A blood test result that places a driver's BAC above the legal limit can feel like the last word in a DUI case. Many defendants assume there is nothing left to contest once a blood draw has been taken and a number comes back from the lab. That assumption is not accurate, and acting on it means walking away from defenses that are real, available, and in some cases quite strong.
Blood tests in California DUI cases are subject to specific collection protocols, laboratory procedures, and documentation requirements. When any part of that process is not handled correctly, the result can be affected in ways that matter directly to the defense, which is why a criminal defense law firm requests the full testing file rather than taking the number at face value.
Why Blood Tests Are Used in California DUI Cases
Under California's implied consent law, a driver who is lawfully arrested for DUI must submit to a chemical test, and how implied consent law impacts DUI cases reaches well past the roadside. The driver can usually choose between a breath test and a blood draw. In certain situations, an officer may specifically require a blood test: when drug impairment is suspected rather than alcohol, when the driver is unconscious and cannot complete a breath test, or when the driver is being treated at a hospital after an accident.
Blood tests carry a reputation for precision because they measure alcohol content directly from the bloodstream rather than estimating it from a breath sample. They can also be independently retested after the fact, which a breath sample cannot. Jurors tend to take blood test results at face value for that reason. It is also why we examine every step of how a sample was collected, transferred, stored, and analyzed in any DUI case involving a blood draw.
Where Errors in Blood Testing Can Occur
The accuracy of a blood test result depends on a series of procedural steps that are entirely within human control. One documented source of error is using an alcohol-based swab to clean the skin before inserting the needle. Another is the type of preservative and anticoagulant placed in the collection vial. When the wrong type is used, or when the correct concentrations are not present, the blood sample can ferment after it has been drawn. Fermentation produces alcohol that was not in the defendant's bloodstream at the time of the arrest, which means the resulting BAC reading can be higher than what was actually present.
Storage is another area where errors occur. Blood samples must be refrigerated within a defined time window after collection and kept at the correct temperature until the lab processes them. If that protocol is not followed, the sample can degrade or ferment during storage. The only way to establish whether storage was handled correctly is through the documentation, and when that documentation is incomplete or inconsistent, the reliability of the result is genuinely open to question.
Chain of Custody and What It Means for the Defense
Chain of custody is the documented record of every person who handled the blood sample, when they handled it, and under what conditions. Every transfer of the sample from the collecting officer to the evidence technician to the lab is supposed to be logged and signed off. When that documentation has gaps, when forms are missing signatures, or when the sample cannot be fully accounted for during a specific period, it raises legitimate questions about whether the sample that was tested is the same one drawn from the defendant.
A broken chain of custody does not automatically exclude the blood test result from evidence, but it creates a meaningful challenge to the prosecution's ability to demonstrate that the sample was properly handled and correctly identified throughout the entire process. Requesting chain of custody records is a standard part of how we review any DUI case involving a blood draw. When those records show problems, it opens a concrete line of attack.
What the Lab Process Can Reveal
Crime lab analysis of DUI blood samples follows defined procedures, but labs are run by people using equipment that requires regular maintenance and calibration. Analyst errors, instrument malfunctions, sample contamination, and procedural shortcuts can all affect the final BAC reading. Accredited labs are required to maintain calibration records, quality control logs, and documentation of each analyst's training and qualifications. When those records are not current, complete, or consistent, the validity of the analysis is open to challenge.
A DUI defense attorney can request the full testing file from the lab: calibration logs for the equipment used, the qualifications of the analyst who ran the test, quality control data from the relevant testing batch, and the specific methodology applied to the sample.Reviewing that documentation is a standard part of what we do in any case involving a blood draw, because the problems are not always visible from the final result alone.
Legal Grounds for Challenging a Blood Test
There are established legal avenues for challenging blood test evidence in a California DUI case. A Fourth Amendment challenge focuses on whether the blood draw itself was lawful. Courts have ruled that warrantless blood draws violate constitutional protections in certain circumstances, and when a draw was taken without a valid warrant and without an applicable exception, a motion to suppress can exclude the result from evidence entirely, which is one of the clearer answers to how to beat a DUI charge.
California law also gives defendants the right to request an independent blood split test. When a defendant makes that request, a portion of the original sample is preserved and sent to an independent laboratory of the defense's choosing. If that lab's result differs from the prosecution's result, the conflict in the evidence becomes something we can work with at trial. The blood split option is one of the most direct ways to challenge the reliability of the original analysis when the facts of a case support it.
How We Review Blood Test Evidence in West Covina DUI Cases
Reviewing the blood test documentation starts on day one of any DUI case we take on that involves a blood draw. Tammy Higgins spent nearly 18 years in California criminal courts, including 16 years as a public defender across Orange County, Los Angeles County, and San Bernardino County, where she handled cases involving forensic and scientific evidence across every charge type. That background matters directly when evaluating what the lab records show and where a challenge is actually worth pursuing.
We request the full documentation package in every blood test case: chain of custody records, lab calibration logs, analyst credentials, collection records, and quality control data. When the records reveal problems, we bring in forensic experts where the evidence warrants it. We represent clients facing DUI charges across West Covina, the San Gabriel Valley, and Los Angeles County, and we approach the blood test evidence in every case with the same level of scrutiny.
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