According to a new survey by Quest Diagnostics Inc., the country’s largest drug-test processor, drug use in the workplace has significantly declined. In 1988, 13.6% of employment-related drug tests were positive. By last year, however, just 4.6% of the approximately 6 million tests conducted by the company turned up positive. The study also reveals that in Southern California, amphetamines were the drug of choice for workers who tested positive. But the survey suggests there is a growing trend among workers to cheat on drug tests. The testing firm found 7,800 cases in 1999 where individuals substituted or adulterated samples in order to pass a test, a slight increase over the prior year. Quest Diagnostics attributed the cheating to the fact that there are many new products that make it easier for workers to substitute samples or mask positive results.