Employers across the United States are ordering fewer tests for marijuana in prehire drug screens even as the number of positive drug tests is on the rise — a trend that is raising concerns about the potential implications for workplace safety.

Screening dips in states that have approved marijuana for recreational use are particularly troubling, as they indicate that employers are forgoing drug testing due to legal risks and staffing issues.

Nationwide, 99% of drug tests in 2016 included screening for marijuana, and in 2017 that figure dropped to 98.4%, according to data compiled by Barry Sample, Seneca, South Carolina-based senior director of science and technology for the employer solutions business at Quest Diagnostics Inc.

States with legalized marijuana saw the highest drops in screening for the drug. According to Mr. Sample, 98.1% of employment drug tests in Colorado screened for marijuana in 2016, a figure that dropped to 96.2% in 2017; and 98.2% of tests in Washington state screened for marijuana in 2016, dropping to 97% of tests in 2017.

About 70% of drug tests ordered in the workplace are for prehire screenings, he said.

Such figures, set alongside recent data on an increase in positive drug tests in the workplace, are a safety concern, experts say, with 30 states and the District of Columbia legalizing medical marijuana and nine states approving its use recreationally.

“This is a situation we have to pay attention to,” said Jim Smith, president of the American Society of Safety Engineers and the regional leader of risk control safety services at Arthur J. Gallagher & Co. in Boca Raton, Florida.

The well-publicized report released by Quest on May 8 shows the highest positive drug test percentages in 10 years.

Overall, the report showed positive urine drug testing among the combined U.S. workforce in 2016 was 4.2% — an increase over the 2014 rate of 4%. Positive tests for marijuana use continued to climb in both the federally mandated, safety-sensitive and general U.S. workforces, according to the study.

In saliva testing, marijuana positivity increased nearly 75%, to 8.9% of the general U.S. workforce in 2016 from 5.1% in 2013. Marijuana positivity also increased in both urine testing — 2.5% in 2016 vs. 2.4% in 2015 — and hair testing —7.3% in 2016 vs. 7% in 2015 — in the same population.

While some drug screens can gauge for levels of intoxication — indicating whether a worker is intoxicated at the time of being in the workplace or at the time of an accident — marijuana can only test for presence in the body, Mr. Sample said, adding that the science to gauge marijuana intoxication is “a very long way off.”

“Trying to develop a recognized national standard for impairment for alcohol took some time,” he said. “In terms of marijuana, there is no data that would help guide or adequately say what impairment is.”

Testing for marijuana, unlike that for alcohol, is a predicament for employers because there is no way to scientifically tell whether a person ingested the drug that same day, over the weekend or over the past month, experts have noted.

Companies — except, for example, those in the transportation industry required to test for drugs by federal regulations — are stepping away from drug screens as a way to avoid issues filling positions, especially in states with approved recreational and medical marijuana.

Another issue is that some states prohibit employers from firing or not hiring an employee with a medical marijuana card, said Adam Kemper, Fort Lauderdale, Florida-based senior counsel with Greenspoon Marder L.L.P. The issue could put the employer in violation of the federal Americans with Disabilities Act, he said, adding that the issue has employers perplexed.

“I think a lot of employers are stepping away from (workplace drug) testing because they don’t want to get into knowing whether their employees are using and then considering whether or not they have to take action, or whether taking action would be an ADA violation,” he said. “If the employee is performing, maybe the employer doesn’t want to intrude on the employee’s life. That’s the reality a lot of times. You have employers looking at performance (on the job) only.”

Dr. Todd Simo, Charlotte, North Carolina-based chief medical officer for the drug screening firm HireRight L.L.C., isn’t convinced employers will pull back from testing for drugs — even with what he called slight dips.

“Companies spend three times on medical care costs for workers on illicit drugs,” Dr. Simo said, noting other benefits for companies who opt to drug-test their workers such as improving productivity or decreasing absenteeism.

The dip in testing for marijuana, which Mr. Smith of ASSE called “alarming,” could also be the result of the U.S. Occupational Safety and Health Administration’s electronic record-keeping regulation, which discourages employers from conducting drug testing post-injury.

“I don’t think drug testing is going away,” he said. “I know some people have stopped doing it as a result of OSHA,” which is making “the assumption that people will not report (injury) claims because of drug testing.”

In 2017, ASSE recommended that OSHA rescind the electronic recordkeeping rule, which requires certain employers to annually electronically submit injury and illness data that they are already required to record on their on-site OSHA Injury and Illness forms. Employers have objected to the rule’s limitations on incident-based employer safety incentive programs and/or routine mandatory post-accident drug testing programs. They were also troubled by the anti-retaliation provisions in the rule, which established a new, citation-based pathway for employee complaints.

The rule “created a lot of confusion for employers,” Mr. Smith said.