An incompetent doctor is successfully sued for malpractice because of negligence that killed a patient. The doctor moves to a distant state, goes to a hospital, presents credentials, says nothing about the lawsuit and joins the staff.

It happens.

To prevent this, a new federal operation has begun, a computerized National Practitioner Data Bank that from Sept. 1 will contain the records of doctors:

Who are successfully sued for malpractice, including cases settled out of court.

Whose licenses are revoked or temporarily suspended by any state's licensing agency.

Who suffer an adverse action by a hospital -- such as a suspension or loss of permission to practice some kinds of medicine -- or by a medical group or health plan with a peer review system.

Hospitals, state licensing agencies and many medical organizations and health plans will be able to get any existing file on any doctor or dentist or, if a malpractice claim has been paid, on other health professionals. Hospitals will be required to query the data bank before taking on a new practitioner, and to query it every two years on every staff member.

Nothing like this has existed before. The Federation of State Medical Boards has maintained a data bank on its members' disciplinary actions, but "it doesn't work well enough," Daniel Cowell, the physician directing the federal effort, said last week.

Many state licensing boards -- including the District of Columbia's -- are too under-financed and under-staffed to get and disseminate all the information they should, let alone do the investigating and disciplining they are supposed to do.

Now the new data bank -- operated by the Unisys Corp. under a five-year, $15.9 million contract -- will make the reporting of adverse actions mandatory for a wide range of organizations. Federal officials expect 50,000 to 60,000 reports a year, half of them notices of malpractice payments, half on other reports of misconduct or incompetence.

"Incompetent practitioners will no longer be able to escape detection by moving from state to state," said Health and Human Services Secretary Louis W. Sullivan. "As a physician, I welcome this new safeguard of the health of all America."

It is a safeguard that is long overdue.

Seventy percent of disciplinary actions against doctors involve an event in a hospital, yet "quality measurement by most hospitals is generally poor" and in many hospitals "so primitive it is embarrassing," said a report this spring in Inquiry, a Blue Cross-Blue Shield publication.

"Physician credentialing" -- checking the past records of doctors -- is often weak in HMOs (health maintenance organizations) and other health plans, industry officials have acknowledged.

HHS Inspector General Richard Kusserow said this year that the number of doctors disciplined each year is far smaller than the number of potential cases reported. He said state agencies' efforts are hampered by inadequate authority as well as by skimpy financing.

The Federation of State Medical Boards disclosed that, for the first time in five years, these overworked agencies had failed to increase disciplinary actions, despite huge jumps in complaints.

The Public Citizen Health Research Group listed 6,892 "questionable doctors" who were disciplined a total of 9,510 times by state and federal agencies. This represented only one doctor in 100, yet, this group said, "those dangerous doctors who fall through the cracks continue to kill, maim, defraud and otherwise injure their unknowing patients."

Hospitals and medical societies have peer review bodies -- tissue committees, mortality, professional conduct and credentialing committees. Doctors do far more examining of each other than any other professional group.

But for years these bodies have been cowed by fear of lawsuits by the doctors they accuse or investigate. It is one thing for doctors to know that a colleague is incompetent. It is another to prove it in court.

The same 1986 act that created the new data bank gave hospitals and doctors who investigate colleagues immunity from suits, as long as they exercise due process. But this part of the act isn't working despite cases in which investigating doctors have prevailed, says Arnold Relman, editor of the New England Journal of Medicine. "Most doctors are {still} afraid to participate in quality review and peer review activities," he reports.

Will the data bank do better? It offers hope, but it is not a cure-all.

Only hospitals must use it to check on doctors. For others, even state medical boards, such checks are voluntary, though reporting their own severe disciplinary actions is compulsory.

A doctor who hears that he or she may be investigated might resign and move on before the action starts -- and never be caught.

The bank will not include all malpractice claims, only those that end in a payment. Doctors of course maintain -- correctly --

that many claims are unjustified and the majority never result in a payment. Still, a user of the bank might want to know about a doctor who is subjected to claim after claim.

By no means are all doctors who lose malpractice suits bad doctors. A high rate of claims, or even lost suits, may mean only that a doctor treats high-risk patients. Surgeons, obstetricians and anesthesiologists are sued far more often than family doctors. At the same time, the main cause of significant awards is true negligence.

Though not required to do so, the Defense Department, Public Health Service and Drug Enforcement Administration have agreed to take part in the data bank. The Department of Veterans Affairs, though the operator of the nation's largest hospital system, is still arguing over several issues.

Health care professionals other than doctors and dentists are so far included only if they are successfully sued for malpractice, or a payment is made to settle a claim.

Lawyers may get information only regarding suits against hospitals, not doctors, and only if the hospital failed to query the bank about an involved doctor.

The bank will include only actions started or completed from Sept. 1 -- though there are sources of past information, some available to hospitals or health plans, some not.

Many of the above gaps are the result, justified or not, of lobbying by medical and hospital groups.

"This system is a milestone," said Cowell, director of quality assurance in HHS's Health Resources and Services Administration. "It is an alerting system whose time has come, but it is no panacea. Those who want to check on a doctor should use many sources, not this alone."

There is also the system's worst gap of all: the remaining weakness of many state medical licensing boards to discipline doctors. For this, blame state governments -- and the District's.

There is one more large gap, if you're a patient. The public may not have any of the data bank's information, even about doctors who commit the worst acts.

Medical groups contend that even large malpractice awards do not prove negligence. True, but the public still gets little or no information about even the most outrageous practitioners.

Patients can contribute, however, to this bank's effectiveness. They can complain to hospitals, state agencies, state or local medical societies -- or all of these -- when they feel they have been wronged.

Complaints do get read. Some lead to investigation. Some end in discipline.