The rising blood alcohol defense argues that your BAC was lower while you were driving than when you were tested, because your body was still absorbing alcohol. Blood alcohol keeps climbing after the last drink until it peaks. If the test came during or after that climb, the reading can overstate your level behind the wheel.
The number on your police report was measured at the station, the jail or the hospital. It was not measured at the moment the officer pulled you over. That gap can be 30 minutes or it can be several hours, and it matters. As forensic toxicologist A.W. Jones put it in a 2010 review in Forensic Science International, "A blood sample for forensic analysis might not be taken until several hours after an offence was committed. The courts usually want to know the suspect's blood-alcohol concentration (BAC) at some earlier time, such as the time of driving."
The gap can cut either way. Sometimes your level was higher while driving than at the test. Sometimes it was lower. Which one depends on where your body was in processing alcohol, and that is the fact at the center of this defense.
How alcohol moves through your body
Alcohol follows a curve. After you drink, it is absorbed into your blood and your level rises. It reaches a peak. After the peak, your body clears it at a fairly steady rate and your level falls.
The peak is usually close to the last drink, but not always. In a 2026 study in the Journal of Analytical Toxicology, O'Connor, Wang and Vargas had 42 adults drink self-chosen 80-proof mixed drinks with food available. The time from the final drink to peak breath alcohol ranged from 23 minutes before the last drink to 76 minutes after it, with a mean of 17 minutes and a median of 13. The researchers wrote that "most participants reached peak concentration shortly after drinking ceased, although delayed peaks of more than 1 hr were observed in some individuals," and some showed "occasional multi-peak profiles."
An older study points the same way. Watkins and Adler (1993, Journal of Forensic Sciences) had nine subjects drink 0.69 g/kg of alcohol quickly, once after a large meal and once after a fast of about six hours. The average time to maximum breath alcohol was 41 minutes in both conditions.
The rate of decline varies from person to person. Jones's 2010 review puts the physiological range of alcohol elimination from blood at 10 to 35 mg/100mL per hour, which is 0.010 to 0.035 in BAC terms per hour. On an empty stomach the rate falls around 10 to 15. In people who have eaten it tends to be 15 to 20. In alcoholics during detoxification it might be 25 to 35. The review calls 15 "a good average value" for moderate drinkers, but says 19 "is more appropriate" for apprehended drivers, "since many of these individuals are binge drinkers or alcoholics." It also reports that the decline is slightly steeper in women than in men.
Food matters here too. Watkins and Adler found the average elimination rate was lower after a meal than after fasting (0.017 vs 0.020 BrAC per hour). The O'Connor study found a mean elimination rate of 0.0186 g/210 L per hour, with individuals ranging from 0.0134 to 0.0261.
What retrograde extrapolation is
Retrograde extrapolation is the method experts use to estimate your level at the time of driving from a test taken later. The basic math is simple: take the test result and add the number of hours that passed multiplied by an elimination rate.
The UNC School of Government's North Carolina Criminal Law Blog describes the method as an estimate "calculated from a single reading taken some hours after the incident, assuming an average elimination rate and that the defendant was in the elimination versus the absorption phase."
That last assumption is the whole ballgame. The math only works if you had already peaked before you drove. If you were still absorbing alcohol when you were stopped, your level was climbing between the stop and the test. Your level at the time of driving was then lower than the test result, not higher. Adding hours of elimination on top of it would push the estimate in the wrong direction.
To see how much the chosen rate matters, take a hypothetical test result of 0.07 two hours after a stop. Using an elimination rate of 0.015 per hour, the estimate at the time of driving is 0.10. Using 0.010, it is 0.09. Using 0.035, it is 0.14. Same test, same time gap, very different answers. And none of those numbers is correct if the person was still absorbing alcohol at the time of the stop.
Prosecutors use the same math
Retrograde extrapolation is not only a defense tool. Prosecutors use it too, sometimes to turn a test result under 0.08 into an estimate over the limit.
That is what happened in State v. Babich (North Carolina Court of Appeals, 2017). According to Shea Denning's summary on the UNC School of Government blog, the defendant blew a 0.07 one hour and 45 minutes after the stop. The State's expert estimated her level at the time of the stop at 0.08 to 0.10. To get there, the expert assumed she had already peaked, and the expert conceded that no facts supported that assumption. The Court of Appeals held the testimony inadmissible under the Daubert "fit" test.
Per the blog's summary, the rule from Babich is that a retrograde extrapolation opinion assuming the defendant was in the elimination phase must be based on facts that support that assumption. Denning wrote that the decision "may be big." It is worth knowing the backdrop: earlier North Carolina appellate decisions, including State v. Green (2011), State v. Taylor (2004) and State v. Catoe (1985), had accepted retrograde extrapolation testimony, according to the same blog. Babich did not ban the method. It required the key assumption to rest on evidence.
What courts require: the Mata factors
The leading decision on the reliability of this evidence is Mata v. State, decided by the Texas Court of Criminal Appeals on June 6, 2001 (No. 133-00, opinion by Judge Keasler).
The court stated the core scientific limit plainly: "A single test will not reflect where the person is on his BAC curve. In other words, it will not indicate whether the person is in the absorption phase, at his peak, or in the elimination phase."
Mata told trial courts to weigh three things when deciding whether an extrapolation is reliable:
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The length of time between the offense and the test or tests.
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The number of tests given and the length of time between each test.
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Whether, and to what extent, the expert knew any individual characteristics of the defendant when making the estimate.
The court then listed the personal facts that matter: "the person's weight and gender, the person's typical drinking pattern and tolerance for alcohol, how much the person had to drink on the day or night in question, what the person drank, the duration of the drinking spree, the time of the last drink, and how much and what the person had to eat either before, during, or after the drinking."
The court also said that "the expert's ability to apply the science and explain it with clarity to the court is a paramount consideration." In Mata itself, the court held the State failed to prove by clear and convincing evidence that its expert's extrapolation was reliable. It pointed to inconsistent testimony and to the fact that there was only one test, taken more than two hours after the offense.
How state law can limit the timing argument
Many states have narrowed the timing question by writing a time window directly into the DUI statute. Rules vary by state, and your state's statute decides whether your level at the moment of driving is even what the prosecution has to prove.
Two-hour laws: Washington, Nevada and Arizona
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Washington: Under RCW 46.61.502(1)(a), a person is guilty if they have, "within two hours after driving, an alcohol concentration of 0.08 or higher." Under subsection (4)(a), samples taken more than two hours after driving "may be used as evidence" of a 0.08 or higher level within the two hours.
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Nevada: NRS 484C.110(1)(c) covers a person "found by measurement within 2 hours after driving" to have 0.08 or more.
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Arizona: ARS 28-1381(A)(2) makes it unlawful to have "an alcohol concentration of 0.08 or more within two hours of driving" where the concentration "results from alcohol consumed either before or while driving."
Under these provisions, the question is your level within the two-hour window, not your exact level at the moment you were driving. That changes what the prosecution must prove, and it shrinks the room for an argument that you were lower at the wheel and higher at the test.
Drinking after driving: Washington and Nevada
Both states give an affirmative defense for alcohol consumed after driving. In Washington, RCW 46.61.502(3) makes it a defense that the defendant drank enough alcohol after driving and before the test to bring their level to 0.08 or more within two hours after driving. In Nevada, NRS 484C.110(5) makes post-driving drinking an affirmative defense. In both states, the defendant carries the burden of proving it by a preponderance of the evidence.
California's three-hour presumption
California takes a different approach. Under Vehicle Code 23152(b), "it is a rebuttable presumption that the person had 0.08 percent or more... at the time of driving the vehicle if the person had 0.08 percent or more... at the time of the performance of a chemical test within three hours after the driving." The word "rebuttable" matters. The presumption can be challenged, and evidence about absorption and timing is one way to do that.
The science on uncertainty
The research supports a narrow but important point: a single number from a single test hides a lot of uncertainty when it is projected backward.
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Elimination rates span 10 to 35 mg/100mL per hour (Jones, 2010). The fastest rate in that range is more than three times the slowest.
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Peak timing varies. Most people peak soon after the last drink, but some peak more than an hour later, and some show more than one peak (O'Connor et al., 2026).
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Jones's review notes that back calculations in criminal cases have "many proponents and critics."
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Wu and Benowitz (2025, Laboratory Medicine) note that the Academy Standards Board recommends reporting a range of BAC calculations rather than one figure. In their case study, the retrograde calculation produced a range of 2.6 to 3.1 g/L. They concluded: "A prolonged duration between drinking and testing produces a range of BAC results that can cause ambiguities in legal proceedings."
If an expert on either side offers a single precise number for your level at the time of driving, that is a point your lawyer may want to examine closely.
What to gather for your lawyer now
Mata's list of individual facts is, in practice, a checklist. An expert cannot place you on the alcohol curve without these details, and memories fade fast. Write down what you can while it is fresh:
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When you started drinking and when you had your last drink.
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What you drank and how much of each.
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What you ate, how much, and when (before, during or after drinking).
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The time of the traffic stop.
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The time of each breath or blood test, and how many tests were taken.
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Your weight, and your usual drinking pattern.
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Any receipts, bar tabs, texts or card statements that show times.
Do not guess to fill gaps. Give your lawyer what you actually know. The strength of any timing argument, for you or against you, depends on facts in the record.
The bottom line
A test result is a measurement at one moment, and that moment was not when you were driving. Whether that helps you depends on the facts: when you stopped drinking, what you ate, how long the gap was, and how many tests were taken. Courts following Mata and Babich expect estimates to rest on those facts, not on assumptions. A defense toxicologist may be needed to evaluate or challenge the numbers. And your state's statute may decide whether your level at the time of driving is even the question. If you are facing a DUI charge, raise the timing of your test with your attorney early.
Related reading
Sources
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Retrograde Extrapolation Alive and Well, North Carolina Criminal Law Blog, UNC School of Government
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Wu and Benowitz, Theoretical and retrograde calculation of blood alcohol, Laboratory Medicine (2025)
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California Vehicle Code 23152, California Legislative Information
Note: This article contains AI-assisted content and has been reviewed by our editorial team.
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