DENVER (AP) — A marijuana blood standard for drivers appears headed for approval in Colorado — thanks to a single vote change from a Republican senator. DENVER (AP) — A marijuana blood standard for drivers appears headed for approval in Colorado — thanks to a single vote change from a Republican senator.

Sen. Nancy Spence of Centennial voted Tuesday in favor of the plan to consider drivers impaired if they test positive for 5 nanograms or more of THC, the psychoactive ingredient in marijuana, per milliliter of blood.

Spence's decision gave the bill the support it needed to advance on a vote of 18-17 after an emotional debate.

Driving while impaired by marijuana or any drug is already illegal, but supporters of the THC blood limit say law enforcement needs an analogous standard to the blood-alcohol standard to keep stoned drivers off the road.

Spence, who voted last year with critics who said there needed to be more study of the driving-high problem, said after the vote that she's become convinced that the time has come for a bright-line standard to determine legal impairment.

"I'm just sick of the abuse that the state of Colorado has taken from the medical marijuana industry," Spence said.

Spence's vote put her in agreement with sponsoring Sen. Steve King, a Grand Junction Republican who argued that the explosion of pot use in Colorado since the state approved medical marijuana in 2000 made it past time to have a driving blood limit.

"We are well are on our way to a doped-driving epidemic that will match the DUI epidemic that we had 15 and 20 years ago," King argued. He said that the legality of medical marijuana here has led to people thinking it's OK to smoke and drive.

Opponents included lawmakers from both parties. A handful of Democrats and Republicans rose in vain to try and stop the DUI blood standard. Sen. Pat Steadman, a Denver Democrat, made the point that marijuana users who legally use the drug could be unfairly deemed impaired. Unlike alcohol, THC is fat-soluble, so blood limits can remain above the legal limit even when a user is not stoned.

"Some of these people wake up in the morning and roll out of bed at 5 nanograms," said Steadman, who tried and failed to amend the bill to exempt card-holding medical marijuana patients.

The measure now awaits one more formal vote in the Senate, though chambers seldom change course after their initial debate. After a final Senate vote, the bill heads to the Republican House, where a 5-nanogram blood limit was approved last year by a comfortable margin of 51-14.

This year's pot DUI bill originally included other drugs, including some legal mind-altering prescription drugs such as sleep aids. But the bill was pared down to deal only with marijuana. That meant a big drop in how much the state estimated it would cost to enforce.

The White House has urged all states to set a blood-level drugged driving standard, though the federal government hasn't specified what the amount should be.

The fact that Colorado would allow any amount of THC in a driver's blood when marijuana remains illegal under federal law posed a dilemma for some lawmakers. One of the Senate's most conservative members, Berthoud Republican Sen. Kevin Lundberg, pointed out the dilemma before he voted against the blood standard.