Partition Ratio Evidence in DUI Cases

The California Supreme Court rendered a decision on June 09th 2009 which makes it easier to challenge breath test results in DUI trials. In the case of People v McNeal, the Court decided that evidence of partition ratio variability may be admitted to defend against a DUI charge.

Whereas blood testing directly measures a subject's blood-alcohol level, a breath sample must be converted to derive a blood-alcohol percentage. This conversion factor is known as a "partition ratio" and it reflects the relationship between breath and blood alcohol measurements. Breath testing machines in California operate on a partition ratio of 2,100 to 1 which means that the amount of alcohol in 2,100 milliliters of breath is condisered equivalent to the amount of alcohol in 1 milliliter of blood.

It is widely accepted that the actual partition ratio for indivduals can vary quite considerably, depending upon a number of factors including sex, body temperature and medical conditions. It is possible for a study to be undertaken by a toxicologist on an individual to compare their actual breath and blood sample results over time to see what their actual partition ratio is.

Evidence of partiton ratio variability is now considered admissable and relevant to the "DUI" charge (section 23152a) but not to whether a defendant drove a vehicle with a .08% breath alcohol level or higher (section 23152b). Previous case law makes it clear that evidence of partition ratio variability is irrelevant to vehicle code section 23152(b) as the legislature incorporated a 2,100 to 1 partition ratio within its definiton of the offense.

This change or clarification in the law will help defend cases where a defendant is charged only with the generic DUI charge in vehicle code section 23152(a) or in cases where there is a seperate defense to the "per se" charge in section 23152(b). Ofcourse, the case is only helpful to cases based on a breath test.

If you have any questions about this or any orther defenses to DUI charges in California, please contact Nigel Witham at Gold & Witham , Attorneys at Law. Gold and Witham is a DUI Defense Law Firm primarily defending DUI cases in Los Angeles and Orange Counties.


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