When someone is arrested for DUI in Tennessee, the breath or blood test result often becomes one of the most important pieces of evidence in the case. But when that test is taken can be just as important as what the test says.
Blood alcohol concentration, or BAC, changes over time. After a person drinks, their BAC may continue rising before it reaches its peak and begins to decrease. Because chemical testing usually happens after a traffic stop, a later test result may not always reflect the person’s BAC at the time they were driving or in physical control of the vehicle.
That timing issue can have real consequences in a Tennessee DUI case. If a person’s BAC was still rising after the stop, the defense may question whether the State can prove the driver was over the legal limit while driving. Timing may also matter when the test result is close to 0.08%, or near the 0.15% threshold that can trigger enhanced penalties for a first-offense DUI.
In other words, rising BAC can affect how the prosecution uses the test result, how the defense challenges the evidence, and whether the BAC number tells the full story.
This blog explains what rising BAC means, how BAC changes over time, why test timing can matter in Tennessee DUI cases, and how a DUI defense lawyer may evaluate BAC evidence as part of a broader defense strategy.
What BAC Means in a Tennessee DUI Case
Blood alcohol concentration refers to the amount of alcohol in a person’s blood or breath. A BAC result can be important, but it is not the only way the State can try to prove DUI.
Under Tennessee law, a person can be charged with DUI if they drive or are in physical control of a vehicle while under the influence of alcohol, drugs, or another intoxicant that impairs their ability to safely operate the vehicle. Tennessee law also makes it unlawful to drive or be in physical control of a vehicle with an alcohol concentration of 0.08% or more. For commercial motor vehicle cases, the BAC threshold is 0.04% or more.
That means Tennessee DUI cases often involve two related but different theories.
Impairment-based DUI focuses on whether alcohol, drugs, or another substance affected the person’s ability to drive safely. In these cases, the State may rely on the officer’s observations, driving behavior, field sobriety tests, bodycam footage, statements, and other evidence.
Per se DUI focuses on the BAC number itself. If the State alleges that a driver’s blood or breath alcohol concentration was at or above the legal limit, the chemical test result may become central to the case.
This distinction matters because BAC can affect both how the State tries to prove the charge and how the defense evaluates the evidence. BAC can also matter for sentencing. For a first-offense DUI in Tennessee, the general minimum jail sentence is forty-eight consecutive hours. But if the person’s BAC is 0.15% or more, Tennessee law requires a minimum of seven consecutive days instead.
How BAC Rises and Falls Over Time
BAC is not a fixed number. After a person drinks alcohol, their blood alcohol concentration usually moves through three general stages: absorption, peak, and elimination.
During the absorption stage, alcohol enters the bloodstream through the stomach and small intestine. This does not happen all at once. According to the National Highway Traffic Safety Administration, alcohol can generally be measured within 30 to 70 minutes after a person has had a drink. That means a person’s BAC may continue rising for a period of time after they stop drinking.
Several factors can affect how quickly BAC rises, including how much alcohol the person drank, how quickly they drank, their body weight, sex, and whether they had food in their stomach. NHTSA notes that drinking faster generally leads to a higher BAC than drinking over a longer period of time, and that body weight and sex can also affect BAC levels.
Eventually, BAC reaches a peak. After that, the body begins eliminating alcohol from the system. But elimination takes time. A person cannot quickly lower their BAC by drinking coffee, taking a shower, or trying to “walk it off.” The body has to process the alcohol.
This matters in DUI cases because a breath or blood test reflects a specific point in time. When testing happens after an arrest, the result may need to be evaluated in light of when the person drank, when they drove, and when the test was actually administered.
What Is Rising BAC?
“Rising BAC” refers to a situation where a person’s blood alcohol concentration is still increasing after they stop driving. This can happen when someone drinks shortly before getting behind the wheel and their body is still absorbing the alcohol at the time of the traffic stop.
For example, imagine a driver is stopped at 11:30 p.m., but the breath test is not given until 12:20 a.m. If the test result is 0.09%, the legal question may not be as simple as whether the person tested over 0.08% at 12:20 a.m. The defense may also examine whether the person’s BAC was still rising and whether it may have been lower at the time of driving.
A rising BAC issue does not automatically defeat a DUI charge. Instead, it is a factual issue about timing, absorption, testing, and proof. The key question is whether the chemical test result accurately reflects the person’s BAC when they were driving or in physical control of the vehicle.
Why Timing Can Impact DUI Charges
Timing can be a major issue in Tennessee DUI cases because there may be a delay between the alleged driving and the breath or blood test. A driver may be stopped, questioned, investigated on the roadside, arrested, transported, and then tested later. By the time the test is given, the person’s BAC may be different than it was earlier in the encounter.
This can affect how the State tries to prove the case. In a per se DUI case, the BAC number may be central to the charge. If the result is close to 0.08%, the timing of the test may become especially important. In an impairment-based DUI case, the State may rely more heavily on other evidence, such as driving behavior, officer observations, field sobriety tests, statements, or video footage.
Timing may also matter when enhanced penalties are at issue. If a delayed test result is close to 0.15%, the defense may closely examine whether the BAC was still rising after the traffic stop and whether the result fairly reflects the relevant time.
The key point is that BAC evidence is time-sensitive. A breath or blood test can be important evidence, but it should be evaluated in context.
BAC Is Only One Part of the DUI Evidence
Although BAC evidence can be important in a Tennessee DUI case, it is rarely the only evidence that matters. A defense lawyer may review the full timeline of the stop, investigation, arrest, and testing process to determine whether the evidence supports the charge.
For example, the defense may look at the reason for the traffic stop. If the officer did not have a lawful basis to stop the vehicle, evidence gathered afterward may be challenged. The defense may also review whether the officer’s observations actually support impairment. Common observations like odor of alcohol, red or watery eyes, or slurred speech may have innocent explanations, and they do not always prove that a person was impaired while driving.
Field sobriety tests may also create defense issues. These tests can be affected by medical conditions, fatigue, age, footwear, weather, road conditions, lighting, nervousness, or unclear instructions. Bodycam or dashcam footage may show whether the tests were properly explained, whether the driver performed better than described in the report, or whether the officer left out important context.
The chemical test itself may raise additional questions. In a breath test case, the defense may examine whether the machine was properly maintained, whether the officer followed required testing procedures, and whether anything could have affected the reliability of the result. In a blood test case, the defense may review how the blood was drawn, stored, transported, and tested, as well as whether the chain of custody was properly documented.
Rising BAC is one defense angle within this broader analysis. Ultimately, a DUI case is not just about a number on a test result. It is about whether the State can prove the charge beyond a reasonable doubt using reliable evidence.
Talk to a Knoxville DUI Defense Lawyer
If you were charged with DUI in Knoxville or East Tennessee, the BAC number may not tell the whole story. The timing of the traffic stop, arrest, and breath or blood test can all affect how the evidence should be interpreted.
At Barnes & Fersten, our Knoxville DUI defense lawyers review the details behind the BAC result, not just the number itself. That may include examining when testing happened, whether the BAC result was close to a legal threshold, whether proper procedures were followed, and whether the officer’s observations match the video and other evidence.
Rising BAC issues are fact-specific. In some cases, the timing of the test may raise important questions about whether the State can prove the BAC at the legally relevant time. In other cases, the defense may focus on the stop, field sobriety tests, breath machine records, blood draw procedures, chain of custody, or other weaknesses in the evidence.
A DUI charge can carry serious consequences, including jail time, license issues, fines, and long-term effects on your record. If you are facing a DUI charge in Knoxville or East Tennessee, call Barnes & Fersten at 865-805-5703 or fill out our contact form to schedule a free consultation.
Attorney At Law, Managing Partner
Brandon D. Fersten is an esteemed Knoxville attorney practicing DUI, criminal defense, and juvenile law. Known for his empathetic approach and commitment to his clients, he brings a record of favorable case outcomes including dismissals and not guilty verdicts at jury trials resulting in Brandon being recognized as one of the “Top 40 Under 40” in Criminal Defense, U.S. News’ Best Lawyers: “Ones to Watch,” and Super Lawyers’ “Rising Stars”. Brandon’s professional accolades, combined with his passion for justice, position him as a reliable criminal defense advocate in the East Tennessee legal landscape, including Knox County, Blount County, Sevier County, Loudon County, Roane County, Anderson County, Cumberland County, Hamblen County, Monroe County, and McMinn County.
