The nation should drastically change the way it pays and trainsits doctors - and sharply limit payments to specialists - to encourage physicians to give primary care rather than specializing, a national study group said yesterday.

The 14-member committee of the Institute of Medicine urged a 10-year moratorium on increasing the number of medical students - in effect, putting a lid on the size of current medical classes and halting the plans to start new medical schools now under way in some states.

The committee said it urged this moratorium because there are so many medical students now in the pipeline that the numbers of doctors will increase by 60 percent in 12 years. Every new doctor generates so many fees and tests that he or she adds $250,000 a year to the nation's health bill, the committee reported.

Nonetheless, the percentage of doctors who gave primary care - meaning basic personal and family phycians rather than continuing to leave specialists - will increase only slightly, continuing to leave serious shortage, the committee found.

In a series of recommendations certain to be opposed by thousands of specialists, the group said:

Health insurance plans and government agencies should refused to pay for specialized care unless it had been declared necessary by a primary care doctor. This could deny payment for all Medicare, Medicaid and privately insured patients unless their regular doctor refers them to a specialist. Primary care doctors should be paid as much as specialists for the same work. Currently, some specialists charged considerably more for even simple procedures than general doctors. As one example, said Dr. E. Harvey Estes of Duke University, study group chairman, a general doctor may be paid $20 to sew up a cut on a child's leg, but a plastic surgeon may get $100 for the same service.

Doctors in all part of the state should receive equal fees, instead of the current situation in which insurers and government agencies commonly pay city doctors more than smalltown and rural doctors - 22 percent more nationally for Medicare patients. The federal government and other forces should exert maximum pressure on medical schools and hospitals - and giving them subsidies - so that by 1990 perhaps 60 to 70 percent of all doctors will be giving primary care rather than today's 44.8 percent.

Without such interventions, the committee said, the proportions of primary care doctors will rise to only 50 percent by 1990, though the number of all doctors will rise in number from 340,000 to 559,800.

"The primary care doctor is your personal physician," Estes said. "He or she is the physician-manager of your care, the person you turn to for input into the medical system, for advice on where to go and how to deal with your problems."

Primary care doctors include some broader specialists. As generally defined, they include general practitioners, family practitioners (general practitioners who take more training and are certified by a national crediting body), internists or specialistsin internal medicine (but only internists who do not mostly "sub-specialize" in some field like heart or intestinal disease), pediatricians and obstetrician-gynealogist who give their patients their overall care.

But "execellent" primary care, the committee said, can also be given by medical groups (in which several kind of specialists cooperate) or by well-trained nurse-practitioners backed by supervising physicians.

A medical group or hospital should in fact be paid the same fee whether a service is given by a doctor, nurse-practitioner or trained physician's assistant, the committee recommended. It urged states to authorize nurse-practitioners and physicians' assistants to make diagnoses and prescribe drugs "when appropriate."

But it said these nurses and doctors' helpers - who generally get from one to two years' medical training - should not work without physician supervision. And it said the number of them being trained should not be increased until the country sees how many are needed.

Committee member Dr. Loretta Ford, University of Rochester nursing dean, objected to such recommendations. She said the report failed "the unmet needs" of many Americans by recommending "giants steps" to train hundreds of health teams to deliver their care.

Whoever gives Americans' primary health care, the report said, should be "accessible" to every patients - or provide competent alternative care - 24 hours a day.

The report listed five such practical tests for good primary care: accessibility, comprehensiveness (giving almost all the care a person needs), coordination (working with others but "managing" or coordinating the care), continuity (staying with the patient throughout health or disease) and accountability (answering patients' questions, keeping up with medical science, maintaining malpractice insurance).