The rate of American workers testing positive in workplace drug tests hit a 14-year high in 2018, driven by an uptick in marijuana use, a new analysis found.

Last year, 4.4% of workers and job applicants tested positive for drugs, a 5% increase over 2017 and the highest mark since 2004, according to an analysis of more than 10 million workplace drug tests released Thursday by Quest Diagnostics, one of the largest drug-testing laboratories in the country. Despite the increase, the rate remains significantly lower than that in the late 1980s and early 1990s, when rates were in the double digits .

As an increasing number of states legalize medical or recreational marijuana, the rate of positive marijuana tests continues to rise, increasing 16% from 2.4% in 2014 to 2.8% last year in the general U.S. workforce.

Positive tests for marijuana have also increased in employees in safety-sensitive jobs, such as airplane pilots, law enforcement and train conductors, the analysis shows. In the safety-sensitive workforce, the rate of such positive tests remains under 1% but has jumped 24% from 0.71% in 2014 to 0.88% in 2018, the analysis found.

"As marijuana policy changes, and employers consider strategies to protect their employees, customers and general public, employers should weigh the risks that drug use, including marijuana, poses to their business," Barry Sample, senior director of science and technology at Quest Diagnostics, said in a statement.

Opiate positivity, however, continued to decrease in 2018. Positive tests for opiates in the general workforce dropped by the largest margin in three years in 2018 and was down 27% from its peak in 2015, according to the report. The rate of positive tests for heroin and cocaine also fell last year.

The analysis also indicates that more employees and job applicants may be attempting to "cheat" drug tests. From 2017 to 2018, the percentage of urine specimens rejected due to inconsistencies – suggesting attempts at altercation or substitution – jumped 80% in safety-sensitive workforce and 40% in the general workforce.

