California Supreme Court Opens Door for Challenges to Breath Tests in DUI Cases?

By Javier Lavagnino, Esq. on July 09, 2009 | Last updated on March 21, 2019

Breath test results in California DUI cases could be seeing more challenges in courts soon, thanks to a decision today by the state's high court. The Supreme Court ruled that defendants can challenge the accuracy of blood-alcohol concentration (BAC) results from a breath test device. So what does this mean for DUI defendants?

Well, first off, the ruling affects only those cases involving "generic" DUI charges, as opposed to "per se" DUI charges. What's the difference? Well, one key difference between the two is that the per se DUI charge does not require the prosecution to prove that a defendant was actually "impaired". In other words, driving with a BAC at or above .08 constitutes the offense in per se cases. However, for generic DUI charges, proof of impairment is required.

So here's what happens in such generic DUI cases. If a defendant tests for a BAC of .08 or above, a presumption that they were intoxicated arises. A defendant then has the opportunity to try and rebut (or disprove) that presumption, which may be pretty tough to do if it an actual blood test was administered. But if a breath test (e.g., Breathalyzer) was used, today's ruling makes it possible to challenge the way a person's BAC was calculated using that test.

This is because Breathalyzers other breath-test machines have to mathematically convert the amount of alcohol in a person's breath to the amount of alcohol in their blood. Breath-testing machines in California all use the same, fixed "conversion factor" or breath-to-blood ratio, to calculate a BAC, but as the Supreme Court noted these "ratios can vary widely, both in the general population and within an individual." Defendants can now provide evidence about these variable ratios to challenge breath-test results.

This might make people wonder whether this means getting off the hook is easy? Well, not remotely. As was seen in today's case, other evidence comes into play in DUI cases to establish that someone was actually impaired, including "the driver's appearance, an odor of alcohol, slurred speech, impaired motor skills, slowed or erratic mental processing, and impaired memory or judgment."

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