Under pressure to slash medical outlays, the Department of Health and Human Services is cutting a big batch of inner-city and rural areas off its list of "medically underserved areas," which could knock out federal funding for nearly 100 of the nation's 800 community health centers.

An area is classified as medically underserved if it has a high concentration of poor people and the elderly, a high infant mortality rate and a proportionately small number of doctors who provide primary care.

If a community qualifies, groups can then seek federal funding for a health care center. The centers are open to anyone, but low-income people are charged reduced rates or have their care covered by Medicaid.

For millions of people, these centers are the sole source of medical care because there are simply no other facilities in their communities. Last year, the federal government provided $327 million in funds to the centers, which served about 5 million people.

Now the Reagan administration wants to chop this appropriation by more than one third, although there is some resistance on Capitol Hill. Sen. Ernest F. Hollings (D-S.C.) plans to offer an amendment to the HHS appropriations bill Thursday to restore part of the slash.

Meanwhile, HHS is paring down its list of "underserved areas," using 1978 data in some cases and 1970 population figures in others (it can't use the 1980 population figures because of court challenges to the census).

So far, it looks like the major community health centers in the D.C. metropolitan area--Shaw, Upper Cardozo and East of the River--will survive. The center at Hadley Memorial Hospital has had its funding reduced and plans to shift its focus to infant mortality problems. The Kaiser Georgetown Community Health Plan, which served about 2,000 people under the program, is dropping out of the program, but not because of reclassification.

However, areas like New York's Harlem, Boston's Roxbury, black sections of Baltimore and Hispanic sections of Miami have already been tentatively reclassified and thus would be ineligible for the money. Altogether, about 95 of the 800 community health centers would be knocked out if a proposed list is finally adopted, saving substantial amounts of money. Funding for others, even those still on the list, might have to be cut if the department's approporiation is reduced to what the president wants.

"Anyone who has driven through Roxbury, Harlem and the Bronx can see that these are underserved areas," protested Dan Hawkins of the National Association of Community Health Centers. "Where are these folks going to go for their care?"

Hawkins contends the 1978 physician-population ratios provided by the American Medical Association are flawed, and that HHS should have waited a few more months so that it could use 1980 census data, rather than the out-of-date 1970 figures.He said his group has surveyed nearly half of the centers that would be cut and has been told by "virtually every one" that they will challenge the data by bringing in their own numbers to prove that they have been misclassified. "In some cases, the AMA showed a doctor in the neighborhood and we know for a fact that he's not there anymore."

An HHS official said that in reclassifying the areas, "we are using the best data we have available" at present.