In July 2009, the California Supreme Court made a decision in People v McNeal that can be an important weapon in a criminal defense attorney's DUI defense arsenal.

According to the decision, defendants who have been charged with Vehicle Code §23152(a) may now present evidence of one's partition ratio to contest the accuracy of Breathalyzer results. The partition ratio is the conversion ratio that the breath machines use to convert the amount of alcohol in a breath sample to the amount of alcohol in the blood. The standard partition ratio is set at 2100:1, defined as milliliters present in the breath: milliliters present in the blood.

Even though scientists still believe that using a 2100:1 ratio is an acceptable representation of a person's partition ratio, it is believed that the majority of the population has a 2300:1 ratio. This would result in a higher than actual blood alcohol content on the Breathalyzer machine.

One problem with this decision is that court declined to provide guidance as to how a criminal defendant could establish their own personal partition ratio. In addition, the court could determine that this evidence is inadmissible at trial because in order for scientific evidence to be allowed it must have gained general acceptance in the particular scientific filed and currently there are no established procedures for determining a person's partition ratio.

Given the problems with this decision, it is important that anyone facing a DUI charge work with an experienced DUI attorney.