The staff of a regional health planning agency, asserting that Northern Virginia has an oversupply of hospital beds, has sharply criticized a proposal by a large national health care chain to build a 200-bed hospital in Reston.

A 44-page staff report, presented to the board of Health Systems Agency of Northern Virginia, said population in western Fairfax County will not grow fast enough to support a full-service, for-profit community hospital proposed by Humana Inc.

Although the agency, whose recommendations on hospital construction are usually followed by the state health commissioner, has not acted on the staff report, the study will be a key factor guiding the agency's 30-member board when it meets next week to make its final recommendation.

While the report does not take a stand on whether Humana should be allowed to build the hospital, it says the project would be an extremely costly way to provide care for people in Reston who now have to travel as much as 45 minutes to get to one of the two nearest full-service hospitals--the 695-bed Fairfax Hospital at Rte. 50 and the beltway, or the 125-bed Commonwealth Doctors on Chain Bridge Road in Fairfax.

A preliminary study by the staff had indicated it would be far cheaper to expand the number of beds at existing hospitals in Fairfax County and that emergency cases still could be handled at the Ambulatory Care Center Emergency Services System (ACCESS) in Reston.

The critical report is viewed as a blow to Humana, the Louisville-based firm that is the nation's second largest for-profit health care chain. The firm has been working hard to gather political and citizen support for its proposed "Cameron Woods Hospital," a $34 million health care facility that would be located near the Dulles Airport Access Road.

For the company, which tonight has a chance to argue its case in a public hearing by the health systems agency, population growth projections in western Fairfax will be a key factor in determining whether it can win approval for its hospital. Health planning experts say that if construction of hospital beds outpaces population growth it increases costs for existing hospitals, boosts Medicaid and Medicare costs and increases private health insurance premiums.

The two sides are far apart on population growth estimates for the Reston area. Humana says that the population there will grow by 20,000 over the next five years, or about the same rate it has grown since 1978. But the health agency staff report says the population will grow at half that rate.

"Humana . . . uses inflated population projections to help justify its proposal," said the report. The study went on to say that the current 2,100 medical-surgical beds in Northern Virginia are "more than adequate to meet regional needs for at least the next five years."

"The HSA board is on record that they feel there is a surplus of beds and nothing submitted to us by Humana convinced us that that was not the case," said George L. Barker, associate director of HSA. "Unless they submit some new information at the hearing that alters our conclusion about the oversupply of beds , it would make more sense to increase bed capacity at existing hospitals if more beds are needed."

"We have serious problems with their staff's analysis," countered Thomas McCandlish, a Richmond lawyer who represents Humana. "We have not just concocted some statistics out of thin air." He said the company's estimates showing much faster growth were taken from Fairfax County and state population projections.