FILE - In this June 6, 2017 file photo, a reporter holds up an example of the amount of fentanyl that can be deadly after a news conference about deaths from fentanyl exposure, at DEA Headquarters in Arlington Va. Some large law enforcement agencies have recently abandoned the routine chemical field tests out of concern that officers could be exposed to opioids that can be absorbed through the skin or inhaled. Even a minute amount of the most potent drugs, such as fentanyl, can cause violent illness or death. (AP Photo/Jacquelyn Martin, File)

FILE - In this June 6, 2017 file photo, a reporter holds up an example of the amount of fentanyl that can be deadly after a news conference about deaths from fentanyl exposure, at DEA Headquarters in Arlington Va. Some large law enforcement agencies have recently abandoned the routine chemical field tests out of concern that officers could be exposed to opioids that can be absorbed through the skin or inhaled. Even a minute amount of the most potent drugs, such as fentanyl, can cause violent illness or death. (AP Photo/Jacquelyn Martin, File)

ST. LOUIS (AP) — Police who find suspected drugs during a traffic stop or an arrest usually pause to perform a simple task: They place some of the material in a vial filled with liquid. If the liquid turns a certain color, it’s supposed to confirm the presence of cocaine, heroin or other narcotics.

These chemical field tests have been standard procedure for decades, with officers across the country using them every day. Prosecutors rely on the results to jail suspects and file criminal charges.

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But some large law enforcement agencies have recently abandoned the routine tests out of concern that officers could be exposed to opioids that can be absorbed through the skin or inhaled. Even a minute amount of the most potent drugs, such as fentanyl, can cause violent illness or death.

Police are instead sending suspected drugs to crime laboratories, which have quickly become over-burdened, delaying many cases.

“We instituted the precautions for self-preservation, frankly,” said James Shroba, the agent in charge of the Drug Enforcement Administration office in St. Louis. Agents, he said, began finding fentanyl in everything they seized, including marijuana, cocaine and methamphetamine.

Over the past 18 months, field testing has been banned by the DEA, state police in Oregon, Arizona, Michigan and Missouri, and several big-city departments, including New York and Houston.

No police deaths have been blamed on fentanyl, a synthetic opioid developed for cancer patients and others suffering severe pain. But dozens of officers have become ill, including 18 in one raid last year in Pittsburgh.

Illegal raw fentanyl powder can be 50 times more potent than heroin and is often mixed with other street drugs. Synthetic drugs were blamed for more than 20,000 U.S. overdose deaths in 2016 — double the number from 2015. Prince and Tom Petty are among its victims.

The field test provides only a preliminary finding that must be confirmed by a lab. In fact, a 2016 New York Times report called into question the accuracy of field tests, saying they often produce false positives.

Still, field test results often convince suspects to plead guilty even before the initial indications are checked by scientists, prosecutors said.

For cases that go to trial, full lab results can take months, which puts some suspects back on the streets for long periods.

State police in Oregon and Missouri stopped field testing last month.

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Paige Clarkson, who is in charge of drug prosecutions in Marion County, Oregon, which includes Salem, has been trying to focus on rehabilitation for low-level drug offenders. She worries that the long wait for lab results makes it harder to help defendants.

“If we don’t have a confirmatory test and cannot enter into a criminal-justice process, we lose our window to get those people into treatment,” Clarkson said.

In Audrain County, Missouri, prosecutor Jacob Shellabarger said he’s concerned that the delays invite more crime if suspects are set free to rob and steal to support their addictions.

Indiana state police put out a bulletin in February 2017 urging law enforcement agencies to avoid field testing “unless the circumstances make it absolutely necessary,” and many heeded the warning.

The state crime lab received 14,266 drug samples last year — over 2,000 more than 2016. Police are also confiscating more drugs as the opioid crisis worsens, state police spokesman John Perrine said.

In Arizona, state troopers stopped field testing in November 2016. By August of last year, the backlog at the state crime lab reached 2,300 cases.

“We had to do something,” lab superintendent Vince Figarelli said.

So he and his colleagues came up with a plan: Move the field test to the lab. Since October, suspicious material brought to the lab initially gets a preliminary color test similar to what officers previously performed in the field.

If that test is positive but the suspect pleads not guilty, the substance gets a full analysis. If the suspect pleads guilty based on the preliminary finding, no further testing is necessary.

The backlog has been cut by two-thirds since October. Figarelli believes it may be eliminated by spring.

Other agencies are adapting too. The DEA has agreements with state and local labs to perform “presumptive tests” in controlled environments in cases where immediate results are deemed necessary, Shroba said.

U.S. Customs and Border Protection, which inspects goods entering the country, recently introduced a hand-held device that allows officers to get an instant read on the chemical compounds inside packaging, sparing them from direct contact with a substance, said spokesman Jaime Ruiz.

If the reading is inconclusive, data is sent from the device to an agency lab in Los Angeles for a more definitive conclusion within 24 hours.

In Allen County, Indiana, which includes Fort Wayne, police take suspicious powder to the department for preliminary testing, with negligible lag time compared to field testing, but in a much safer environment, prosecutor Karen Richards said.

Prosecutors acknowledge the drawbacks of abandoning field testing, but they understand the necessity.

“Our position is always officer safety is the number one priority,” Clarkson said.