The percentage of positive drug tests among American workers has increased for the first time in more than a decade, fueled by a rise in marijuana and amphetamines, according to a leading provider of diagnostic information services.

In a report released in August, New Jersey-based Quest Diagnostics said its Drug Testing Index shows a 5.7% year- over-year positivity increase for combined national workforce urine drug tests for the first time since 2003. The figure represents an increase to 3.7% in 2013, from 3.5% in 2012, for 7.6 million de-identified urine and drug tests collected by Quest labs across the country.

The samples were drawn from the U.S. general workforce, i.e., private sector workers, and from employees subject to federal drug testing rules. This latter group includes workers in safety-sensitive jobs, such as truck drivers, train operators, and airline and nuclear power plant workers. The two segments together represent the combined national workforce. Overall, Quest’s analysis involved 8.5 million urine, oral fluid, and hair workplace test results.

Marijuana continues to be the drug of choice, with its positivity rate increasing 6.2% in the combined U.S. workforce, 5.6% in the safety-sensitive workforce, and 5% in the general U.S. workforce.  These increased positivity rates are consistent with findings from the 2012 National Survey on Drug Use and Health, which showed an increase in self-reported past-month marijuana use between 2007 and 2012.

When the urine test data for the general workforce was examined at the state level, it indicated marijuana positivity rate increases of 20% and 23% in Colorado and Washington, respectively, where recreational drug use is legal.  In contrast, among the U.S. general workforce in all states, the average increase was 5%. Oral fluid tests for marijuana went up 27% between 2012 and 2013, Quest said.

Amphetamine use, specifically methamphetamines, also is rising. Amphetamine positivity results reached their highest levels on record, and methamphetamine positive rates were at their highest levels since 2007, according to the company. The increases were seen across all three specimen types of the drug.

The study registered a decrease in one area. Positive test rates dropped 8.3% in 2013 from 2012 for prescription opiates such as hydrocodone and oxycodone, which are used for pain management.

"After years of declines, the prevalence of positive workforce drug tests is increasing," said Quest’s Dr. Barry Sample. "This increase indicates that employers should be aware of the potential for drug use by their workers and the risk that represents for the health and safety of their employees and the public."

Sample also noted that the federal government has not legalized marijuana. He continued, “Employers generally have the authority to restrict the 'recreational' use of marijuana by employees and impose sanctions, including termination, on employees with positive drug tests in all 50 states."

The data suggests employers should brace themselves for the likelihood of an increase in positive drug test results. Employers should review their policies and be familiar with laws governing drug testing and marijuana use in all states where they operate.