In a per se driving under the influence (DUI) trial, expert witness testimony on the reliability of breath-testing machines generally was properly excluded. Appellant was charged with the per se DUI offense (Veh. Code, § 23152, subd. (b)). At trial, he offered testimony by Dr. Hlastala concerning the effects of alcohol on the respiratory system and the reliability of breath-testing machines in general. On objection by the prosecution, the court ruled that testimony would not be allowed as to properly working machines, although evidence could be presented that the particular machines malfunctioned. The jury convicted appellant. The Supreme Court found no error with the trial court ruling as to the exclusion of testimony. Section 23152, subdivision (b) prohibits driving with either the specified blood-alcohol level or the specified breath-alcohol level. The statute defines the .08 percent breath-alcohol offense in light of the standard partition ratio, which the Legislature adopted based on studies correlating breath samples to the concentration of alcohol in the blood. The expert’s testimony challenging the reliability of breath-testing machines generally because they fail to measure the alcohol content of the air from the alveolar region of the lungs was fundamentally at odds with the statutory per se offense. The statute and California Code of Regulations, title 17, section 1219.3 simply require a breath specimen consisting of the last portion of expired breath that is captured by an approved breath-testing machine that is properly calibrated and employed. A defense expert may not invite a jury to ify the Legislature’s determination about the fundamental reliability of approved breath-testing models. Other proposed defense expert testimony about physiological factors that may affect the results of breath-testing machines generally essentially constituted partition ratio variability evidence, which is barred in per se DUI prosecutions. (People v. Bransford (1994) 8 Cal.4th 885.)

[Editor’s Note: Appellant was also charged with driving under the influence in violation of Vehicle Code section 23152, subdivision (a) (known as the “generic” offense) but the jury did not reach a verdict on this count. Because there was no conviction on the generic charge, it was not necessary for the court to address the propriety of the court’s ruling as to that count. (See People v. McNeal (2009) 46 Cal.4th 1183 [partition ratio variability is relevant and admissible to rebut presumption underlying a generic DUI charge when prosecution relies on the results of a breath-test machine].)]