The nasty secret of health care economics is that a lot done for patients is useless or dangerous, and costly, and that much that could help them, at relatively low cost, isn't done. Occasionally we get a glimpse into the issue of quality and costs in medicine, but not often.

Surveying the field of health care studies, researchers at Rand Corp., the California think tank, found a "surprisingly small amount of systematic knowledge on the quality of health care delivered in the United States," much of it dating from the 1980s and early 1990s. But after scrutinizing the reviews that have been done, they concluded that "Whether care is preventive, acute, or chronic, it frequently does not meet professional standards."

The number of studies was small, for sure, but perhaps not so surprising. Providers of treatments, good and bad, generally are not receptive to independent scrutiny that might question their professional judgment, integrity and means of livelihood. Orthopedic surgeons went on the warpath in 1994 when a federal study raised questions about the efficacy of surgery for lower back pain. In sympathy with the irate doctors, Congress whacked the budget of the sponsor of the unwelcome research, the Agency for Health Care Policy and Research in the Department of Health and Human Services.

In that way, messages are sent to researchers and their patrons. The Rand study -- titled "How Good Is the Quality of Health Care in the United States?" -- candidly pointed out that the existing studies provide only "snapshots" of the American medical landscape. Even so, the review emerged with the "dominant finding" of serious deficiencies in medical service based on the available studies.

For example, a study of seven managed-care organizations concluded that 16 percent of hysterectomies in 1989-90 were carried out for "inappropriate reasons." Another 25 percent were done for reasons of "uncertain clinical benefit." A study in 1990 of 1,335 patients who underwent coronary angiography concluded that 4 percent of the procedures were inappropriate and 20 percent were "equivocal." A study in 1988 of 386 cases of coronary artery bypass surgery reported 14 percent as inappropriate and 30 percent equivocal.

Bountiful prescribing of antibiotics for colds -- useless in almost all cases -- was found in a study of some 1,400 patients in 1994. The Rand review found big gaps in the provision of inexpensive, reliable preventive measures. A 1993 study of 8,000 senior citizens reported influenza vaccination for only 52 percent, and merely 28 percent vaccinated for pneumonia. Among 21,600 women over 50, only 58 percent underwent clinical breast examination and 46 percent received mammograms in 1992. Of 128,400 women over 18, 67 percent reported a pap smear in the previous three years.

The picture that emerges, spotty as it is, indicates that a substantial number of patients are needlessly subjected to serious surgery. Surgeons respond that the chart-reading critics of medical treatment didn't have the benefit of hands-on contact with the patient and can be mistaken in their assessments of surgical necessity. Moreover, a certain amount of surgery of equivocal necessity is deemed desirable for erring on the safe side.

But what's striking about health care -- at $1 trillion a year, the biggest industry in America -- is how little is known about its workings. The difficulty of measuring is compounded by technological dynamism, heavily financed by the federal government and industry. New instruments for diagnosis and treatment and new drugs regularly come on the medical market and are welcomed as beneficial before substantial knowledge of their benefits and risks has accumulated. Ironically, the arrival of new, usually costlier medical treatments is accelerating as the population of the uninsured has risen to more than 43 million. The result is a net gain in the ranks of people who can't afford the wonders of modern medicine.

The Rand study provides a needed glimpse into the rarely examined problem of what the nation is receiving for its huge expenditures on health care. Yes, as we're often told, American medicine is the best in the world. Ailing rich foreigners flock to American medical care. But good as it is, it still could be a lot better. A useful first step would be to supplant snapshots with accurate portraits of how the system actually performs.

Daniel S. Greenberg is a science journalist.