Police officers often catch drivers driving erratically but find a breathalyzer test shows that they have not been drinking. Drugs may be involved -- but how to confirm that suspicion quickly?

Enter the Drug Recognition Experts -- police officers specially trained to evaluate drivers who may be under the influence of methamphetamine, marijuana and other drugs, both legal and illegal, that may impair driving.

At the scene, in a hospital or at a police station, DRE officers evaluate suspected impaired drivers who pass alcohol sobriety tests through interviews, balance tests and analysis of vital signs such as pulse, blood pressure and pupil dilation.

Police say the officers, who also perform normal police duties, are key to getting drug-impaired drivers off the streets immediately, without waiting for toxicology test results.

But because DRE officers must use observations and indirect evidence to support their conclusions about drug use instead of instant test results that are available for alcohol use, their use has drawn criticism from some defense attorneys. Fresno defense attorney Eric Schweitzer, for example, calls the DRE program "pseudoscience."

"You have cops in lab coats masquerading as scientists," he said.

Schweitzer cited a 2009 California Court of Appeals ruling that threw out the conviction of a man charged with driving under the influence of methamphetamine because there was insufficient evidence that he was driving unsafely.

DRE officers had noted that he had stopped past the limit line at an intersection and showed telltale signs of fidgety behavior, sweating and a rapid pulse. Even though there also was evidence of the drug in his system, the court found that was not sufficient evidence he was impaired.

Capt. Andy Hall, who heads the Fresno Police Department's traffic unit, agrees that DRE officers are not scientists. But he maintains the training they receive at the California Highway Patrol Academy enables them to recognize the symptoms displayed by a drug-impaired driver.

The department's 20 DRE officers, like others certified in the state, undergo two weeks of classes at the CHP Academy and about another week of field training where they learn to spot the symptoms of drug-impaired drivers. Officers also must regularly update their training to stay certified.

Other Valley cities, including Clovis and Visalia, have trained DRE officers while some law enforcement agencies depend on CHP officers.

The first DRE programs were started in the 1970s by the Los Angeles Police Department to train officers in a standardized 12-step evaluation to help determine whether a driver is under the influence of drugs, determine the type of drug and rule out medical conditions that might be the cause of the impairment.

The LAPD touts two studies -- one in 1984 by Johns Hopkins University and one in 1985 by the National Highway Traffic Safety Administration -- as proof of the program's value: Both found DREs could successfully distinguish between those drivers impaired by drugs and those who were not.

Further evidence: The Los Angeles City Attorney's Office estimates that 95% of those charged with driving under the influence of drugs in Los Angeles are convicted.

Schweitzer, however, said DRE programs are flawed because police are not in a position to determine what level of a drug makes a driver impaired. He cited marijuana, which he said does not metabolize in the body at a constant rate.

Some defense lawyers also point out that the symptoms of impairment can be mimicked by natural causes, including fatigue, allergies, sickness and nerves.