The second chemist sent the vials back to Dookhan to resolve the discrepancies, asking her to repeat the screening test the lab used to tentatively identify the drugs in an evidence bag. When she resubmitted them, the machine showed the vials contained pure THC.

The incident, detailed in a 100-page State Police report obtained by the Globe last week, illustrates one of the many ways Dookhan was able to circumvent safeguards intended to ensure that drug evidence was properly handled and analyzed by workers in a now-closed lab formerly run by the state Department of Public Health.

Forensics specialists interviewed by the Globe say the lab’s procedures appear to have been fairly standard, including having two chemists test every sample, but they were still not enough to prevent an ambitious chemist’s rampant breaches of lab protocol, apparently to boost her performance record. In the process, investigators say, Dookhan has jeopardized the reliability of drug evidence used in 34,000 cases during her nine-year career.

The 34-year-old chemist was arrested Friday and charged with two counts of obstruction of justice and one of falsifying her academic record, in allegedly lying under oath about having a master’s degree in chemistry.

Dookhan was “dry-labbing” her screening tests. Put simply, she was skipping a critical first step, according to her admission to investigators, and instead often made a preliminary identification of drugs simply by how they looked and by the type of suspected drug that was checked off on a control card that accompanied the sample.

Typical lab protocols require an initial screening test, called a color test, in which a chemist applies a specific liquid to each drug sample to determine its identity by the color it turns.

That result is crucial for properly performing a second, more definitive test. It tells the chemist doing that second test what “control” drug to use to compare with the sample being analyzed.

“Dry-labbing is probably the most sinful thing that a chemist can do because it is essentially cheating,” said Thomas E. Workman, a criminal defense lawyer who teaches courses on scientific evidence at the University of Massachusetts Law School.

Workman would like to see cameras added to crime labs to record screening tests, with footage available on the Internet to prosecutors and defense lawyers to help ensure that proper procedures are followed.

Use of cameras is not standard practice in crime labs, said Ralph Keaton, executive director of the American Society of Crime Laboratory Directors/Laboratory Accreditation Board, an agency that certifies hundreds of crime labs nationally, including those run by Massachusetts State Police.

Instead, well-run labs use quality managers who check daily to ensure that staff members have properly calibrated machines and that protocols are being followed, said Ralph Timperi, who stepped down in April 2005 after 18 years as ­director of the Jamaica Plain state lab complex, which ­included the drug testing lab.

“There are different kinds of checks and balances, and a super­visory one is critical,” Timperi said. “You need someone walking around and observ­ing what people are doing and looking for problems.”

It is not clear whether the drug lab continued to use quality managers after Timperi’s depar­ture. Alec Loftus, a ­Patrick administration spokesman, declined a request for a copy of the lab’s policy and procedures manual, saying it was protected as part of the criminal investigation of the lab by State Police and Attorney General Martha Coakley.

But the State Police report suggests Dookhan herself may have, at times, served in another quality assurance role, as the lab’s quality control chemist, who typically runs daily tests to ensure scales are calibrated and machines are running properly.

“These machines could have been used by other chemists, who did not even know that the machines were not properly verified,” said Workman.

That possibility, Workman said, would call into question a much larger universe of drug tests beyond the 60,000 Dookhan is believed to have run during her tenure.