Arizona V. Cooperman: DUI Partition Ratio relevant, competent evidence to show lack of DUI Impairment.
Blood Alcohol Content (BAC) refers to the concentration of alcohol in the blood that can currently be measured either by a DUI blood test or a breath test. Interestingly, however, the results of a breathalyzer test for DUI may not always be the same as the results from a blood test. This may be the case even if the blood and breath are tested at the same time.
Partition Ratio in DUI Breathalyzer Tests
Breathalyzer tests produce a numerical score, only by mathematically converting the breath sample to a Blood Alcohol Concentration level. This conversion process is known as the "partition ratio" when the conversion factor is used. The conversion is considered problematic by some because it is not necessarily reflective of the actual partition ratio for an individual; actual partition ratios for individuals can vary. Factors that may cause the ratios to vary include but are not limited to body temperature, medical conditions and gender. This means a breathalyzer test for an individual may not accurately translate to a blood alcohol concentration level indicative of impairment.
Case Background: State of Arizona v. Cooperman
In a recent case, the Arizona Supreme Court addressed whether partition ratio evidence could be admitted in a DUI case where (1) the state chose to bring in breath test results to prove a defendant had a .08 percent or more BAC within two hours of driving, and (2) evidence related to how much partition ratios varied in the population was relevant to the defendant's level of impairment. The defendant here wanted to show how the partition ratio varies in the general population in order to introduce doubt as to whether the results of the breath test showed impairment.
In this case, a motorist charged with one count of driving while "impaired to the slightest degree" in violation of A.R.S. 28-1381 (A) 1; and the other was for having an alcohol concentration of .08 percent or more within two hours of driving in violation of A.R.S. 28-1381 (A) 2.
The Prosecution generally attempts to prove beyond a reasonable doubt that the latter charge (.08 percent BAC) by presenting a jury with evidence of a defendant's blood alcohol content and establishing a DUI test sample was taken within two hours of the defendant driving. When a person's BAC is .08 percent or higher, the presumption is that a person is under the influence, in violation of Arizona's legal limit. However, to have a level below the .08 percent BAC does not however, create a presumption. If the officer had probable cause to believe that a motorist's was still impaired, even though their BAC was below .08 percent, they may bring charges in violation of "impaired to the slightest degree". The impairment, however, cannot be presumed, and must be decided in connection with other probable cause evidence.
The prosecution in this case attempted to prevent the defendant from submitting evidence that showed the partition ratio used to convert the breath reading to a blood reading was variable, meaning inconsistent, or liable to change. The prosecution argued that it planned only to introduce the breath test results for proof that the defendant's BAC exceeded .08 percent; but not to prove the first charge of "impairment to the slightest degree". Since the prosecution was not going to introduce the breath test for the impairment to the slightest degree charge, they argued the defendant could not present the partition ratios related to that breath test to cast doubt on whether or not the defendant was impaired at all.
Experts for both parties testified regarding the partition ratio. The defendant again tried to introduce exculpatory evidence of the partition ratio to that would cast doubt on whether or not he was impaired to the slightest degree. The State argued the defendant's evidence was irrelevant and had the potential create unfair prejudice. The court ruled that partition ratio evidence was in fact relevant whenever breath test results are brought forward by the State. The court of appeals affirmed this ruling. The State then appealed to the Arizona Supreme Court.
Arizona Supreme Court Analysis
The Arizona Supreme Court held that evidence is relevant where it can make a material fact in a case more or less probable. If evidence is relevant, it is permitted at trial, unless there is specific rule or provision in the law that prohibits it. In this case, the State was required to prove that the defendant was impaired because he drank alcohol. Therefore evidence of his impairment was relevant.
The AZ Supreme Court recognized the strong correlation between Blood Alcohol Content levels and intoxication. The prosecutors had argued that they had the unilateral ability to invoke the presumption that the defendant was under the influence and the partition evidence was irrelevant because they chose not to invoke the presumption of impairment to the slightest degree.
The Arizona Supreme Court disagreed with this approach by the prosecution. They held that there is nothing that precludes a DUI defendant from presenting partition ratio evidence to show he was not impaired in an impairment case. In fact, they cited specific Arizona Law, A.R.S. 28-1381(H) which specifically provides that any "competent evidence" on the issue of the question of the defendant's impairment in DUI charges brought against them.
In conclusion the Arizona Supreme Court cited Sandstrom v. Montana, 442 U.S. 510, 524 (1979). Which holds the need to satisfy constitutional requirement presumptions in criminal cases must be rebuttable, enabling either side to provide evidence or argument that challenges or opposes the presumption. Thereby The Arizona Supreme Court affirmed decisions made by all previous courts, Municipal, Superior, and Appeals Court of Arizona, which was to allow the exculpatory evidence regarding partition ratio variability to be admitted to show the defendant's lack of impairment.