The National Institute on Drug Abuse is investigating "potential problems" in a widely used federal drug testing procedure after learning that a government-certified laboratory wrongly reported workers had tested positive for illegal methamphetamine use when they in fact had been using over-the-counter cold or asthma medicines, agency officials said yesterday.

The laboratory, which officials declined to identify, was suspended earlier this month shortly after federal officials discovered it had mistakenly identified a truck driver as a methamphetamine user. The worker, being tested under the Department of Transportation's drug testing program, was removed from his job last summer as a result of the test.

But the truck driver "vehemently denied" ever using the illegal drug and filed a labor grievance. The investigation discovered that the worker had actually been taking large quantities of ephedrine, a decongestant found in several over-the-counter cold and asthma medicines.

Institute officials and other drug testing experts said yesterday the case could have broad implications for the drug testing industry. Further review determined that four of six other positive methamphetamine tests reported by the same lab since last May had been similarly misidentified.

Dr. Joseph Autry, NIDA's director of applied research, said the agency is investigating whether the false tests reflect inherent flaws in a procedure approved by the Department of Health and Human Services for use in confirming methamphetamine test results.

That procedure is widely used by both private and public sector drug testers, including at least seven other laboratories certified by the NIDA to test urine samples of federal employees and workers in industries regulated by the Department of Transportation and the Nuclear Regulatory Commission.

"This is something we're taking very seriously," said Autry, who oversees the federal drug testing program. "We don't know if the lab did something a little different than other labs . . . or something is wrong in the method itself."

Autry said yesterday there have been no indications of problems in HHS guidelines used to confirm the presence of marijuana or cocaine, by far the most commonly abused drugs. But Arthur Spitzer, legal director of the local chapter of the American Civil Liberties Union, said the incident illustrates the group's concerns in opposing drug testing in general.

"The drug testing lobby has always assured us there is nothing to worry about, and time after time it's turned out these tests are not as good as they were cracked up to be," he said.

Autry said a preliminary review showed that "under 100" federal employees or regulated workers have been reported as testing positive for methamphetamines under the same confirmatory procedure now in question. The agency has notified all 57 NIDA-certified labs not to use that procedure and has begun independent retesting of all the urine specimens previously reported positive for drug use.

But Autry and other experts said yesterday there is no way to know how many unregulated private drug testing laboratories may be using the same procedure. "It's scary," said Donald Ian MacDonald, White House drug adviser under President Ronald Reagan and now chief executive officer of Employee Health Programs, a Bethesda firm that reviews drug test results for private companies.

Methamphetamine, known to users as "speed" or "crank," is an illegal stimulant that federal officials say has become increasingly abused in recent years, especially in rural areas in the West and South. MacDonald said as many as 75 percent of urine samples that initially show traces of the drug turned out to be false positives when strict confirmatory tests were performed.