How Blood Alcohol Testing Actually Works (And Where It Can Go Wrong)

When a prosecutor tells the jury that your client’s blood alcohol concentration was 0.11%, they make it sound simple. Scientific. Irrefutable.

But after 17 years running the exact tests that crime laboratories use to generate those numbers, I can tell you: blood alcohol testing is anything but simple. And understanding where it can go wrong is often the difference between a conviction and an acquittal.

Two Methods, Two Sets of Problems

Law enforcement uses two primary methods to measure blood alcohol: breath testing at the roadside or station, and blood draws analyzed in a crime laboratory. Each has its own vulnerabilities.

But that’s just the beginning. Breath test results can be affected by:

• Mouth alcohol from recent drinking, mouthwash, or GERD (acid reflux) that brings stomach contents back up

• Breathing patterns during the test—hyperventilating lowers readings, holding your breath raises them

• Instrument calibration errors if maintenance schedules weren’t followed

• Environmental factors like temperature and humidity

• Interfering substances in the breath (some industrial solvents, diabetes-related acetone)

I’ve seen cases where variance between two breath samples was so large it should have invalidated both results—but the officer didn’t know to question it, and the prosecution presented the higher number as fact.

Cheryl Kim Straub-Lopez  |  TOXPERT  |  Forensic Toxicology Expert Witness

Blood Testing: More Reliable, But Not Bulletproof

Blood testing is generally more accurate than breath testing, but it comes with its own potential problems:

• Sample contamination from improper blood draw technique or alcohol swabs (though this is rarer than defense attorneys sometimes claim)

• Fermentation in improperly preserved or stored samples, which can actually create alcohol that wasn’t there

• Chain of custody breaks where samples are unaccounted for

• Laboratory errors in analysis, calibration, or data interpretation

The fermentation issue is particularly important. If blood samples aren’t properly preserved with sodium fluoride and kept refrigerated, microorganisms can actually produce alcohol in the tube. I’ve reviewed cases where BAC results were almost certainly elevated by this phenomenon—but without an expert explaining it, juries had no idea.

The Time Factor: What Your BAC Actually Was While Driving

Here’s something most people don’t realize: the BAC result on the lab report isn’t necessarily what your client’s BAC was when they were actually driving.

If the blood draw happened two hours after the traffic stop, the result reflects what was in their blood at that moment—not earlier. Prosecutors use “retrograde extrapolation” to calculate backward, but this calculation requires assumptions about absorption rate, elimination rate, food consumption, and timing of last drink that are rarely known with certainty.

The “rising BAC” defense is based on real science: if someone consumed alcohol shortly before driving, their BAC could have been lower while driving and higher when tested. I’ve worked cases where the math clearly supported this scenario—but the prosecution presented their extrapolation as if it were established fact.

I spent years writing the quality assurance protocols that define these uncertainty thresholds. Understanding measurement uncertainty—and explaining it to juries—is one of the most powerful tools in a defense attorney’s arsenal.

When You Need an Expert

Not every DUI case needs a toxicology expert. But when the evidence is close to the legal limit, when the timeline suggests rising BAC, when there are questions about the testing procedure, or when you need someone who can explain the science to a jury in terms they’ll understand—that’s when an independent expert makes the difference.

I spent 17 years inside state crime laboratories producing the evidence that prosecutors rely on. Now I help defense attorneys understand, challenge, and explain that evidence. If you have a case where forensic toxicology matters, I’m happy to discuss it.