A D.C. Superior Court procedure requiring drug testing for most adults arrested in the District was challenged yesterday by a lawyer who said the practice is "flagrantly illegal" and the test not "sufficiently reliable" to stake a defendant's liberty on its results.

If successful, the motion filed on behalf of a defendant in a criminal drug case eventually could result in challenges by hundreds of other defendants now in jail who underwent urinalysis tests showing drug use, court officials said yesterday. They called the challenge a "very serious issue."

Since the urinalysis testing began in March 1984, more than 60 percent of the defendants appearing in D.C. Superior Court have tested positively for drug use.

Court officials said such information has been extremely useful in determining how well defendants will conform to pretrial release conditions or whether they are good probation risks once convicted. Those who test positively are often required to undergo testing as a condition of their release before trial, officials said.

"The research has shown that to the extent you can keep people off drugs they become better risks to both appear in court and stay out of trouble," said Jay Carver, director of the Pretrial Services Agency, which administers the tests.

Peter H. Meyers, the attorney who filed the motion, said the drug testing violates federal drug abuse laws and that the urinalysis test, EMIT, is not reliable enough to use without additional tests to confirm the results.

Under controlled circumstances, EMIT has shown a 97 to 99 percent accuracy, company officials said yesterday. But Meyers said in his motion that other tests of EMIT show a much greater margin of error, in some cases more than 25 percent, and that the company that produces EMIT recommends in its packaging instructions that the results be confirmed by an additional test.

The armed forces require additional confirmation before any disciplinary action can be taken against military personnel, Meyers wrote.

Meyers also said that the testing violates the confidentiality requirements of federal drug abuse laws.