A new study shows that workplace drug tests are increasingly turning up positive and heroin has cropped up more since opioid pill use has declined.

Quest Diagnostics, a company that pools data and translates it mainly for doctors and hospitals, reported that an analysis of nearly 11 million workforce drug test results released on Thursday shows a steady increase in U.S. workforce positive results. After years of declining, the number is now at a 10-year high, the company reported.

"Our nationally representative analysis clearly shows that drug use by the American workforce is on the rise, and this trend extends to several different classes of drugs and categories of drug tests," said Barry Sample, senior director of science and technology at Quest Diagnostics Employer Solutions.

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While amphetamine positivity increased 44 percent and marijuana positivity increased 26 percent since 2011, heroin positivity in that period increased 146 percent, according to urine results.

Federally mandated post-accident drug tests in 2015 show a 6.2 percent raise in drug positivity compared to the year before.

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"This report shows a welcome decline in workplace drug test positives for certain prescription opiates but a disturbing increase in heroin positives,” said Dr. Robert DuPont, former director of the National Institute on Drug Abuse. “This rise in heroin should concern both policymakers and employers. Substance abuse is a safety risk for everyone. This new workplace evidence is an additional sign of the rising national heroin problem, this time in the workplace.”

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More than 9.5 million urine, 900,000 oral fluid and 200,000 hair laboratory-based tests performed nationally by Quest Diagnostic for employers in 2015 showed an overall increase in illicit drug use of 4 percent – a 2.6 percent change over 2014 results. Drug use by employees in the U.S. has not been this high since 2005, according to Quest.