The current DUI statute also provides an affirmative defense to the (A)(2) charge. The law states, " C. It is an affirmative defense to a charge of a violation of subsection A, paragraph 2 of this section if the person did not have an alcohol concentration of 0.10 or more at the time of driving or of being in actual physical control of a vehicle. If a defendant produces some credible evidence that the defendant's alcohol concentration at the time of driving or being in actual physical control of a vehicle was below 0.10, the state must prove beyond a reasonable doubt that the defendant's alcohol concentration was 0.10 or more at the time of driving or being in actual physical control of a vehicle."

Typically, the affirmative defense is shown by conducting a retrograde extrapolation on the breath test results. Expert testimony is used to describe how alcohol appears in the blood within minutes after ingestion. The concentration increases slowly while alcohol is being absorbed from the stomach and the small intestine into the bloodstream. When completely absorbed, a peak alcohol level is reached in the blood, and the post absorption phase begins. The alcohol level slowly decreases until no alcohol is present.

Alcohol in the body is absorbed into the bloodstream, distributed throughout the body's water, and eliminated by oxidation and excretion. Elimination of alcohol varies in individuals. Experts testify that elimination rates of 0.012 to 0.015 percent w/v per hour seem to be acceptable values once absorption is complete. These figures may vary by as much as 30 percent between individuals.

Experts also testify that in the absorptive phase, use of the 2100:1 breath-to-blood ratio may result in alcohol concentration results that are 50% too high.

Any discussion of the affirmative defense necessarily mandates a detailed discussion of blood alcohol in the human body. This inevitably concerns many factors and the use of values, rates, temperatures, and other variables when calculating alcohol cencentrations at different times.

What a difference 30 days meant for two defendants raising the same issue before the Arizona Court of Appeals. One issue, two opposite rulings.

In State v. Petersen, the defendant offered to stipulate that his license was suspended when he was again cited for DUI, and that he had previously been convicted of two prior DUIs. His stipulation was contingent on the jury not hearing about the above facts.

The trial court found the prejudice from the aggravating circumstances was obvious, and that if the facts were admitted, the state would not need to present evidence of them.

The state opposed the stipulation and filed a special action and their requested relief was granted.

A mere 30 days later the same court came down with just the opposite opinion in State v. Root.

The issue in Root was, "did the trial court err by denying the motion to preclude all evidence of the defendant’s prior DUI convictions once the defendant had agreed to stipulate to their existence in order to satisfy an element of the charged offense?"