When most people think about DUI evidence, they picture a roadside breathalyzer test. But in many DUI cases in Fullerton and throughout California, blood test results play an equally important — and often more significant — role in the prosecution's case. Blood tests are generally considered more accurate than breath tests, and prosecutors lean on them heavily to establish that a driver's blood alcohol concentration exceeded the legal limit. However, blood test results are far from infallible, and an experienced defense attorney knows exactly where the science and the procedures behind them can break down.
When Blood Tests Are Used in DUI Cases
Law enforcement may request a blood test rather than — or in addition to — a breathalyzer in several situations. If a driver is suspected of being under the influence of drugs rather than alcohol, a blood test is often the only way to detect the substances involved. Blood tests are also commonly used when a driver is involved in an accident and transported to a hospital, when a breathalyzer is unavailable or produces an inconclusive result, or when an officer suspects the presence of both alcohol and drugs. In some cases, a warrant may be obtained to draw blood from a driver who refuses to submit voluntarily.
California's implied consent law requires drivers lawfully arrested for DUI to submit to a chemical test. Refusing a blood or breath test carries its own automatic license suspension and can be used as evidence against a defendant in court.
How Prosecutors Present Blood Test Evidence
Once a blood sample is drawn and analyzed, the resulting BAC figure becomes a centerpiece of the prosecution's case. Prosecutors present blood test results alongside the arresting officer's observations — such as slurred speech, the smell of alcohol, and poor performance on field sobriety tests — to paint a comprehensive picture of impairment. A BAC at or above 0.08% creates a legal presumption that the driver was impaired, which shifts the burden toward the defense to present countervailing evidence.
In cases involving drugs rather than alcohol, toxicologists are called to testify about the substances detected in the blood, their typical effects on driving ability, and the significance of the concentration found. These cases are often more complex because there is no universally established impairment threshold for most drugs the way there is for alcohol.
Where Blood Test Evidence Can Be Challenged
Despite their reputation for reliability, blood test results are vulnerable to challenge at multiple points in the process. A thorough defense attorney will examine every stage of the blood draw, storage, and analysis for potential problems, including:
- Chain of custody errors: Blood samples must be properly labeled, stored, and transferred according to strict protocols. Any break in the chain of custody raises questions about whether the sample tested was actually the defendant's blood and whether it was contaminated or tampered with.
- Improper blood draw procedures: Blood draws must be performed by qualified personnel using approved methods. Errors in technique can affect the integrity of the sample.
- Fermentation and preservation issues: If a blood sample is not properly preserved, the blood can ferment and produce alcohol on its own — artificially inflating the BAC reading. Defense experts can test the preserved sample to look for evidence of fermentation.
- Lab errors and equipment calibration: Crime lab analysts must follow precise procedures, and testing equipment must be properly calibrated and maintained. Documentation of equipment maintenance and analyst qualifications is subject to scrutiny.
- Rising blood alcohol: Alcohol continues to be absorbed into the bloodstream after a person stops drinking. If there was a significant delay between when the driver was stopped and when the blood was drawn, the BAC at the time of the draw may have been higher than it was while the person was actually driving.
Why the Defense Needs Its Own Expert
In cases where blood test results are central to the prosecution, retaining an independent forensic toxicologist to review the lab's procedures and findings can be invaluable. Defense experts can identify flaws that are not apparent from the raw numbers alone and provide credible testimony to counter the prosecution's scientific evidence.
Law Office of Michael L. Fell defends clients against DUI charges throughout Orange County, including Fullerton. Law Office of Michael L. Fell will examine every aspect of the blood test evidence in your case and build the strongest possible defense on your behalf. Call (949) 585-9055 today for a free legal consultation.