Written by Jarrett Ambeau, trial attorney, court-qualified expert in forensic DNA interpretation, and holder of a Master of Science in forensic DNA and serology, at The Ambeau Law Firm.
A number on a machine is only as good as the machine.
The state treats a blood-alcohol or drug result like a verdict. A number appears, and the case is supposed to be over. But that number comes from a machine, run by a person, using a process with real room for error. Each step is a place to fight.
Let’s say Jill blows a 0.09 at the station. The legal limit is 0.08. That razor-thin result carries a felony-level punishment on a repeat charge. It also depends on calibration, timing, and technique that almost no one checks. We check.
Chemical evidence is intimidating because it wears a lab coat. But a result is a measurement, and every measurement carries uncertainty. The defense job is to expose the uncertainty the state would rather ignore.
What is forensic toxicology?
Forensic toxicology is the testing of blood, breath, or urine to measure alcohol or drugs. Breath testing estimates alcohol from a breath sample. Blood testing uses lab instruments to measure concentration directly.
Both methods rely on assumptions. A breath machine assumes a standard ratio between breath and blood alcohol, and that ratio varies from person to person. Blood testing assumes proper collection, preservation, and instrument calibration. Break any assumption and the number stops meaning what the state says it means.
The instruments are not magic. A breath machine infers alcohol from an infrared or fuel-cell reading. A blood lab uses gas chromatography, a process that must be calibrated, maintained, and run by a trained analyst. Every one of those steps can introduce error, and every one leaves a paper trail we can demand.
The science: where the number comes from and where it fails
Start with breath. The machine does not measure blood. It measures breath and converts, using an assumed ratio of 2,100 to 1. That ratio is an average. A real person can run higher or lower, which means the “blood alcohol” number can be wrong before anyone touches the machine.
Breath testing has other traps. Mouth alcohol from a recent burp, acid reflux, or dental work can spike a reading far above the true level. That is why the law requires an observation period, and why a skipped or sloppy observation is a real defense.
Now blood. A vial needs the right preservative and anticoagulant. Without enough preservative, bacteria can ferment the sample and create alcohol after the draw. Warm storage speeds that up. So a sample that read under the limit at the roadside can read over the limit by the time the lab runs it, purely from bad handling.
And there is timing. Alcohol absorbs over time. If a person was still absorbing when tested, the level at the station can be higher than it was behind the wheel. The legal question is the level while driving, not the level an hour later at the jail. Rising-alcohol analysis can move the whole case.
Where toxicology results go wrong
Breath machines need regular calibration and maintenance. Skip a log, use an expired solution, or test a person who just burped, and the reading climbs. Mouth alcohol alone can spike a result well past the true level.
Blood tests have their own failure points. A vial without enough preservative can ferment. A warm storage room can do the same. A chain-of-custody gap raises the question of whether the sample tested was even the right one. Each failure is a thread we pull.
The law: confrontation, foundation, and admissibility
A lab result does not walk into court on its own. Under Melendez-Diaz v. Massachusetts, 557 U.S. 305 (2009), a forensic lab report is testimonial, and the defendant has a Sixth Amendment right to confront the analyst who produced it. The state cannot just hand the jury a certificate.
The Supreme Court tightened that further in Bullcoming v. New Mexico, 564 U.S. 647 (2011). A substitute analyst who did not run the test generally cannot stand in for the one who did. That means we get to cross-examine the person who actually handled the sample, and that cross-examination is where errors surface.
Reliability also matters. Expert testimony about the result is screened under Code of Evidence Article 702 and the standard from State v. Foret, 628 So.2d 1116 (La. 1993), which adopted Daubert v. Merrell Dow, 509 U.S. 579 (1993). If the state cannot show the machine was calibrated and the method properly applied, the foundation fails.
How we challenge chemical evidence: law and science together
We demand the records the state would rather skip. Calibration logs, maintenance history, the analyst’s training, the observation-period documentation, and the full lab file. Gaps in those records create real doubt about the result.
On the law, we invoke the confrontation right under Melendez-Diaz and Bullcoming to insist that the actual analyst testify. We challenge the foundation when the state cannot prove calibration and proper method. And we litigate any unlawful stop or blood draw as a suppression issue.
On the science, we retain independent toxicologists to review the data and explain rising alcohol, fermentation, mouth alcohol, and machine error to the jury. When the state cannot prove the number was reliable, the number cannot carry the case.
At trial, we translate the science into plain stakes. A single missed maintenance log, a single warm storage room, a single rushed observation period, any one of them can move a 0.09 below the limit. That is the difference between a conviction and a dismissal.
Which cases turn on toxicology?
Chemical testing drives DWI and drugged-driving cases, and the stakes climb fast. A repeat DWI, or one involving injury or death, becomes a serious felony with prison exposure. In vehicular homicide and negligent injury cases, the blood result is often the entire prosecution.
Drug testing raises its own problems. Distinguishing impairment from mere presence of a substance is not simple, and the science is far less settled than the state suggests. In a serious drug case, that gap can be decisive.
Frequently asked questions
The breath test says I was over the limit. Is that the end?
No. Breath results depend on calibration, technique, and personal physiology. Each of those can be challenged, and each can move the number.
Can a blood test be wrong?
Yes. Bad preservation, poor storage, or fermentation can raise a blood-alcohol reading after the draw. Proper handling has to be proven, not assumed.
Does the actual lab analyst have to testify?
Generally, yes. Under Melendez-Diaz and Bullcoming, you have a right to confront the analyst who ran the test, not just a stand-in.
Do you use outside experts?
Yes. We work with independent toxicologists who review the lab file and testify to the real sources of error.
Authoritative resources
The National Institute of Standards and Technology publishes work on the reliability of forensic testing methods, including the measurement issues that affect toxicology.
Related forensic evidence challenges
Forensic evidence rarely stands alone. We challenge the full range of scientific proof the state uses, including bloodstain pattern analysis, digital forensics and phone extractions, and fingerprint and latent print analysis. For the full picture, see our overview of what a forensic science lawyer does.
How The Ambeau Law Firm can help
If a chemical result is the case against you, that result can be tested and beaten. We pull the records, enforce your right to confront the analyst, expose the error, and refuse to let an unverified number decide your future. Contact The Ambeau Law Firm to talk about your defense.