I am Joel Brand, and I defend DUI cases across California. One of the most common questions I hear from people who were just arrested is simple: "I had to pick between a breath test and a blood test at the station. Did I choose wrong?" The honest answer is that neither test is automatically better or worse for every person in every case. The right answer depends on the specific facts of your arrest, your body, the equipment involved, and what a skilled defense attorney can do with the results. This post walks you through what each test actually produces, where each one can go wrong, and what your choice means right now.
Why You Were Asked to Choose in the First Place
After a DUI arrest in California, the law gives you the right to choose between a breath test and a blood test at the station or jail. This is the evidentiary chemical test, which is different from the preliminary alcohol screening device an officer may have used on the roadside. Under California Vehicle Code section 23612, you have the right to select your test. If you refused both options entirely, that refusal carries its own serious consequences. Assuming you submitted to one of them, the test result is now the centerpiece of the prosecution's case against you.
What a Breath Test Actually Measures
The breath machine at the station does not measure the alcohol in your blood directly. It measures alcohol in your deep lung air and then applies a mathematical conversion factor to estimate what your blood alcohol concentration might be. That conversion assumes a standard partition ratio of 2100 to 1, meaning 2100 milliliters of air contains the same alcohol as 1 milliliter of blood. The problem is that ratio varies significantly from person to person. If your actual ratio is lower than the assumed standard, the machine will report a higher BAC than you actually had. This is one of the foundations of the mouth alcohol defense and challenges based on the machine's calibration history.
Where a Breath Test Can Break Down as Evidence
Breath machines must be maintained, calibrated, and operated correctly. If the device was not serviced on schedule, if the officer did not observe you for the required continuous observation period before the test, or if you have a medical condition like GERD, acid reflux, or diabetes, the reading can be artificially elevated. Residual mouth alcohol from a recent burp, belch, or vomit can also contaminate the sample. Breath results are not preserved after the test. Once you blow into that machine, the actual air sample is gone. There is nothing left to retest. That matters a great deal to your defense.
What a Blood Test Actually Measures
A blood draw gives the lab a physical sample of your actual blood. A technician then analyzes it, usually using gas chromatography, to determine the concentration of alcohol present. Because the sample is preserved, it can be retested. Your attorney can request that the retained portion of your blood sample be sent to an independent laboratory for analysis. That independent test sometimes produces a lower number than the government's result, and it can expose problems with how the sample was stored, handled, or analyzed.
Where a Blood Test Can Break Down as Evidence
Blood evidence is only as reliable as the chain of custody that surrounds it. If the sample was not refrigerated properly, if the tube contained too little or degraded preservative, or if the sample fermented while sitting in storage, the measured BAC can be higher than your true BAC at the time of driving. Contamination, mislabeling, and lab error are all real possibilities. The rising BAC defense can also apply to blood results. If you were stopped shortly after your last drink, your BAC may have still been climbing when police pulled you over and was lower while you were actually driving.
The Timing Question That Applies to Both Tests
Both breath and blood results reflect your BAC at the moment of testing, not necessarily the moment you were driving. There is always a gap between when you were stopped and when the test was administered. Alcohol absorbs into the bloodstream at different rates depending on what you ate, your body weight, your metabolism, and other factors. If your BAC was still rising during that window, the number on the test result may overstate what was in your system while you were behind the wheel. This is a legitimate scientific argument, and it applies whether you chose breath or blood.
Does Your Choice Affect the DMV Side of Your Case?
Yes, in a meaningful way. The DMV administrative process runs separately from your criminal court case. At your DMV hearing, the test result is one of the key pieces of evidence the DMV uses to justify suspending your license. If the result can be challenged on technical grounds, that challenge can be raised at the hearing as well. A breath result, because no physical sample survives, is harder to independently verify. A blood result, because it can be retested, sometimes gives the defense more to work with at both the court and DMV levels.
What If You Refused Entirely?
If you declined both tests, that is a separate and more serious situation. California's implied consent law under VC 23612 means that a refusal triggers an automatic longer suspension and can be used against you in court. The refusal sentencing enhancement under VC 23577 can add mandatory jail time if you are convicted. If you were taken to a hospital and a blood draw was done there, different legal rules may apply to how that evidence was obtained and whether it can be challenged under post-arrest refusal law.
What a Defense Attorney Does With Your Test Result
Regardless of which test you took, the result is not the end of the conversation. A defense attorney reviews the DMV discovery packet, the arresting officer's report, the machine's calibration records or the lab's chain-of-custody documentation, and the timeline of your arrest. In some cases the result can be suppressed entirely if the stop or arrest was unlawful under a 1538.5 motion to suppress. In other cases the result is admissible but its accuracy is genuinely disputed. Either path can affect whether the charge is reduced, dismissed, or resolved differently than you fear right now.
The Honest Bottom Line
If you chose breath, you are not automatically in worse shape. If you chose blood, you did not automatically hand the prosecutor an easy win. Both tests have real scientific limitations, procedural requirements, and documented failure points. What matters most right now is not which test you took but how quickly you move to protect your rights. The ten-day window to request a DMV hearing is running, evidence preservation deadlines are approaching, and the sooner an attorney reviews your specific results and arrest circumstances, the more options you have. You can read more about the top DUI defenses and how they apply to test results on this site.
You can get a free written case analysis right here on this page. Call me directly at (888) 271-6644. I answer my own phone, 24/7. You can also read more from the DUI blog for practical information on every stage of a California DUI case.