The Colorado House gave initial approval late Monday to a bill setting a limit of 5 nanograms of THC, the psychoactive ingredient in pot, per milliliter of blood.

The bill cleared two Senate committees but was voted down by the full Senate on Tuesday afternoon.

The vote was 17-17 on Tuesday and Sen. Nancy Spence (R-Centennial) was absent. She voted in favor of the bill during the regular session, but the bill needed a majority to survive.

The pot DUI bill was also rejected in the Senate last year.

In the regular session, the bill died in the House because legislative work was stopped for a filibuster on the civil unions bill. That bill died again during the special session on Monday night.

Sponsors of the marijuana DUI bill described it as an analogy to the blood-alcohol standard for drivers. Opponents say blood THC levels don't gauge impairment.

Researchers who study the effects of marijuana say that THC measured in blood can reliably tell whether someone has used in recent hours.

"Absolutely it is good science," Marilyn Huestis, senior investigator with the National Institute on Drug Abuse, said.

Huestis argues that the limit set in the bill (5 ng/ml in whole blood) was too lenient, but a good start to addressing the problem of drugged driving.

Researchers do admit the tests aren't perfect because much like testing for blood alcohol, some of the heaviest users may not be impaired while testing above the legal limit.

"A chronic user who used three hours ago might get caught at this [standard,] but a chronic user who used a month ago would not get caught," Dr. Christian Thurstone, who works with drug addiction for Denver Health Medical Center, said.