American workers are failing drug tests more than ever, and it may be due to an increase in heroin use, according to a study released by Quest Diagnostics on Thursday.

After surveying more than 11 million drug tests over the past three years, researchers found four percent more tests came up positive for illicit drugs in 2015 after almost a decade of declining numbers. Results also indicated that amphetamines, marijuana and heroin use by workers has increased in since 2011.

This comes just a few months after the Drug Enforcement Agency released an updated National Heroin Threat Assessment Summary, which noted the dramatic increase in heroin use in the U.S. since 2007. The report also detailed the influx in heroin-related overdoes, which increased to 10,574 in 2014 compared to 2010’s reported 3,036. During Quest’s study, the medical screening lab found that heroin use by workers increased 146 percent between 2011 and 2015.

In 2015, the DEA had a record-breaking drug bust in New York City after confiscating about 70 kilograms of Mexican heroin – which is worth roughly $50 million – and $2 million in cash,  the Associated Press reported.

However, the study showed a decline for prescription opiates. However, the Quest release suggested that the decrease in opiates could be because of the increase in heroin.

Quest Diagnostics pulled more than 9.5 million urine, 900,000 oral fluid and 200,000 hair results from drug tests conducted by Quest for employers in 2015.

A study conducted by the Nation Institute of Drug Abuse in June 2015 highlighted the overall increase of illicit drug use throughout the country. In a survey of approximately 67,800, researchers found that the use of narcotics had increased to 9.4 percent of the population. However, the research in NIDA’s study noted the surge in drug use could have been linked to the increase in marijuana use.