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You’re pulled over on a Texas highway. You’ve had some drinks, but you don’t feel intoxicated. The officer puts you through field sobriety tests. You may or may not pass them. You’re arrested. You’re taken to the police station. Breath or blood tests are administered. The test shows a blood alcohol concentration (BAC) above the legal limit of .08 percent. In the following days and weeks, you’re facing DUI charges. The prosecutor has a test result showing you were over the limit. What you might not realize is that one of the most potent defenses in that situation is the simple fact of when the test was administered.
At Shane Phelps Law, we defend people in trouble. We know what it’s like to be charged with a crime you believe you didn’t commit or that prosecutors are overcharging. We know the fear and uncertainty that comes with a DUI charge. What we also know is that breath and blood tests don’t measure your BAC at the moment you were driving. They measure your BAC at the moment you give the sample. When there’s a gap between when you were driving and when you were tested, that gap becomes an opportunity.
This defense strategy—the rising BAC defense and the challenge to retrograde extrapolation—is how we fight back.
The Fundamental Problem with BAC Testing
Here’s the critical issue that most people don’t understand: breath and blood tests measure your BAC at the moment you provide the sample. They do not, cannot, and have never measured your BAC at the moment you were driving. If you were pulled over and tested an hour later, the test measures your BAC an hour later. During that hour, your body was still absorbing and metabolizing alcohol.
Blood alcohol concentration changes over time. When you drink alcohol, it doesn’t instantly saturate your bloodstream. Alcohol is absorbed through the stomach and small intestine over time. That absorption process typically takes 15 to 90 minutes after you finish drinking, depending on a variety of factors, including the alcohol content of what you consumed, whether you ate, your body composition, and your metabolism.
As alcohol is being absorbed, your blood alcohol concentration is rising. During this “absorption phase,” your BAC is not constant—it’s increasing. Once absorption is complete, your BAC plateaus briefly, and then begins to decline as your body metabolizes and eliminates the alcohol.
This timeline is crucial. If you were tested during the absorption phase, your BAC at the moment of the test was higher than your BAC while you were driving. If your BAC was rising during the time between when you drove and when you were tested, then the test result is not a valid indicator of your BAC while driving.
The prosecutor, however, assumes that your BAC was constant or that it had already peaked and was declining when the test was taken. This assumption forms the basis of their case against you. It’s an assumption we challenge.
Understanding Retrograde Extrapolation
Retrograde extrapolation is the process of estimating what someone’s BAC was at a particular time in the past, based on a measurement taken at a later time. The prosecution uses retrograde extrapolation to argue that if your BAC was .08 at the moment of testing, and your BAC was declining at a certain rate after that moment, then your BAC must have been even higher at the moment you were driving.
Here’s how it typically works: The police officer arrests you at, say, 11:00 PM. You’re transported to the station and a breath or blood test is administered at 12:30 AM. The test shows a BAC of .10 percent. The prosecutor argues: “The BAC was declining at a rate of approximately .015 percent per hour. If it was .10 at 12:30 AM, then at 11:00 PM—one and a half hours earlier—it must have been approximately .125 percent.”
This retrograde extrapolation is presented to the jury as a scientific fact. It’s not. It’s a calculation based on assumptions and estimates that are often wrong.
Attacking Retrograde Extrapolation: The Rising BAC Problem
The fundamental problem with retrograde extrapolation in your case might be that your BAC wasn’t declining when the test was taken. You might have been in the absorption phase—your BAC might have been rising.
Think about this scenario: You finish drinking at 10:30 PM. Your absorption phase doesn’t complete until 11:45 PM. But you’re pulled over at 11:00 PM, right in the middle of your absorption phase. You’re tested at 12:30 AM. Your BAC is still rising at the moment you were driving, but it’s peaked and begun declining by the moment you were tested.
If the prosecutor uses retrograde extrapolation to argue backwards from your 12:30 AM test to your 11:00 PM BAC, they’re making a critical error: they’re assuming your BAC was declining during the entire period. In fact, your BAC was rising from 11:00 PM to 11:45 PM, then began declining from 11:45 PM to 12:30 AM. The decline from 11:45 to 12:30 is one thing, but the calculation assumes a constant decline from 11:00 to 12:30. That’s wrong.
Here’s what this means: if your BAC was rising at the moment you were driving, then your BAC while driving was lower than what the test showed at 12:30 AM. It might have been below the legal limit. You might not have been legally impaired.
This is the rising BAC defense: the argument that your BAC was rising, not declining, at the moment you were driving. Therefore, any retrograde extrapolation used by the prosecution is fundamentally flawed.
Factors Affecting Absorption and Metabolism
The absorption and metabolism of alcohol is affected by numerous factors. The prosecution typically assumes a generic absorption and elimination rate, but the science shows these rates vary considerably from person to person. Key variables include:
Body weight and composition: Heavier people have a larger volume of distribution for alcohol, meaning the same amount of alcohol results in a lower BAC. Conversely, lighter people have higher BACs from the same amount of alcohol. Body composition matters too; alcohol distributes in water but not in fat, so a muscular person has a lower BAC than a person with high body fat, even at the same weight.
- Gender: Women typically have a lower percentage of body water than men and have lower levels of alcohol dehydrogenase (the enzyme that metabolizes alcohol), resulting in higher BACs for the same amount of alcohol.
- Food consumption: Food slows alcohol absorption. If you ate before or while drinking, your absorption phase is longer and your peak BAC is lower.
- Alcohol content and drink size: Strong drinks (like liquor) are absorbed faster than weak drinks (like beer). The more alcohol you consume, the longer the absorption phase.
- Medications: Some medications affect alcohol metabolism. Medications that inhibit alcohol dehydrogenase slow metabolism and increase peak BAC.
- Stress and exercise: These factors can affect absorption and metabolism.
- Stomach contents and gastric emptying rate: Individual variation in how quickly food and drink move through your stomach affects absorption.
The critical point is this: the prosecution typically assumes a standard absorption and metabolism rate applied to all people. But your absorption and metabolism rates are unique to you. We use this variability to challenge their assumptions.
The Expert Witness Challenge
Retrograde extrapolation calculations are only admissible in court if the expert witness doing the extrapolation is qualified and using accepted scientific methods. This is where our defense becomes powerful. The expert who conducted retrograde extrapolation relied on assumptions. We can challenge those assumptions.
Our own forensic toxicology expert can testify about the variability in human alcohol metabolism. They can explain why the prosecutor’s assumed rates don’t apply to you. They can demonstrate how a rising BAC scenario would explain the test results. They can show that retrograde extrapolation is not reliable because it’s based on assumptions that may be wrong.
The cross-examination of the prosecutor’s expert becomes a teaching moment for the jury. Through careful questioning, we reveal the assumptions underlying their calculations. We demonstrate that those assumptions aren’t supported by the science. We show that alternative scenarios are equally or more plausible.
Timing and Transportation Time
The time between your arrest and your test is crucial. Texas law and the Constitution require testing to occur without unreasonable delay. But what constitutes reasonable delay? If you were arrested at 11:00 PM and tested at 12:30 AM, ninety minutes have passed. Depending on your circumstances, that gap could be critical.
During that time, you were likely in police custody, perhaps at the station, perhaps in a holding cell. You were being transported, processed, booked, advised of your rights, and read the implied consent warnings. All of this time allows continued absorption and metabolism of alcohol.
The longer the delay, the more opportunity for your BAC to change. A substantial delay strengthens our argument that retrograde extrapolation is unreliable because your BAC was likely still changing during the delay, in unpredictable ways.
The Partition Ratio Problem
Blood breath tests (as opposed to actual blood tests) create an additional layer of uncertainty. Breath tests measure the concentration of alcohol in your breath, then calculate what your blood alcohol concentration likely is. This calculation relies on something called the partition ratio—a conversion factor that translates breath alcohol to blood alcohol.
The law typically assumes a partition ratio of 2100:1, meaning there’s approximately 2,100 units of alcohol in your blood for every one unit in your breath. But this ratio varies considerably among individuals. Some people have ratios as low as 1300:1, while others have ratios as high as 3200:1. That’s a huge range, and it directly affects whether the breath test reading translates to a BAC above or below the legal limit.
If the prosecutor is relying on a breath test and assuming a standard partition ratio, we can challenge that assumption. If your partition ratio is lower than the assumed rate, your actual blood alcohol would be lower than the breath test suggests. You might not have been legally intoxicated.
Blood Tests and Blood Alcohol Content
If you submitted to a blood test instead of a breath test, we have different arguments. Blood tests directly measure BAC, which is more accurate than breath tests. However, blood tests are subject to other challenges: Were the samples stored properly? Was there proper chain of custody? Were the testing procedures correct? Were the results recorded accurately? Can we get the actual blood sample retested?
We can also raise questions about how long the blood test took, where it was conducted, and whether there were delays in testing. We can challenge the lab’s procedures and the technician’s qualifications. We can demand independent retesting.
Putting It All Together
The rising BAC defense is not a get-out-of-jail-free card. But it’s a powerful tool when the facts support it. If there’s a significant gap between when you were driving and when you were tested, if you consumed alcohol close in time to driving, if you hadn’t eaten, or if other circumstances suggest your absorption phase was incomplete when you were driving—then the rising BAC defense is worth exploring seriously.
This defense requires careful investigation. When exactly did you stop drinking? What did you eat? When were you pulled over? When were you tested? Were there delays in processing? These facts matter tremendously.
We investigate these questions thoroughly. We work with forensic toxicology experts. We challenge the prosecution’s assumptions about absorption and metabolism. We present the rising BAC defense to juries with clarity and force. And we help you defend your freedom.
You Have Options
If you’re facing a DUI charge in Texas, you don’t have to accept the prosecution’s theory of the case. There are defenses. There are ways to challenge the evidence. One of those ways is understanding that your BAC at the moment of testing wasn’t necessarily your BAC while driving. And that difference might mean the difference between conviction and acquittal.
We’ve helped Texans defend themselves against DUI charges. We know the science. We know the law. We know how to fight.
Call Shane Phelps Law at 979-596-6843. If you’re facing DUI charges, you need someone in your corner who understands the science of blood alcohol, who’ll challenge the evidence aggressively, and who’s committed to your defense. Let’s talk about your case and how we can help protect your future.
