How to Challenge DUI Blood or Breath Test Results in Court

If you’ve been arrested for DUI in California, the prosecution will often rely heavily on chemical test results to build its case. Breathalyzer readings and blood test reports are frequently presented as objective, scientific proof of intoxication. But these tests are not infallible. In many cases, they can be challenged, and sometimes excluded, if they were improperly administered or if the results are unreliable.

At Kosnett Law Firm, our Los Angeles DUI defense practice often involves a detailed review of how chemical tests were conducted and whether the results can withstand scrutiny in court. Understanding how these tests work and where they can fail is critical to building a strong defense.

The Legal Foundation for Chemical Testing

Under California’s implied consent law (Vehicle Code §23612), drivers lawfully arrested for DUI are required to submit to a chemical test of either their breath or blood. These post-arrest tests are distinct from the roadside Preliminary Alcohol Screening (PAS) device. Once a person is arrested, refusing a chemical test can lead to license suspension and additional penalties.

However, just because a test was administered does not mean the results are automatically valid. Prosecutors must establish that the test was properly conducted, that the equipment was functioning correctly, and that the results accurately reflect the driver’s blood alcohol concentration (BAC) at the time of driving.

Challenging Breath Test Results

Breath tests are commonly used because they are fast and non-invasive. But they are also vulnerable to a range of scientific and procedural issues. One of the most common challenges involves improper calibration or maintenance of the breathalyzer device. California regulations require that breath-testing equipment be regularly tested and maintained. If records show lapses in calibration or servicing, the reliability of the results can be called into question.

Another issue is the 15-minute observation period. Before administering a breath test, officers are supposed to observe the subject continuously to ensure they do not eat, drink, smoke, or regurgitate, any of which could affect the reading. If this observation period was not properly conducted, the results may be unreliable.

Breath tests can also be affected by residual mouth alcohol, which can come from recent drinking, certain medical conditions, or even products like mouthwash. This can artificially inflate BAC readings. Additionally, factors such as body temperature, breathing patterns, and certain health conditions (like acid reflux or diabetes) can skew results.

A strong defense may involve cross-examining the officer about testing procedures, reviewing maintenance logs, and consulting with forensic experts to explain how these variables could have impacted the results.

Challenging Blood Test Results

Blood tests are often viewed as more accurate than breath tests, but they are not immune to error. In fact, they introduce a different set of potential issues related to collection, storage, and analysis.

One major area of challenge is the chain of custody. The prosecution must show that the blood sample tested in the lab is the same sample taken from the defendant and that it was properly handled at every stage. Breaks or inconsistencies in the chain of custody can undermine the integrity of the evidence.

Another critical issue is fermentation or contamination. If a blood sample is not properly preserved with anticoagulants and preservatives, it can ferment over time, producing alcohol and artificially increasing the BAC reading. Improper storage temperatures can also affect the sample.

Defense attorneys may also examine whether the blood draw was conducted by a qualified professional in accordance with California law. If the procedure was improper or unsanitary, the results may be subject to challenge.

Finally, the laboratory analysis itself can be scrutinized. Crime labs must follow strict protocols, and errors in testing procedures, equipment calibration, or analyst interpretation can all affect the outcome. Reviewing lab records, audit reports, and analyst credentials can reveal weaknesses in the prosecution’s case.

The “Rising Blood Alcohol” Defense

Even when test results are accurate, they may not reflect the driver’s BAC at the time of driving. Alcohol takes time to absorb into the bloodstream. In some cases, a person’s BAC may have been below the legal limit while driving but rose above 0.08% by the time the test was administered.

This is known as the “rising blood alcohol” defense, and it can be particularly effective when there is a significant delay between the traffic stop and the chemical test. Expert testimony is often used to explain how absorption rates, body weight, and food consumption can affect BAC levels over time.

Violations of Constitutional Rights

In some cases, DUI test results can be challenged on constitutional grounds. If the initial traffic stop was not supported by reasonable suspicion or if the arrest lacked probable cause, any evidence obtained afterward, including chemical test results, may be suppressed.

Additionally, while breath tests are generally allowed as a search incident to arrest, warrantless blood draws can raise Fourth Amendment issues unless valid consent or exigent circumstances existed. If law enforcement violated constitutional protections, the results may be excluded from evidence.

Why These Challenges Matter

Chemical test results often form the backbone of a DUI prosecution. If those results are weakened or excluded, the prosecution’s case can fall apart or become significantly less persuasive. This can lead to reduced charges, more favorable plea negotiations, or even dismissal.

Challenging DUI test results requires a detailed understanding of both the science involved and the legal standards governing admissibility. It also requires access to records, expert analysis, and strategic cross-examination.

Work with an Experienced DUI Defense Attorney in Los Angeles

If you’re facing DUI charges in Los Angeles, it’s critical to have a defense attorney who knows how to dissect chemical test evidence and identify weaknesses in the prosecution’s case. At Kosnett Law Firm, we thoroughly investigate every aspect of DUI arrests, from the legality of the stop to the reliability of test results. We work with forensic experts, review maintenance and lab records, and challenge improper procedures to protect our clients’ rights. Whether it’s a breath test or a blood test, we know how to build a defense that puts the prosecution’s evidence to the test.

If you’ve been arrested for DUI in Los Angeles, contact Kosnett Law Firm today for a free consultation. We can evaluate your case, explain your options, and fight for the best possible outcome.