Drug use among American workers declined dramatically over the past 25 years, although the rate of positive test results for certain drugs, including amphetamine and opiates, continues to climb, according to a landmark analysis of workplace drug test results by Quest Diagnostics.

The special 25th anniversary Drug Testing Index (DTI) coincides with the passage of the Drug-Free Workplace Act in 1988, which was a catalyst for greater awareness of the problem of workplace drug use and the implementation of workplace drug education and monitoring programs, including drug testing, by federal agencies and private employers in the United States.