The D.C. police department's drug screening program has emerged as an increasingly important internal center of power that can exercise control over officers from the beginning of their careers to the end, according to police union officials and department sources familiar with the program.

And with the pending retirement by 1992 of 2,351 of the District's 3,880 police officers -- as an estimated 600 new recruits a year are needed to fill those slots -- the clinic's role as a clearinghouse will become even more critical.

Several recent and controversial incidents have opened a window onto the normally secretive workings of the drug screening program. This week, for example, officials at the clinic admitted that for the last two years they have been secretly testing the urine submitted by women police recruits for pregnancy, in addition to the routine drug screening required of all police department applicants.

And during the summer, two clinic workers went to Mayor Marion Barry and U.S. Attorney Joseph E. diGenova with allegations of widespread improprieties and tampering with urine samples by clinic and police officials.

A report on the results of an internal investigation into the allegations is expected to be released in two weeks, according to sources close to the investigation.

In a recent interview, D.C. Police Chief Maurice T. Turner Jr., who initiated the screening program in 1982, said that the high-level police panel he appointed to conduct the investigation had told him that the integrity of the drug screening program was intact.

"I think the system is a good system. Obviously, there were some things that happened that shouldn't have happened and we're going to call a spade a spade and take action," Turner said, adding that he felt there was the need for personnel changes at the clinic and promised that apparent irregularities in the system would be explained in the report.

But union officials and police department sources close to the investigation expressed doubt that an internal review could be unbiased about some of the allegations, especially a charge that some test results were manipulated under orders from officials to advance the careers of some favored officers.

In their letters to Barry and diGenova, the clinic whistle-blowers -- Officer Vernon Richardson and civilian Marguerite Anastasi -- alleged that on May 30, 1985, a lieutenant up for promotion to the rank of captain returned in civilian clothes to the clinic for an unprecedented second chance at a drug test after a sample of his urine taken earlier that day tested positive for marijuana.

Richardson was ordered to conduct the extraordinary second test by clinic officials and members of the department's internal watchdog -- the Internal Affairs Division -- after they received several calls from a high-ranking police official whose identity was not known to the whistle-blowers, according to the letters.

The letters also alleged that the second sample was not tested at the clinic -- as is routine -- but was flown to a North Carolina laboratory by a police sergeant for a routine confirmation test. Normally the samples are sent by overnight mail. When the sample was returned from the lab to the clinic, it was reported free of drugs and the lieutenant received his captain's bars.

The whistle-blowers also alleged that clinic and police officials were engaged in "a systematic effort to subvert the integrity of the drug testing procedures" and "gross misconduct and possible criminal violations" including bribery, tampering with evidence and violations of standards of conduct.

Gary W. Hankins, chairman of the Fraternal Order of Police Labor Committee, sees the clinic "as a tool of management to further its own ends rather than as a medical facility."

"It's been turned into Gulag, where dissent is treated as insanity," he said, alluding to author Alexander Solzhenitsyn's term for misuse of the Soviet prison system.

"What has made this such a terribly important thing to the FOP is that once the protocol was broken to benefit an official, it demonstrates that the possibility exists again to break the protocol for personnel reasons," Hankins said. "We can't have any faith in its integrity before or after that incident. We don't know if that incident is an isolated case and that's the scary part."

Robert E. Deso, the attorney for the Fraternal Order of Police, who wrote the seven-page letter detailing the allegations and represents Richardson and Anastasi, also finds great fault with the way the clinic has been used.

"The department has gone to great lengths to maintain the secrecy of the drug testing program," he said. "That secrecy, combined with the potential to terminate or protect the career of an officer or an official, places great power in the hands of the administrators who run the program.

"There is no other area of discipline in the department where the penalty for a first infraction is automatic termination and there's no type of case more difficult to defend that a drug case because the only evidence is documents produced by the department."

Drug screening tests are required of police recruits, those up for promotion and those suspected of substance abuse. The tests also are a routine part of yearly physicals given to officers 35 years old and older.

Applicants who test positive for drugs are not hired. Police officers are subject to termination after one positive urine test is confirmed by an outside lab. At least 40 officers and 50 probationary officers -- those in their first year of service -- have lost their jobs because of allegedly tainted urine in the last five years.

In the interview, Turner defended the termination of police officers based on results of the screening program.

"We think we have a drug screening process that will withstand any challenges from a court," he said. "The integrity of that unit is there, it's . . . a good process and we're going to continue it."