The California Supreme Court joined courts in Vermont and Arizona in finding that partition ratio evidence is admissible in defending against a generic DUI allegation. All states have separate charges for driving under the influence of intoxicating substances (generic DUI law) and driving with a blood alcohol level over .08 (per se DUI law). Often people are charged with both generic and per se violations.

Anyone driving with a blood alcohol level of .08 or above is guilty of violating the per se law. In other words one cannot legally drive if eight hundredths of one per cent of one’s blood is alcohol. The most common test for being under the influence determines the amount of alcohol in one’s breath. To determine if one is in violation of the law it is necessary to convert the amount of alcohol in one’s breath to the amount in the blood. By statute, in the per se law the conversion factor, known as the partition ratio is 2100:1 which means that the amount of alcohol in 2100 liters of breath is the same as in one liter of blood. But the partition ratio is different for different individuals and at different times for the same individual. Just the same, as far as the per se law is concerned, evidence that the defendant has a different partition ration or that 30 per cent of people, in general have a different partition ratio, is immaterial since by law you are in violation if you have a blood alcohol level of .08 or greater using a 2100:1 ratio.

But under the generic driving under the influence law:

To be ‘under the influence’ within the meaning of the Vehicle Code, the liquor or liquor and drug(s) must have so far affected the nervous system, the brain, or muscles as to impair to an appreciable degree the ability to operate a vehicle in a manner like that of an ordinarily prudent and cautious person in full possession of his faculties.

Some years ago the California Supreme Court and other courts determined that evidence of an individual’s partition ratio is immaterial in a per se case because the law sets the ratio at 2100:1. But now in People v. McNeal the court decided that since the presumption that you are under the influence uses a 2100:1 ratio, a defendant may use evidence of his/her unique ratio or of the inaccuracy of partition ratios in general to rebut the presumption that he/she is under the influence in generic DUI cases.

Timmie Lance McNeal was arrested and charged with generic driving under the influence law and violating the per se law. At trial the evidence showed that he ran two stop lights. At the time of his arrest his eyes were watery and bloodshot. His speech was slurred and he smelled of alcohol. His blood alcohol ratio was measured at .10 using a breath test. The arresting officer testified that he failed all of the field sobriety tests. After the evidence was in his lawyer requested permission to reopen the defense case with evidence regarding the partition ratio. There was no showing of whether he/she wanted to provide evidence in general or more specifically related to McNeal. The jury acquited him on the per se violation and found him guilty on the generic allegation. The court denied permission. He motion for a new trial was denied and the appeal followed.

The California Supreme Court ruled that evidence regarding the partition ratio both as it specifically applied to McNeal and in general where 30 per cent of all people have a partition ratio other than 2100:1 is admissible to counter evidence obtained by a breath test using a partition ratio of 2100:1. But the court refused to reverse McNeal’s conviction because there was overwhelming evidence of his guilt and evidence of the partition ratio was unlikely to sway the jury.