By Deandra Grant, J.D., M.S. (Pharmaceutical Science), ACS-CHAL Forensic Lawyer-Scientist
Driving at 11 pm and blood drawn at 12 am with a result of 0.10. On its face, the case looks straightforward. But Texas law does not ask what your BAC was when the test was administered. It asks what your BAC was, and whether you were intoxicated, at the time you were driving.
Those two moments are not the same moment. And in cases where alcohol was still being absorbed into the bloodstream when the test was taken, the number on the report may reflect a BAC the driver had not yet reached while behind the wheel.
This is the rising blood alcohol defense, and it is not a technicality. It is a pharmacokinetic reality that has been litigated successfully in Texas courts and that, when the facts support it, can fundamentally challenge the prosecution’s most important evidence.
"Deandra Grant Law – Criminal & DWI Defense handled my case with diligence and professionalism. Deandra Grant's reputation is stellar and now I know why. She has a team of individuals who provide quality service."
- N. Coulter
"Deandra Grant Law – Criminal & DWI Defense fights hard for their clients and is always willing to go above and beyond. They are the best firm for DWI cases in DFW and beyond. Definitely hire them to represent you in any pending cases."
- P. Williams
"Deandra Grant made a tough situation so much better. She listened to my concerns and helped me so much with my case. I would recommend her to anyone needing legal services."
- M. Haley
The Science of Alcohol Absorption: Two Phases That Define the Defense
To understand why the timing of a chemical test matters, you need to understand how alcohol actually moves through the body. This is clinical pharmacokinetics, and it has direct application to DWI defense.
The absorption phase. After alcohol is consumed, it moves from the stomach and small intestine into the bloodstream. This process takes time. It is not instantaneous. The rate of absorption depends on a range of variables: whether food is present in the stomach (food significantly slows absorption by delaying gastric emptying), the type and concentration of the alcoholic beverage consumed, the speed of consumption, the individual’s body composition, and certain physiological and metabolic factors. During this phase, BAC is rising which means it has not yet reached peak concentration.
The post-absorptive phase. Once absorption is substantially complete and peak BAC is reached, the body begins eliminating alcohol primarily through hepatic metabolism. This is the elimination phase, and it follows a relatively predictable pattern described by Widmark’s model which is a roughly linear decline in BAC over time, typically at a rate of 0.010 to 0.025 grams per deciliter per hour depending on the individual.
The critical point for DWI defense is this: the rising BAC defense applies specifically during the absorption phase, before peak concentration is reached. If a driver consumed alcohol shortly before driving (ex. within the last 30 to 90 minutes, depending on the circumstances) and was tested 45 to 90 minutes after being stopped, it is entirely pharmacologically plausible that their BAC was below 0.08 while they were operating the vehicle and above 0.08 by the time the test was administered.
Peak BAC typically occurs somewhere between 30 minutes and 90 minutes after the last drink, depending on the factors above. The range is wide because the variables are significant. A person who drinks quickly on an empty stomach may reach peak concentration much faster. A person who drinks slowly over the course of a dinner with food may reach peak concentration much later (potentially after the traffic stop, after the field sobriety tests, and after law enforcement has administered the breath test).
Why Texas Law Creates Space for This Argument
Texas Penal Code §49.01 defines intoxication as either having a BAC of 0.08 or above, or not having normal use of mental or physical faculties. The per se BAC definition focuses on the level of alcohol in the blood, but Texas law requires proof that the intoxication existed at the time of driving, not merely at the time of testing.
A breath or blood test administered after the stop is evidence of what BAC was at the time of testing. It is not automatically proof of what BAC was at the time of driving. The prosecution must connect the result to the moment of operation — and when the test was taken during or near the absorption phase, that connection is the very thing in dispute.
This is where Texas appellate decisions on retrograde extrapolation become directly relevant. Courts have addressed the reliability requirements for expert testimony about BAC at an earlier time point, and those requirements reflect the scientific reality that extrapolation. particularly backward extrapolation across the absorption phase, requires data the prosecution frequently does not have.
The Retrograde Extrapolation Problem During the Absorption Phase
Retrograde extrapolation is the mathematical process of estimating what a person’s BAC was at an earlier time based on a later measurement. Prosecutors sometimes use this to argue that a defendant’s BAC was above 0.08 at the time of driving even if the test was delayed. Defense teams use it to argue the opposite (that a BAC measured after an absorption-phase delay was lower at the time of driving than at the time of testing).
Here is the fundamental problem that applies specifically to the absorption phase: Widmark’s elimination model assumes you are in the post-absorptive, elimination phase. The model applies a fixed elimination rate and works backward in time from the measured result. This is a reasonable approximation when the person is clearly past peak BAC and in the elimination phase.
But during the absorption phase, BAC is not declining at a predictable rate. It is rising. You cannot apply a post-absorptive elimination rate to an absorption-phase curve and produce a reliable estimate. The math requires knowing when peak absorption occurred, which in turn requires knowing the precise drinking pattern, the stomach contents, the absorption rate for that individual, and the exact timing of consumption. Without that data, any backward calculation that assumes an elimination-phase curve is being applied where it does not belong.
When a prosecution expert attempts to extrapolate backward through the absorption phase by arguing that the driver’s BAC at the time of driving was similar to or higher than the tested value, they are making assumptions the pharmacokinetic science does not support without complete drinking history data. Challenging those assumptions, exposing the missing data, and presenting the alternative absorption-phase explanation through a qualified toxicology expert is the mechanism by which the rising BAC defense is built at trial.
For a deeper look at retrograde extrapolation methodology, see Managing Partner Deandra Grant’s published article in Counterpoint: The Journal of Science & the Law.
When the Defense Is Most Viable
Not every DWI case supports a rising BAC defense, and it is important to evaluate the facts carefully rather than assert the argument speculatively. The defense is strongest when several conditions converge:
Short time between last drink and driving. If the driver consumed their last drink within 30 to 60 minutes of getting behind the wheel, the probability of still being in the absorption phase during the stop is meaningful.
Delayed chemical testing. The longer the gap between the stop and the test (whether due to field sobriety test administration, transport to the station, the observation period before a breath test, or a warrant blood draw) the more time alcohol had to continue absorbing after driving ended. Delays of 45 minutes to two hours or more create the most favorable conditions for the rising BAC argument.
Test result near the threshold. A result of 0.09 in a case where the driver may have been in the absorption phase is more defensible than a result of 0.15. The closer the result to 0.08, the smaller the gap between the measured BAC and the BAC at the time of driving needs to be.
Documented drinking history. The defense is supported by contemporaneous documentation (ex. receipts, witness accounts, surveillance footage, credit card records) that establishes what was consumed and when. Without that evidence, the argument becomes more speculative and less persuasive.
Physical evidence inconsistent with intoxication. If the dashcam video shows normal driving in the period before the stop, the field sobriety test video shows a person who appears coordinated and coherent, and the officer’s observations are not strongly consistent with impairment, that evidence supports the argument that the driver was not intoxicated at the time of driving regardless of what the test showed later.
Case Results
How the Defense Is Built
Option 1: Build the Affirmative Rising BAC Case
Timeline reconstruction. The defense develops a precise timeline of the driver’s drinking history, food consumption, physical activity, and the sequence of events from the last drink through the traffic stop, field sobriety tests, and chemical testing. Every time entry in the police report, every timestamp on dashcam or body camera footage, and every contemporaneous document the driver can provide contributes to this reconstruction.
Toxicology expert analysis. A qualified forensic toxicologist evaluates the timeline, applies the relevant absorption and elimination parameters to the individual’s characteristics, and provides an opinion about the probable BAC at the time of driving. This expert analysis is what converts the scientific theory into admissible testimony.
Challenging the prosecution’s extrapolation. If the prosecution has retained a toxicology expert to testify that the driver’s BAC was above 0.08 at the time of driving, cross-examination focuses on whether that expert accounted for the possibility that the driver was still in the absorption phase. What data did they use for the drinking timeline? What assumptions did they make about when peak absorption occurred? Do they acknowledge the absorption-phase limitation on Widmark extrapolation? These are the questions that expose the weaknesses in a prosecution expert’s methodology when the scientific foundation is insufficient.
Option 2: The State Can’t Prove Intoxication at the Time of Driving
The burden is on the State to prove their case, not on the defense to disprove it. Cross-examination may simply consist of pointing out all the assumptions and potential error in the State expert’s opinion and alleged retrograde extrapolation. When the State cannot establish with reliable data what the defendant’s BAC was at the moment of driving, because they lack the drinking history, the absorption parameters, and the timeline necessary to run the math correctly, that failure of proof is the defense. The rising BAC defense does not always require a competing expert. Sometimes it requires only a cross-examination that exposes what the prosecution’s expert doesn’t know.
Speak With Deandra Grant Law
The rising BAC defense is a pharmacokinetic argument. Making it effectively requires a defense attorney who understands the science at a level that allows for genuine expert cross-examination and not just the legal argument, but the underlying chemistry and physiology.
My Master’s Degree in Pharmaceutical Science and ACS-CHAL Forensic Lawyer-Scientist designation give me the foundation to evaluate whether the rising BAC argument applies to a specific case, to identify the weaknesses in a prosecution’s retrograde extrapolation testimony, and to work with qualified toxicology experts in a way that produces effective trial preparation rather than a superficial credentialing exercise. My Master’s thesis was on retrograde extrapolation and it was later published in two parts in Counterpoint: The Journal of Science & the Law. Part 1 and Part 2.
If you were arrested for DWI in Texas and there was a significant gap between your last drink and the chemical test, or between the stop and the testing, that timing deserves careful forensic analysis before any decision is made about your defense.
Call (214) 225-7117 or visit texasdwisite.com to schedule a confidential consultation.
Firm Accolades
Better Business Bureaus
D Magazine
