The federal government's drug-testing program is riddled with inequities because of widespread differences in the way agencies are testing employees and selecting workers that must be tested, according to study by the General Accounting Office.

The GAO study found that out of 18 agencies surveyed, the proportion of workers who are subjected to random urinalysis drug testing ranged from less than 1 percent of the workforce to 100 percent. Even in agencies with comparable missions, the discrepancies were large: Whereas one out of every four workers in the Department of Navy are designated for testing, only 2.4 percent of workers in the Department of Army are subjected to drug testing.

The GAO findings show federal workers "are not being treated fairly or equitably," said an aide to Sen. Barbara A. Mikulski (D-Md.), who requested the report.

The report recommends that a single federal agency be designated to oversee the federal drug-testing effort -- a conclusion that is scheduled to be presented at a hearing today of the Senate Appropriations subcommittee on treasury, Postal Service and general government.

The study comes at a time when some critics are again questioning the value of widespread drug testing in the federal government, because there appears to be little evidence that drug abuse is a widespread problem among federal workers.

Of 40,000 federal workers who had their urine tested between April 1 and Sept. 30 last year, only 200 -- about one-half of 1 percent -- tested positive for any illegal drug, according to Joseph Autry, director of the division of applied research of the National Institute on Drug Abuse, who oversees the federal drug-testing effort.

In contrast, tests in the private sector have often shown about one out of every 10 workers testing positive. Autry said that the low number of positive results among federal workers -- about half of which were for marijuana use -- shows that the testing program is acting as a "deterrent" to drug abuse. Critics charge that it proves that drug testing was never needed in the first place.

The federal effort dates back to September 1986, when President Ronald Reagan signed an executive order requiring the heads of each federal agency to develop a program and select categories of workers, such as those in "safety sensitive" positions or with access to classified information, for drug testing.

But the move to implement the executive order has been stalled by lawsuits and opposition by federal employee unions, producing large differences among agencies, the GAO found. While some agencies and Cabinet departments have vigorously installed testing programs, others have barely gotten such programs off the ground.

The Defense Contract Audit Agency, for example, had developed a plan to randomly test 5,300 of its workers. But it was unable to reach agreement with a union that represents 700 employees and, as a result, has deferred testing of any workers and may not begin testing for another three years, the GAO study said.

The study also found large differences in the amounts agencies are paying for drug tests as well as examples of agencies failing to comply with testing procedures aimed at protecting workers' rights.

In one case, a certified drug-testing lab had falsely identified a worker as an illegal drug user, but the Department of Health and Human Services failed to notify federal agencies of the lab's mistake as it was required to do, the study found.

The GAO study concluded that a single agency overseeing the drug-testing program could ensure more uniform and equitable results for workers. But Mark Roth, general counsel for the American Federation of Government Employees, said such a move could end up hurting federal workers because the duties employees perform and the mission of various agencies "is often radically different."

"The bases for GAO's conclusions are not sound," Roth said. "Placing the federal drug-tesing program under a single agency and removing agency discretion may well accomplish nothing and could even result in constitutional violations."