IN THE SUPREME COURT OF CALIFORNIA
A defendant accused of driving under the influence of alcohol can be charged under two separate code sections. The “generic DUI” provision prohibits driving “under the influence” of alcohol. (Veh. Code, § 23152, subd. (a) (hereafter § 23152(a).)1 The “per se DUI” provision prohibits driving with a blood-alcohol level of 0.08 percent or more. (§ 23152, subd. (b) (hereafter § 23152(b).) This case concerns how a generic DUI charge can be proven, or defended, at trial.
The Vehicle Code requires all drivers who are lawfully arrested for DUI to submit to chemical testing of the blood or breath to determine the alcohol content of their blood. (§ 23612, subd. (a)(1)(A).)2 Whereas a blood test directly measures the subject‟s blood-alcohol level, a breath sample must be converted to derive a blood-alcohol percentage. The conversion factor, known as a “partition ratio,” reflects the relationship between alcohol measured in a person‟s breath and alcohol in the blood. Breath-testing machines in California incorporate a partition ratio of 2,100 to 1, which means the amount of alcohol in 2,100 milliliters of breath is considered equivalent to the amount of alcohol in 1 milliliter of blood. It is undisputed, however, that partition ratios can vary widely, both in the general population and within an individual.
Defendant was charged with generic and per se DUI after he produced a breath sample indicating a blood-alcohol concentration of 0.10 percent. By statute, if a chemical test within three hours of driving measures a driver‟s blood alcohol at 0.08 percent or more, the driver is presumed to have been driving “under the influence” of alcohol. (§ 23610, subd. (a)(3) (hereafter § 23610(a)(3).) Defendant claims he was wrongly prevented from introducing evidence about partition ratio variability to rebut this presumption. In People v. Bransford (1994) 8 Cal.4th 885, 887-888 (Bransford), we confronted a similar claim in the context of the per se DUI offense. We concluded evidence about partition ratio variability is irrelevant in those cases because the Legislature incorporated a 2,100-to-1 partition ratio within its definition of the offense. (Id. at pp. 892-893.)
However, a generic DUI charge is defined differently, and the presumption is not part of that definition. A generic DUI charge requires proof that the defendant‟s ability to drive safely was impaired because he had consumed alcohol. We conclude this difference is significant and hold that competent evidence about partition ratio variability may be admitted to defend against a generic DUI charge. Reversal is not required, though, because any error in this case was harmless under People v. Watson (1956) 46 Cal.2d 818, 836.