PHOENIX - In what would be a precedent-setting case, the state's high court was asked Friday to decide, in essence, whether someone who smokes marijuana - even legally - can ever drive in this state.

In a petition to the Arizona Supreme Court, the attorney for Hrach Shilgevorkyan does not deny his client had at some point inhaled marijuana before his December 2010 arrest by a Maricopa County sheriff's deputy who pulled him over for speeding. That was confirmed by a blood test that showed evidence of carboxy-THC, a metabolite of the drug.

But Michael Alarid III said this particular metabolite can remain in someone's system for up to a month after marijuana has been used. But it becomes "inactive," he asserts.

Alarid pointed out that the blood test found no evidence of tetrahydrocannabinol, or THC, the principal psychoactive ingredient in marijuana, as well as hydroxy-THC, which is what THC breaks down into. State law clearly makes the presence of either chemical presumptive evidence of impairment.

Hanging in the balance is whether marijuana users in Arizona can be convicted of driving under the influence for some activity that has occurred weeks before.

The issue takes on even more importance because Arizona allows those with a doctor's recommendation to legally use the drug. While that does not grant anyone permission to drive while impaired, a Supreme Court ruling against Alarid could effectively preclude any of the state's nearly 38,000 registered medical-marijuana users from driving at any point given the long-term presence of carboxy-THC in their blood.

Alarid called that result "absurd," given the lack of evidence of impairment.

Central to the debate is the law that makes it illegal to drive where there is any drug defined in the criminal code or its metabolite in the person's body.

THC is on that list. And hydroxy-THC is a metabolite.

Prosecutors argued the law includes all possible metabolites of THC. But a justice of the peace dismissed the charges, a decision upheld by Maricopa County Superior Court Commissioner Myra Harris.

"This court has not been persuaded that the Legislature necessarily intended to include all possible derivatives of drugs - particularly inactive end products that no longer affect an individual," Harris wrote. "Where, as here, there is no showing of a drug or metabolite that causes impairment, the driver should not be subjected to criminal penalties."

Last month, however, the state Court of Appeals decided otherwise.

Judge Michael Brown, writing for the panel, said the laws on impaired driving "must be interpreted broadly."

That leaves the question in the hands of the high court.

Alarid said the state's own expert conceded that carboxy-THC does not cause impairment. He said it would be "absurd" to allow someone testing positive for this chemical to be convicted of driving under the influence.

But Maricopa County Attorney Bill Montgomery, whose office is prosecuting the case, said there is no good way to determine whether someone is "impaired" because of marijuana use.

"We don't know at what rate most of the population would be able to break down marijuana to when it's in its non-psychoactive elements," he said. And that, Montgomery said, makes it legally acceptable for the Legislature to decide that the proper standard should be zero.

But Alarid told the justices that standard pre-dates Arizona and 17 other states adopting medical-marijuana laws, which raise questions about what constitutes "illicit drug use."

Anyway, he said, there is an argument to be made that anyone who has a medical need for marijuana probably should not be driving in the first place.

He said someone with chronic pain - the most frequent reason cited by those seeking permission to use the drug - probably would need to be using some other prescribed drug. And he said most of those have warnings against using them while driving or operating machinery.

Alarid added the argument is, the law makes it illegal to drive with THC and "its metabolite" in the blood. That, he said, means a singular metabolite, in this case, hydroxy-THC.

"Based on the plain meaning of the statute, (it) does not prohibit driving with carboxy-THC in the body," he said told the court. The justices have not decided whether to accept the case.