San Diego DUI Defense Attorney Anna R. Yum Explains How Blood Alcohol Tests Work in California DUI Charges

SAN DIEGO, CA - Blood alcohol testing plays a central role in Driving Under the Influence prosecutions in California, yet the procedures governing how samples are collected, stored, and analyzed must meet strict regulatory standards before results can be used as evidence. San Diego DUI defense attorney Anna R. Yum of the Law Offices of Anna R. Yum (https://www.annayumlaw.com/blood-alcohol-test-san-diego-dui-charge/) is providing guidance on how blood alcohol tests work in DUI cases and when test results may be challenged.

According to San Diego DUI defense attorney Anna R. Yum, a blood alcohol test measures the concentration of alcohol in a person's bloodstream and is used by prosecutors to establish a violation of California Vehicle Code Section 23152. Under California law, a blood alcohol content of 0.08% or higher creates a legal presumption of impairment for standard drivers, while commercial drivers face a threshold of 0.04% and drivers under 21 are held to a 0.01% limit. "Understanding the science behind these tests is essential to building an effective defense," explains Yum. "Many individuals are unaware that the accuracy of blood test results depends entirely on whether proper procedures were followed at every stage."

San Diego DUI defense attorney Anna R. Yum emphasizes that Title 17 of the California Code of Regulations establishes mandatory requirements for every phase of blood alcohol testing. These regulations require that a licensed medical professional draw the sample, that the skin be cleaned with a non-alcohol solution to prevent contamination, and that blood be collected in a vial containing both a preservative and an anticoagulant. Any failure to comply with Title 17 procedures can provide grounds for challenging the admissibility of the results.

Attorney Yum notes that chain of custody documentation is another critical element in blood test cases. Every transfer of the blood sample must be recorded, from the moment it is drawn through its arrival at the crime lab and during storage. A break in the chain of custody can raise questions about whether the sample was compromised, mislabeled, or improperly handled. "If the prosecution cannot demonstrate an unbroken chain of custody, the reliability of the entire test comes into question," she adds.

The Law Offices of Anna R. Yum also highlights the importance of the rising blood alcohol defense in DUI cases. Because blood alcohol content can continue to rise for 30 to 90 minutes after a person stops drinking, a test administered well after a traffic stop may reflect a higher concentration than what was present at the time of driving. Retrograde extrapolation, a mathematical method prosecutors sometimes use to estimate earlier levels, is considered unreliable because alcohol metabolism varies significantly based on individual factors such as weight, food consumption, and liver function.

Individuals charged with DUI in San Diego also retain the right to request independent analysis of their blood sample through a blood split motion filed with the San Diego Superior Court. Under Title 17, a portion of the sample must be preserved for at least 12 months, allowing an independent laboratory to conduct its own analysis. Independent testing may reveal that the actual concentration was below the legal limit, that the sample experienced fermentation due to improper storage, or that contamination affected the results. "Independent retesting is one of the most valuable tools available to someone facing DUI charges," Yum observes. "It provides an objective check on the prosecution's evidence."

Yum further explains that the U.S. Supreme Court's ruling in Birchfield v. North Dakota established that warrantless blood draws violate the Fourth Amendment's protection against unreasonable searches. While breath tests may be administered without a warrant under implied consent laws, blood tests require judicial approval due to their more invasive nature. If law enforcement obtained a blood sample without a warrant and without consent, a defense attorney can file a motion to suppress the evidence.

For those facing DUI charges based on blood alcohol test results in San Diego County, consulting with an experienced defense attorney may help identify procedural violations, preserve the right to independent testing, and protect driving privileges.

About Law Offices of Anna R. Yum:

Law Offices of Anna R. Yum is a San Diego-based law firm dedicated to criminal defense with a focus on DUI and drunk driving cases. Led by attorney Anna R. Yum, a former Riverside County Deputy District Attorney and nationally recognized trial lawyer, the firm represents clients throughout San Diego County. For consultations, call (619) 233-4433.

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Email: ayum@annayumlaw.com

Website: https://www.annayumlaw.com/

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Company Name: Law Offices of Anna R. Yum
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Phone: (619) 233-4433
Address:1230 Columbia St #1140
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