You left a restaurant in Lombard, finished a drink shortly before you got in the car, and were stopped ten minutes later. You blew at the station roughly an hour after that, and the number came back at 0.09. The reading is real. The question is whether it describes the person who was driving or the person sitting in a processing room sixty minutes later.
Alcohol does not appear in your blood the moment you swallow it. It has to move through the stomach and small intestine before it reaches the bloodstream, and while that is happening your concentration climbs. Someone who stops drinking shortly before driving can be under the limit behind the wheel and over it by the time a machine measures anything. That gap is what the rising-BAC defense is built on.
What to know
- Illinois law asks what your alcohol concentration was at the time alleged, not what the instrument read later at the station
- Courts treat this as expert territory, and an argument that rests on cross-examination alone rarely survives the objection
- The same foundation rule that limits your expert also limits the State’s, which is the half of this issue most often left on the table
What the Argument Actually Claims
Under 625 ILCS 5/11-501.2, chemical test evidence is admissible to show the concentration of alcohol in a person’s blood or breath at the time alleged. That phrase is doing a great deal of work. The time alleged is when you were driving. The test happens somewhere downstream of that, after the stop, the roadside investigation, the arrest, the drive to the station, and a mandatory observation period.
A rising-BAC argument does not accuse the machine of malfunctioning. It accepts the number and disputes what the number describes. If you were still absorbing alcohol during that interval, the station reading is the peak of a curve you had not yet reached while you were driving.
Two Charges, One Arrest
Illinois can charge driving with a concentration of 0.08 or more and driving under the influence as separate offenses under 625 ILCS 5/11-501. The State does not have to prove impairment to convict on the number alone. A rising-BAC argument aims at the number, so it leaves the impairment count standing. Both have to be answered.
The Timeline Is the Case
Three moments decide whether this argument has anything to work with: when you stopped drinking, when you were driving, and when the sample was taken. The relationship between them matters more than the reading itself.
The argument is strongest when the last drink was close to the moment you got in the car and the test came a long while after. It weakens as that first gap widens. Once alcohol has been fully absorbed, concentration is falling rather than climbing, and a later test understates rather than overstates what you were carrying while driving.
Illinois courts have put a rough boundary on this. In People v. Carlson, 2023 IL App (2d) 210782, the Second District reasoned that a driver who had nothing to drink for roughly three hours and forty minutes before the stop could reasonably be inferred to have finished absorbing, which placed him in the elimination phase rather than a rising one. That is the honest version of this defense. A long evening of steady drinking that ended before you left is the wrong fact pattern. A last round finished on the way out the door is the right one.
Write the Timeline Down Tonight
What you drank, how much, and when you finished are the facts this argument lives on, and they fade within days. Receipts, card statements, texts, and rideshare records fix them in place. Bring all of it to your first meeting rather than trying to reconstruct it later from memory.
Illinois Treats This as Expert Testimony
Working backward from a later reading to an earlier concentration is called retrograde extrapolation, and Illinois courts have consistently held it to be scientific evidence rather than something a witness can reason through on the stand. That holds even when the goal is modest. Saying a concentration was rising is, in substance, saying the person was in the absorption phase, and that conclusion needs the same footing as a specific number.
A DuPage County case shows how this goes wrong. In People v. Garcia, 2017 IL App (2d) 140903-U, arising from a stop in Carol Stream, defense counsel had two readings to work with: a roadside portable test of 0.107 and a station result of 0.12. Counsel asked the arresting officer whether the concentration had been rising. The State objected, and the objection was sustained. The officer was licensed to run a breath instrument, but he could not say whether food in the stomach affects absorption, which is exactly the kind of gap that leaves an opinion without support. The conviction was affirmed. That order was filed under Supreme Court Rule 23 and cannot be cited as precedent, but it is a fair picture of how the objection lands in the Eighteenth Judicial Circuit.
The factors an expert is expected to work through are drawn from People v. Floyd, 2014 IL App (2d) 120507. They fall into three groups.
- About you. Gender, weight, height, age, and mental state at the time.
- About the drinking. Type and amount of alcohol, type and amount of food in the stomach, the pattern of drinking, the time between the first and last drink, and the time between the last drink and the sample.
- About the testing. How many samples were taken, how much time separated them, how long after the offense the first one came, and the applicable absorption and elimination rates.
An opinion assembled without most of these is vulnerable, which is why the credible version of this defense usually starts with an evaluation of whether the facts support an opinion at all.
The Same Rule Runs Against the Prosecution
This is the part that gets overlooked. Floyd was not a defense loss. It was a case where the State reached for extrapolation and lost the ability to use it.
The defendant’s breath test came back below the legal limit. The State called a forensic toxicologist to extrapolate backward and argue the concentration had been at or above 0.08 while she was driving. The expert assumed she was in the elimination phase without accounting for the factors that determine whether anyone is in that phase, and the Second District held that admitting the opinion was error. A calculation built on a single breath test and a set of assumptions is not reliable simply because a credentialed witness performed it.
The practical consequence is that any case where the State intends to extrapolate invites a motion to bar the testimony. That motion does not depend on your own expert saying anything. It depends on what the State’s expert did not know.
A Low Reading Is Not an Acquittal
A breath result under 0.08 does not end a DUI case, because the State can still proceed on impairment. What it does is force the State to either prove impairment through the officer’s observations and the field sobriety tests or reach for an extrapolation that has to survive the same scrutiny your own expert would face.
Where the Second Reading Comes From
One measurement establishes almost nothing about direction. Two measurements taken a known interval apart, with the first reasonably close to the time of driving, are what makes a reliable estimate possible. Finding that second data point is often the difference between an argument and a theory.
If You Blew at the Roadside
Portable breath tests are governed by 625 ILCS 5/11-501.5, which allows the State to use the result only to help decide whether to request a chemical test. The defendant, by contrast, may introduce the result as evidence. If your roadside number was lower than your station number, that comparison belongs to you.
If You Only Blew Once
The remaining sources are timestamps and records. The breath ticket, the squad video, the booking log, and any hospital blood draw all carry times. Where a hospital tested serum rather than whole blood, the conversion matters too. Discovery is where these are found, and they have to be requested.
What This Does Not Do for Your License
Your arrest started a statutory summary suspension that runs on its own schedule, separate from the criminal charge. People assume that an argument strong enough to create doubt about the criminal count will also save the license. It generally will not.
Under 625 ILCS 5/2-118.1, a rescission hearing is limited to four questions: whether you were properly arrested for an offense under Section 11-501, whether the officer had reasonable grounds to believe you were driving under the influence, whether you were warned of the consequences, and whether you refused the test or submitted to one that disclosed a concentration of 0.08 or more. The Illinois Supreme Court set out those categories in People v. McClure, 218 Ill. 2d 375 (2006).
Read the fourth one closely. The question is what the test disclosed, not what your concentration was while you were driving. A rising-BAC argument answers a question the rescission hearing does not ask. That does not make the hearing pointless. It puts the arresting officer on the stand under oath months before trial, and testimony about the timeline given there is testimony that cannot be quietly revised later.
The License Clock Is Already Running
The deadline to petition for rescission runs from the date you were served with notice, not from your first court date, and the suspension takes effect before that window closes. The details of both clocks are set out in our guide to beating a DUI in DuPage County. Confirm your dates before you do anything else.
When It Is Worth Raising
Illinois has no statutory outer limit on how long after driving a test may be given. Delay goes to the weight a court gives the result rather than whether it comes in at all, a rule the appellate courts have applied since People v. Zator, 209 Ill. App. 3d 322 (1991). So the presence of a gap is never the whole argument. What fills the gap is.
| Facts that support the argument | Facts that undercut it |
|---|---|
| Last drink finished shortly before driving | Drinking ended a few hours before the stop |
| A long interval between the stop and the test | A test taken promptly after the stop |
| Two readings with the earlier one lower | A single reading, or an earlier reading that is higher |
| A reading close to 0.08 | A reading well above the limit |
| Documented food, drink, and timing | A timeline that exists only in memory |
Even where the facts line up, this is rarely a standalone path to dismissal. It is more often one component of a broader attack on the State’s proof, and it frequently does its real work in negotiation rather than at trial, where a genuine weakness in the number can support a reduction to reckless driving or a disposition that keeps a conviction off your record. Every case turns on its own facts, and no result can be promised in advance.
Talk Through Your Timeline With a DuPage County DUI Attorney
The window to challenge your license suspension is shorter than most people expect, and the details that support this argument fade quickly.
Frequently Asked Questions
Absorption commonly runs somewhere between thirty minutes and two hours after the last drink, and the range depends on food in the stomach, how quickly the alcohol was consumed, body composition, and what was being drunk. Because the range is wide, the answer in any individual case is a question for an expert rather than a rule of thumb.
No. With no chemical result there is no number to challenge, so the State proceeds on impairment instead. Refusal also carries a longer suspension than failing a test does, and the refusal itself is admissible. That is a different set of problems requiring a different approach.
Not effectively. The statute limits that hearing to four issues, and the one concerning the test asks what the result disclosed rather than what your concentration was while you were driving. The hearing is still worth holding for other reasons, including the sworn testimony it produces.
If the argument is going to be presented as evidence rather than as a suggestion during cross-examination, then in practice yes. Illinois courts require a witness with genuine qualifications in toxicology to offer an opinion about whether a concentration was rising. Whether the facts justify that expense is one of the first things to evaluate.
Not in the same way. THC moves through the body on a different curve than alcohol, and concentration correlates poorly with impairment, so the absorption reasoning above does not transfer. A cannabis DUI is challenged on other grounds.
A reading just over the limit gives the argument more room than a reading well above it, because a smaller correction is needed to place you under 0.08 at the time of driving. It is not decisive by itself. The timeline still has to support it, and the impairment count still has to be answered.
Talk to a DuPage County DUI Attorney
The useful questions right now are narrow. When did you stop drinking, how long was it before the test, and is there a second reading anywhere in the file? Those answers determine whether this argument is worth developing or whether your case turns on something else entirely.
Our DuPage County DUI attorneys handle these cases regularly, including first offense DUI matters where the reading sits close to the limit. Patrick J. Weiland spent nearly a decade as a DuPage County Assistant State’s Attorney and supervised the felony trial division, which means he knows how the State builds and evaluates these files.
Your first consultation is free. Call (630) 261-9098 or schedule your free consultation.