A DUI blood test feels airtight — but it is only as good as how the sample was drawn, stored, and handled.
There are real ways to challenge it, and you have the right to have the sample independently retested.
Chain of Custody
Every hand the sample passes through must be documented. A gap in the chain of custody — a mislabeled vial, a missing log entry, an unaccounted-for period — raises doubt about whether the blood tested was really yours and really unaltered.
Fermentation and Contamination
Blood samples must contain the right preservative and anticoagulant and be stored properly. Without them, a sample can ferment — producing alcohol after the draw and inflating the result. Improper cleaning of the draw site (with an alcohol swab) and other handling errors can also contaminate the sample.
Your Right to Retest
California law preserves a portion of your blood sample, and you have the right to have it independently retested by your own expert. A retest that comes back lower — or reveals fermentation or degradation — can undo the prosecution’s number and open the door to a dismissal or a wet reckless.
Frequently Asked Questions
Can a DUI blood test be challenged?
Yes — and it should be, every time. A blood number isn't gospel; it's only as good as the person who drew it, the container it sat in, and the log that tracked it. The real fight happens on three fronts: gaps in the chain of custody, fermentation or contamination that inflates the result, and your right to have the preserved sample independently retested.
What is blood fermentation in a DUI case?
Fermentation means the sample kept generating alcohol after it left your arm — not before. Skip the required preservative and anticoagulant, or store the vial improperly, and the sample can ferment on its own, inflating the number on the lab report above what your actual BAC was at the time of the draw.
Can I get my DUI blood sample retested?
Yes — California is required to preserve part of the sample specifically so you can do this. Send it to your own expert for independent testing. If the number comes back lower, or shows signs of fermentation or degradation, that undercuts the prosecution's result and can open the door to a better outcome.
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Ahmed S. Hasan
San Francisco Criminal Defense Attorney · State Bar of California #364992
Ahmed S. Hasan is the founder of ASH Legal, a solo criminal defense practice based in San Francisco. The firm runs on flat-fee representation: one lawyer, one fee, from arraignment through resolution. No hourly billing. No surprise invoices.
Ahmed is a graduate of Emory University School of Law and a member in good standing of the State Bar of California (Bar #364992). His practice focuses on San Francisco misdemeanor defense — DUI, domestic violence, petty theft and shoplifting, drug possession, vandalism, trespassing, and assault and battery — with particular attention to diversion-track outcomes that end in dismissal under PC 1001.95, PC 1001.36 (Mental Health Diversion), PC 1001.80 (Military Diversion), and PC 1001.83 (Parental Caregiver Diversion).
Ahmed previously served as a post-bar clerk with the San Francisco Public Defender's Office, the city's largest indigent-defense practice. He is a member of the Asian Pacific American Bar Association of Silicon Valley.
He represents clients at the San Francisco Hall of Justice (850 Bryant Street) and the Civic Center Courthouse. The ASH Legal office is at 15 Boardman Place, Suite 301, San Francisco, CA 94103.
Free 30-minute consultations are available by phone or Zoom. (510) 545-6515 · ahmed@ashlegal.com