By Joanne Kenen

WASHINGTON (Reuters) - For all the talk about aging baby boomers bankrupting the U.S. health care system, the real cost culprits may be tests and treatments of dubious value, according to a government study released on Tuesday.

Medical costs are growing far faster than the population is growing and aging, Congressional Budget Office Director Peter Orszag said in releasing the report. If nothing is done, health care would eat up half the economy in 75 years, he said.

"A lot of what we deliver is of dubious value," Orszag said.

Orszag, who is expanding his agency's health team before an expected deluge of health legislation in the next few years, called for a national push in public and private sectors to learn which treatments work best, and how they stack up against alternatives that may be older or cheaper but just as effective.

"It appears possible to reduce costs without harming outcomes," he said. "The nature of the long-term fiscal problem has been misdiagnosed," with policymakers placing too much emphasis on the aging population and not enough on cost effectiveness and quality.

Orszag cited work by a team of health researchers at Dartmouth College in New Hampshire who have shown that some parts of the country spend far more on health care and specialists than other regions, but do not have better quality or outcomes.

Congress and federal policy makers have focused heavily in recent years on cost projections for a graying population; relatively little federal attention has been paid to research on comparative costs or "evidence-based" medicine, he said.

The aging population is one factor in the cost explosion "but it is not by any means the main factor," he argued.

For instance, he said, technology like magnetic resonance imaging or MRI provides a valuable diagnostic tool -- but the costly screening is now used very widely without a lot of evidence on when it is truly beneficial.

"It gets applied in lots of settings where the benefits are more dubious," Orszag said.

Some of the big private health insurance companies are beginning to collect data comparing various medical therapies and interventions that may help determine how to best diagnose and treat common problems like a back injury, Orszag said, adding that the CBO had asked them to share their findings.

Orszag said politicians were probably overstating how much savings electronic medical records would reap for the health care system, and that despite years of legislation and commissions, politicians had not yet devised a workable solution for fixing Medicare, the state-federal health program for Americans aged 65 and older.

He said that changing payment systems -- which now reward highly specialized doctors who perform many procedures -- would be only one part of the challenge. The cultures of both doctors, who want to do more things, and patients, who demand more and more treatments, would also have to change, he said.

(Editing by Maggie Fox and David Wiessler)