Supreme Court drew a line on testing in Birchfield v. North Dakota (2016). Police can require a breath test without a warrant as part of a lawful DUI arrest, but cannot require a warrantless blood draw. A state may impose civil penalties for refusing a breath test; criminalizing the refusal of a blood test violates the Fourth Amendment.6Justia US Supreme Court. Birchfield v. North Dakota Officers who want blood after a refusal typically need a warrant, which many departments can now get electronically within minutes.
Officer Observations and Field Sobriety Tests Drive Impairment Cases
Impairment prosecutions lean on the three standardized field sobriety tests developed by NHTSA: the Horizontal Gaze Nystagmus test, the Walk-and-Turn, and the One-Leg Stand. NHTSA’s own validation research puts the eye-tracking test at about 88% accurate at identifying drivers at or above 0.08% BAC, the walk-and-turn at about 79%, and the one-leg stand at about 83%.7National Highway Traffic Safety Administration. Standardized Field Sobriety Testing Refresher Manual Under controlled conditions, that leaves roughly one in five to one in eight sober drivers at risk of being misclassified.
Video has changed impairment cases more than any other single factor. Footage showing steady driving, clear speech, and normal movement can undercut a written report that describes something else. The reverse is also true. Video of a driver stumbling out of a car can be devastating even if the BAC later comes back below 0.08%.
Why the Distinction Matters for Your Defense
The practical reason to keep per se and impairment charges straight is that they are defended in different ways, and if you are facing both, each has to be answered on its own terms.
Against a per se charge, the defense goes at the BAC evidence. Was the breathalyzer properly calibrated? Did the officer observe the required waiting period before administering the test? Could a medical condition such as acid reflux have produced a falsely elevated reading? Was the blood sample drawn, stored, and tested correctly, with an intact chain of custody? If the BAC evidence gets suppressed or discredited, the per se charge collapses, because the number was the entire case.
Against an impairment charge, the defense goes at the subjective evidence. Were the field sobriety tests administered according to NHTSA protocol? Given that those tests misclassify sober people at rates between 12% and 21%,7National Highway Traffic Safety Administration. Standardized Field Sobriety Testing Refresher Manual how much weight can the result really carry? Do inconsistencies between the officer’s written report and the camera footage undermine the account? Are there alternative explanations for what the officer read as impairment, such as fatigue, a medical condition, nervousness, or uneven pavement?
When both counts are filed together, winning one does not resolve the other. Suppressing the BAC does not defeat an impairment case built on the officer’s observations. Discrediting the officer does not make a valid 0.10% blood test disappear. The most effective defense identifies the weakest link in each charge and works both at once.