DENVER (AP) - A bill setting a blood standard for THC, the psychoactive ingredient in marijuana, has cleared the Colorado House.

Marijuana activists are fighting the proposal that passed the House Tuesday because they say blood THC tests are an unfair gauge of impairment.

Sponsors of the bill say more drivers are using marijuana and the state needs a marijuana analogy to the accepted blood-alcohol limit.

The pot activists have prevailed twice, but the measure is back before lawmakers in a special legislative session expected to end Wednesday. Marijuana activists hope to sink the third attempt when the bill goes to a vote in the Senate, possibly Wednesday.

Driving while impaired by drugs is illegal in all states. Seventeen states have blood-level limits for drugged-driving convictions.