The DUI Case Runs on Experts
The State’s DUI case is rarely just the arresting officer. Behind that officer stands a line of experts: the analyst who tested your blood, the toxicologist who says what the number means, the people who run and inspect the breath machine, the officer trained to call you impaired by a drug, and, in a crash case, the reconstructionist who says how it happened. Each one is a witness. A witness can be wrong, can use a method that does not hold up, and can be challenged. A real DUI defense goes straight at those experts, and that is a large part of what I do.
The Gatekeeper: Florida’s Daubert Standard
Florida judges are gatekeepers for expert testimony. Under section 90.702 of the Florida Evidence Code, which follows the federal Daubert standard, an expert’s opinion comes in only if it rests on sufficient facts or data, is the product of reliable principles and methods, and applies those methods reliably to the facts of your case. The court weighs reliability with factors from Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993): whether the method can be and has been tested, whether it has been peer reviewed, its known or potential error rate and the standards that control it, and whether it is generally accepted. The standard is not limited to lab science. Under Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999), it reaches technical and specialized witnesses too, which is what brings the drug recognition officer and the accident reconstructionist within its reach. I use this two ways: I retain my own experts to challenge the State’s methods, and I cross-examine the State’s experts against their own data.
The Toxicologist and the Blood or Breath Analyst
The number is the heart of most DUI cases, and the toxicologist is who gives it meaning. Often the State wants to relate a test result back to the time of driving, a process called retrograde extrapolation, and it rests on a stack of assumptions: the result and its timing, your age, gender, and weight, when you had your first and last drink, how much and how strong, and what and when you ate. Change any of those and the answer changes, and many honest experts will not offer an opinion at all because the unknowns are too large. Florida does not force the State to relate the result back, and the delay and the missing inputs go to the weight of the number rather than keeping it out, but the defense is entitled to probe the science, as the Florida Supreme Court recognized in Haas v. State, 597 So. 2d 770 (Fla. 1992), and Miller v. State, 597 So. 2d 767 (Fla. 1991). That is the opening for a rising-alcohol defense, where your level was still climbing and was lower behind the wheel than at the station, and the same scrutiny applies to the gas chromatography behind a blood result. The chemistry is covered on the blood testing and breath testing pages.
The Person Who Drew the Blood
A blood case is only as good as the draw behind it. Florida law limits who may take the sample. Under section 316.1933, only a physician, a certified paramedic, a registered or licensed practical nurse, a person a hospital has authorized to draw blood, or a licensed clinical laboratory director, supervisor, technologist, or technician may withdraw blood at an officer’s request. Beyond who drew it, the how matters: a non-alcohol swab at the site, the right tube with anticoagulant and preservative, proper mixing, careful storage, and an unbroken chain of custody. A draw done wrong can raise or shift the number through clotting or fermentation, and that is a place a careful defense pushes hard. There is more on the blood testing page.
The Breath-Test Program Witnesses
A breath result is not one witness, it is a whole program. The Intoxilyzer 8000 has to be approved, inspected, and maintained on a schedule, the agency inspector has to certify it, and the operator has to run your test the right way, including the observation period before the sample. The records behind all of that, the inspection and maintenance logs and the agency’s own data, are discoverable, and gaps in them are where breath cases come apart. The breath testing page goes through this in detail.
The Drug Recognition Expert
In a drug case, the State often calls a drug recognition expert, or DRE, an officer trained to run a twelve-step evaluation and then offer an opinion that you were impaired by a particular category of drug. The steps run through eye examinations, divided-attention exercises, pulse and vital-sign checks, pupil measurements in light and dark, and a look for injection sites, ending in an opinion that is supposed to be confirmed by a toxicology test. The questions a DRE invites are the same ones section 90.702 asks of any expert: how reliable is the protocol, what is its error rate, and does this officer’s training and method support the conclusion the officer drew. When the toxicology does not match the call, or the protocol was cut short, the opinion is exposed. The field sobriety exercises and drugged driving pages carry more.
The Accident Reconstructionist
In a crash, a serious-injury case, or a DUI manslaughter, the State leans on an accident reconstructionist to say how fast, who did what, and what caused the result, often drawing on the vehicle’s event data recorder, the so-called black box, which Florida courts have allowed as a reconstruction tool. See State v. Abbey, 28 So. 3d 208 (Fla. 4th DCA 2010), and Matos v. State, 899 So. 2d 403 (Fla. 4th DCA 2005). The reason this matters so much is causation. In a DUI manslaughter the State has to prove that the driving caused or contributed to the death, as recognized in Eversley v. State, 748 So. 2d 963 (Fla. 1999), and when that proof fails the charge can drop to a simple DUI, as it did in Pennington v. State, 100 So. 3d 193 (Fla. 5th DCA 2012). A reconstruction is a set of opinions, and opinions can be tested. These cases are covered on the serious and felony DUI page.
| Expert | What they claim | Where it is challenged |
|---|---|---|
| Toxicologist | Your alcohol or drug level, and what it was while driving | Retrograde-extrapolation assumptions, the rising-alcohol defense, and lab method |
| Blood analyst and the person who drew the blood | A reliable blood number | Draw qualifications under 316.1933, site prep, the tube and storage, and chain of custody |
| Breath-program witnesses | A valid breath result | Machine approval and inspection, maintenance logs, and the observation period |
| Drug recognition expert | Impairment by a drug category | Protocol reliability and error rate under 90.702, and a toxicology mismatch |
| Accident reconstructionist | Speed, fault, and cause in a crash | Causation proof, the data behind the opinion, and a competing reconstruction |
How I Use Experts
Two things win these cases. The first is bringing in the right defense expert, a toxicologist or a blood or breath specialist, when one will move the needle. The second is cross-examining the State’s expert against the expert’s own data, which is where a lot of cases turn. Reading that data the way the analyst reads it is what makes the cross land, and it is the reason the forensic-science training matters here.
Related: Blood testing in a DUI, Breath testing in a DUI, Field sobriety exercises, Serious and felony DUI, and Drugged driving.
I started out as an Assistant Public Defender in Florida’s Thirteenth Judicial Circuit, in Tampa, and today I am one of six ACS-CHAL Forensic Lawyer-Scientists in Florida, with NHTSA field-sobriety instructor training and forensic chromatography training from the same kind of program that trains the analysts the State puts on the stand. That is what lets me take the State’s expert apart on the expert’s own ground. More on the forensic lawyer-scientist credential.
Common Questions
Can the State's DUI expert be challenged or kept out of evidence?
Yes. Florida judges are gatekeepers under section 90.702, which follows the Daubert standard, and an expert's opinion is admissible only if it rests on sufficient facts, uses reliable methods, and applies them reliably to your case. In the right case, a motion can limit or exclude the testimony.
What is retrograde extrapolation?
It is the attempt to estimate what your alcohol level was while you were driving from a test taken later. It depends on a stack of assumptions about your drinking and your body, and small changes swing the result, which is why it is open to challenge and why it can support a rising-alcohol defense.
Who is allowed to draw blood in a Florida DUI?
Under section 316.1933, only a physician, a certified paramedic, a registered or licensed practical nurse, hospital-authorized blood-draw personnel, or a licensed clinical laboratory director, supervisor, technologist, or technician may draw it at an officer's request. Who drew it, and how, can be challenged.
What is a drug recognition expert?
A DRE is an officer trained to run a twelve-step evaluation and offer an opinion that you were impaired by a category of drug, meant to be confirmed by a toxicology test. The protocol's reliability, its error rate, and a mismatch with the toxicology are all open to attack.
Do I need my own expert?
Sometimes. It depends on the facts and the science in your case. I retain a defense expert when one will move the needle, and in many cases the larger gains come from cross-examining the State's expert against their own data.
What does an accident reconstructionist do in a DUI case?
In a crash or a DUI manslaughter, the reconstructionist offers opinions on speed, fault, and cause, often using the vehicle's event data recorder. Because the State must prove the driving caused the harm, a reconstruction that does not hold up can shrink the case.
This page is general information about Florida law, not legal advice, and it does not create an attorney-client relationship. Whether an expert can be limited or excluded turns on the facts and the science in your case, and the law can change, so confirm how it applies with counsel. Every case is different, and past results do not guarantee a similar outcome.
