By Nathan Cate, Nashville Criminal Defense Attorney | Cate Law
You blew a .09 on the breathalyzer. The officer says you were swerving. Now you are looking at your charging documents and you see two separate DUI counts — one for being “under the influence” and one for having a blood alcohol concentration above .08. You thought DUI was one charge. It is not. Tennessee recognizes two distinct theories of DUI, and the State can charge you under both at the same time.
Understanding the difference between DUI per se and DUI by impairment is not academic. It determines what evidence matters, what defenses are available, and how your case gets fought in court. I have tried DUI cases in Davidson County, Williamson County, Rutherford County, and across Middle Tennessee for years. The defense strategy changes depending on which theory the State is relying on — and when both are charged, the defense has to address each one separately.
Here is how these two DUI theories work, why they exist, and what it means for your defense.
Tennessee’s DUI Statute: Two Theories Under One Law
Tennessee’s DUI law is codified at Tenn. Code Ann. § 55-10-401. That single statute contains multiple ways the State can prove a DUI. The two most common are:
DUI per se — It is unlawful for any person to drive or be in physical control of a motor vehicle while the alcohol concentration in the person’s blood or breath is .08 or more. That is it. The State does not have to prove you were impaired. It does not have to prove you were swerving, slurring, or stumbling. If the machine says .08 or above, the State has met its burden under this theory.
DUI by impairment — It is unlawful for any person to drive or be in physical control of a motor vehicle while under the influence of any intoxicant, marijuana, controlled substance, drug, substance affecting the central nervous system, or combination thereof that impairs the driver’s ability to safely operate a motor vehicle. Under this theory, the number on the machine is not the point. The question is whether you were impaired — whether your ability to drive was diminished by whatever substance you consumed.
These are separate theories with separate proof requirements. The State can charge you with both, present evidence on both, and ask the jury to convict on either or both. A jury can acquit on the per se theory (finding the BAC test unreliable) and still convict on the impairment theory (finding you were too impaired to drive based on the officer’s observations). Or vice versa.
DUI Per Se: The Number Is the Case
How the State Proves Per Se DUI
The per se theory is built almost entirely on the chemical test result. If you submitted to a breath test and the result was .08 or higher, or if your blood was drawn and analyzed at .08 or above, the State has a straightforward path to conviction under this theory.
The State must prove:
- You were driving or in physical control of a motor vehicle
- At the time of driving, your blood or breath alcohol concentration was .08 or more
That is the entire legal standard. The officer does not need to testify that you appeared drunk. The dashcam does not need to show erratic driving. The field sobriety tests are irrelevant to this theory. The machine result is the evidence.
Tennessee law establishes that the BAC result creates a presumption. Under Tenn. Code Ann. § 55-10-408(b), a BAC of .08 or more creates a presumption that the person was under the influence of an intoxicant. This presumption is rebuttable — the defense can challenge it — but the statute builds the State’s case into the number itself.
Per Se DUI Defense Strategies
Because the per se theory depends on the chemical test result, the defense targets the reliability of that result. If the number falls, the per se theory falls with it.
Challenging the breath test machine. Tennessee uses the Intoximeter EC/IR II for breath testing. These machines require regular calibration, and the Tennessee Bureau of Investigation sets maintenance and calibration schedules. If the machine was overdue for calibration, if the calibration records show the machine was reading outside acceptable tolerances, or if the machine malfunctioned during your test, the result is vulnerable.
Challenging the blood draw procedure. If your BAC was established through a blood test, the defense examines the chain of custody and the laboratory procedures. Was the blood drawn by a qualified person? Was the sample properly preserved with the correct anticoagulant? Was the sample refrigerated? Was the chain of custody maintained from the draw site to the laboratory to the courtroom? A break in any of these steps can undermine the reliability of the result.
Challenging the traffic stop itself. The Fourth Amendment requires that an officer have reasonable suspicion to initiate a traffic stop. If the stop was unlawful — no traffic violation, no reasonable suspicion of criminal activity — then everything that followed, including the breath or blood test, may be suppressible. No admissible test result means no per se DUI.
The rising BAC defense. This is one of the most important defense tools in per se DUI cases, and I will discuss it in detail below because it applies with particular force to the per se theory.
DUI by Impairment: Proving You Were Too Impaired to Drive
How the State Proves Impairment
The impairment theory does not require a chemical test at all. The State can prove impairment entirely through the officer’s observations and testimony. This is why refusing the breathalyzer does not make a DUI case disappear — the State simply shifts to the impairment theory.
The evidence the State uses to prove impairment typically includes:
- Driving behavior. Swerving, drifting between lanes, failure to maintain speed, running red lights, delayed reaction to signals. The officer’s dashcam or body camera footage of the driving pattern is the starting point.
- Physical observations. Bloodshot watery eyes, slurred speech, odor of alcohol, unsteady gait, fumbling with documents, difficulty following instructions. The officer testifies to what he or she observed during the encounter.
- Standardized Field Sobriety Tests (SFSTs). The horizontal gaze nystagmus test, the walk-and-turn test, and the one-leg-stand test. These are the three tests validated by the National Highway Traffic Safety Administration. The officer administers them roadside and testifies about the “clues” observed.
- Statements. “I had three beers.” “I just left the bar.” Anything you said to the officer about what you consumed, when, and how much.
- Chemical test results. If a BAC result exists, the State will use it as corroborating evidence of impairment even though the impairment theory does not strictly require it.
Impairment DUI Defense Strategies
Because the impairment theory rests on human observations rather than a machine number, the defense strategy is different. The goal is to challenge the reliability and interpretation of what the officer saw.
Challenging the field sobriety tests. SFSTs are not pass/fail tests, despite the way officers often describe them. They are standardized clue-counting exercises that must be administered according to NHTSA protocols to have any reliability. Common problems I see in Middle Tennessee DUI cases:
The officer did not administer the tests according to the standardized instructions. The walk-and-turn test requires a straight line, a flat surface, and specific verbal instructions. If the officer told you to walk along an imaginary line on a sloped gravel shoulder at 2:00 AM next to a highway, the conditions undermine the test’s reliability.
The officer counted clues that are not part of the standardized test. NHTSA specifies exactly which observations count as clues for each test. An officer who testifies that you “seemed nervous” or “were swaying slightly” is not describing standardized clues.
Medical conditions, injuries, or physical limitations explain the observations better than impairment. Inner ear problems, knee injuries, back problems, neurological conditions, obesity, age, footwear — all of these affect performance on field sobriety tests and have nothing to do with alcohol.
Challenging the officer’s observations. Body camera footage is the single most valuable tool in impairment DUI defense. Officers write their reports hours after the stop. Memory is imperfect and biased toward confirmation of the arrest decision. Body camera footage shows what happened in real time.
I have had cases where the officer’s report said the defendant was “staggering” and the body camera showed the person walking normally. I have had cases where the report described “heavily slurred speech” and the footage showed clear, coherent communication. The gap between what the officer remembers writing in the report and what the camera recorded is sometimes the entire defense.
Challenging the impairment connection. Even if the State proves you consumed alcohol, it must prove that the alcohol impaired your ability to drive. Having two glasses of wine with dinner and driving home is not illegal. The question is impairment — whether your ability to safely operate the vehicle was diminished. If the driving pattern was normal, the field sobriety tests were adequate, and the only evidence of impairment is the officer’s opinion, the defense has room to argue that the State has not met its burden.
For a deeper look at DUI defense strategies, see our DUI defense page.
When Both Theories Are Charged Together
In most Tennessee DUI cases, the State charges both per se and impairment. This is standard prosecutorial practice. The DA wants to give the jury two paths to conviction.
From the defense perspective, fighting both theories means addressing two different evidence sets. The BAC number for per se. The officer’s observations for impairment. Sometimes the same piece of evidence cuts both ways — a .08 BAC is barely over the limit, which supports a per se defense (the margin of error may encompass .08) and also supports an impairment defense (a .08 BAC in a regular drinker may not produce observable impairment).
From the jury’s perspective, the two theories can create confusion that works in the defense’s favor. If the BAC was .09 but the body camera shows the defendant walking normally, speaking clearly, and performing field sobriety tests adequately, the jury sees a disconnect. The machine says impaired. The video says not impaired. That tension creates reasonable doubt.
Trial strategy in dual-theory DUI cases requires careful sequencing. You want to undermine the BAC result first (attacking per se), then use the strong body camera footage to undermine impairment, and let the jury connect the dots — if the person was not impaired on camera, maybe the machine was wrong too.
The Rising BAC Defense
The rising BAC defense is one of the most misunderstood and underutilized defenses in DUI cases. Here is how it works.
Alcohol takes time to absorb into the bloodstream. If you finished your last drink 20 minutes before driving, your BAC was still rising when you got behind the wheel. By the time the officer administered the breath test — which might be 45 minutes to an hour after the stop, and potentially 90 minutes or more after your last drink — your BAC could be significantly higher than it was when you were driving.
The legal question is your BAC at the time of driving, not at the time of testing. If your BAC was .07 when you were behind the wheel but rose to .09 by the time you blew into the machine, you were not over the legal limit when it mattered.
Retrograde Extrapolation: The State’s Response
To combat the rising BAC defense, prosecutors use retrograde extrapolation — a mathematical calculation that works backward from the test result to estimate what the BAC was at the time of driving. The State calls an expert (usually a forensic toxicologist) who testifies about absorption rates, elimination rates, and the theoretical BAC at the time of the traffic stop.
Retrograde extrapolation has serious scientific limitations. The calculation depends on assumptions about the individual’s absorption rate, metabolism, food consumption, body weight, gender, drinking pattern, and the time of the last drink. Change any of these variables and the calculation changes. The expert is estimating, not measuring.
Defense challenges to retrograde extrapolation focus on the assumptions:
- The expert does not know when the defendant’s last drink was consumed (unless the defendant told the officer, which is why you should not answer those questions)
- The expert does not know the defendant’s individual absorption rate, which varies significantly between people
- The expert does not know whether the defendant had food in their stomach, which dramatically slows absorption
- The expert is applying population averages to an individual, which is inherently imprecise
When the BAC is close to .08, these margins of error are large enough to create reasonable doubt. A .09 test result with a 45-minute delay between the stop and the test, combined with testimony that the defendant finished their last drink shortly before driving, can support a credible rising BAC defense.
Refusing the Chemical Test: Implications for Both Theories
Tennessee is an implied consent state. Under Tenn. Code Ann. § 55-10-406, by driving on Tennessee roads you have impliedly consented to a chemical test if an officer has probable cause to believe you are driving under the influence. Refusing the test triggers a one-year driver’s license revocation — a civil penalty separate from the criminal case.
A refusal eliminates the per se theory. Without a BAC result, the State cannot prove your blood alcohol concentration was .08 or above. But the refusal does not eliminate the impairment theory. The State can — and routinely does — proceed on the impairment theory using the officer’s observations, field sobriety tests, and your statements.
And here is what many defendants do not anticipate: the State can use the refusal itself as evidence of consciousness of guilt. The argument is that you refused the test because you knew you were intoxicated. This is not a per se theory argument — it is an impairment theory argument. The refusal becomes part of the evidence that you knew you were too impaired to pass.
Whether to submit to or refuse a chemical test is one of the most consequential decisions a person makes during a DUI stop, and it should be made with an understanding of how it affects both theories of prosecution.
How Field Sobriety Tests Relate to the Impairment Theory
Field sobriety tests are the backbone of the impairment theory. Without them, the State’s impairment case often reduces to the officer’s subjective opinion — bloodshot eyes, odor of alcohol, slurred speech. Those observations alone are weak. Every criminal defense attorney in Nashville knows that “bloodshot watery eyes” appears in approximately 90% of all DUI reports regardless of the defendant’s actual condition.
The three standardized tests add a veneer of scientific rigor:
Horizontal Gaze Nystagmus (HGN)
The officer moves a stimulus (usually a pen or finger) across your field of vision and watches for involuntary jerking of the eyes. NHTSA identifies six clues (three per eye): lack of smooth pursuit, distinct and sustained nystagmus at maximum deviation, and onset of nystagmus prior to 45 degrees.
HGN is the most scientifically supported of the three tests, but it has significant limitations. Nystagmus can be caused by medications, medical conditions, fatigue, caffeine, and other factors unrelated to alcohol. The test must be administered correctly — the stimulus must be held at the correct distance, moved at the correct speed, and held at each position for the correct duration. Officers in the field frequently deviate from these requirements.
Walk and Turn
Nine steps heel-to-toe along a straight line, turn, nine steps back. The officer looks for eight clues: cannot keep balance during instructions, starts too soon, stops walking to steady, does not touch heel-to-toe, steps off the line, uses arms for balance, improper turn, wrong number of steps.
This test is heavily dependent on conditions. The surface must be flat, dry, and well-lit. The person must be wearing reasonable footwear. The instructions must be delivered correctly. I have had clients fail this test on dark, sloped, gravel shoulders while wearing heels — conditions that would challenge a sober person.
One-Leg Stand
Raise one foot six inches off the ground, point the toe, look at the foot, count out loud for 30 seconds. Four clues: swaying, using arms for balance, hopping, putting the foot down.
Age, weight, physical fitness, medical conditions, and footwear all affect performance on this test. A 55-year-old with a bad knee is not going to perform the same as a 25-year-old athlete, regardless of alcohol consumption.
The critical point is that field sobriety tests only matter under the impairment theory. They are irrelevant to per se DUI. If the State is relying solely on the BAC number, the field sobriety tests add nothing to the per se case. But if the BAC is excluded or the State is running the impairment theory in parallel, the field sobriety tests become the centerpiece of the prosecution’s evidence — and the centerpiece of the defense’s attack.
Penalties for DUI Conviction in Tennessee
Regardless of which theory supports the conviction, the penalties are the same. Tennessee does not impose different sentences based on whether the conviction was per se or impairment-based.
For a first-offense DUI under Tenn. Code Ann. § 55-10-403:
- 48 hours to 11 months 29 days in jail (mandatory minimum 48 hours, or 7 days if BAC was .20 or higher)
- $350 to $1,500 fine
- One-year driver’s license revocation
- Court-ordered drug and alcohol treatment program
- Possible ignition interlock device
For a second offense, the mandatory minimum jail time increases to 45 days, the fine increases to $600 to $3,500, and the license revocation is two years. Third offense: 120 days minimum jail, $1,100 to $10,000 fine, and three-to-ten-year license revocation.
These penalties apply whether the jury convicted under per se, impairment, or both theories. The distinction between the theories matters for the defense at trial — it does not matter at sentencing.
What This Means for Your Defense
If you are charged with DUI in Tennessee, the first question your attorney should answer is: what theory is the State going to rely on at trial? If there is a BAC result at or above .08, expect both theories. If you refused the test, expect the impairment theory alone.
The defense strategy follows from the theory:
- Per se defense targets the chemical test — the machine’s accuracy, the blood draw procedure, the rising BAC timeline, the chain of custody.
- Impairment defense targets the officer’s observations — field sobriety test administration, body camera footage, medical explanations for physical symptoms, the gap between the report and the video.
- Both theories charged requires attacking both evidence sets, and using the tension between them (a low BAC with poor field sobriety performance, or a moderate BAC with excellent on-camera behavior) to create doubt across the board.
The worst DUI defense is a generic one. The best DUI defense is built around the specific theory the State is pursuing, the specific evidence in your case, and the specific weaknesses in how that evidence was gathered. Visit our DUI defense page for more on how we approach these cases, or contact us to discuss your specific situation.
Frequently Asked Questions
Can I be convicted of both DUI per se and DUI impairment at the same time?
The State can charge both theories and present evidence on both, but Tennessee’s protection against double jeopardy means you cannot be convicted and sentenced separately for both arising from the same incident. In practice, if the jury finds you guilty under both theories, the judge enters a single conviction and imposes one sentence. The two theories are alternative paths to the same conviction, not two separate crimes.
If I refused the breathalyzer, can the State still prove DUI?
Yes. Refusing the breathalyzer eliminates the per se theory because the State has no BAC number. But the State can and routinely does pursue the impairment theory based on the officer’s observations, field sobriety tests, your statements, and the circumstances of the stop. The refusal itself can also be introduced as evidence that you believed you were intoxicated.
What is the rising BAC defense and does it work?
The rising BAC defense argues that your blood alcohol concentration was below .08 at the time you were driving but rose above .08 by the time you were tested, due to ongoing alcohol absorption. It is a legitimate defense that applies to the per se theory. Its effectiveness depends on the facts — how close the BAC was to .08, how long after the stop the test was administered, and when you had your last drink. In cases where the BAC is between .08 and .12 with a significant delay between the stop and the test, the rising BAC defense can be powerful.
Do field sobriety tests prove I was impaired?
Field sobriety tests do not prove impairment by themselves. They are one piece of evidence the State uses to argue impairment. The tests are standardized by NHTSA and have known error rates — even when administered perfectly, the walk-and-turn test is only 79% accurate, and the one-leg-stand is only 83% accurate according to NHTSA’s own validation studies. When administered improperly (wrong conditions, wrong instructions, officer counting non-standardized clues), their reliability drops further. A strong defense examines every aspect of how the tests were given and challenges the clues the officer claims to have observed.
Can I challenge the breathalyzer result in court?
Yes, and it is one of the primary defense strategies in per se DUI cases. Breath testing machines require regular calibration and maintenance. The operator must be certified. The observation period before the test must be followed. The machine must be functioning within acceptable parameters. If any of these requirements were not met, the result may be unreliable and potentially inadmissible. Your attorney should obtain the machine’s calibration records, maintenance logs, and the operator’s certification history as part of the defense investigation.
Should I hire a lawyer for a first-offense DUI in Tennessee?
A first-offense DUI in Tennessee carries a mandatory minimum of 48 hours in jail, fines up to $1,500, a one-year license revocation, and a permanent criminal record. The collateral consequences — increased insurance rates, employment implications, professional licensing issues — extend far beyond the courtroom sentence. A DUI conviction cannot be expunged in Tennessee. Given the permanent nature of the consequences and the technical complexity of challenging BAC evidence and field sobriety tests, having an experienced defense attorney review your case is important for understanding your options and building the strongest possible defense.
Charged with DUI in Nashville or Middle Tennessee? Call (615) 664-8083 for a free consultation.