The average cost of a day's care in a Washington hospital is rising 50 percent faster than the national average and is expected to top $300 a day by early 1979.

The cost here rose 18.5 percent in the year that ended last Sept. 30, increasing from $239.62 to $283.91 a day.

This steep rate in the increase -- far greater than the rise in the Washington suburbs and still continuing, by the best evidence -- was disclosed by area Blue Cross officials yesterday.

They said it emphasizes the need to abandon unneded hospital beds and eliminate unneeded hospital building and expansion throughout the metropolitan area. They said one prime cause for the high costs -- empty, unused beds or whole wards -- soon will become epidemic throughout the area because of too much building.

The full cost of a day's care includes not only the hospital's room rate (which may amount to less than half the real cost), but also th price of tests, drugs and special treatment.

In suburban Maryland, where hospital rates are controlled by a state ratesetting agency, the cost of care increased by just 8.2 per cent, from $196.82 to $213 daily, in the same period.

In the Northern Virginia suburbs, it went up 13.2 percent, from 192.45 to $217.92 daily. This was just slightly above the national average -- up 12.1 percent from $177.42 to $198.90 a day.

Blue Cross and other authorities said higher average in the city costs as compared to the suburbs are because the District has six large adbanced-care, teaching centers: George Washington, Georgetown and Howard University hospitals, Childrens' Hospital, Washington Hospital Center and D.C. General Hospital.

All care for many of the sickest patients, often from other jurisdictions in the area. "They are giving more services than the average," said Barry Wilson, Blue Cross vice president.

Care at Children's Hospital cost an average $696 a day in the year that ended June 30, according to government sources. Childrens' hospitals generally are among the most expensive nationwide.

Care at Georgetown Hospital, the most costly of the university hospitals, is $381 a day.

Low occupancy in District hospitals has caused high unit costs, since many expenses remain the same even when some beds or wards are not being used.

Between July 1, 1977 and June 30, 1978, occupancy in te entire Washington area dropped from an average 80.7 to 78 percent, Wilson reported. In 15 District hospitals, it dropped 3.2 points, from 80.7 to 77.5 percent.

As of the end of September, Washington Hospital Cneter was only 71 percent filled, on the average. It was the site of a month-long nurses' strike last may and June from which it has still not fully recovered.

Occupancy for the year was only 61 percent at Columbia Hosptial for Women; 70 percent at Children's, 72 percent at D.C. General, 73 percent at Georgetown.

It was 73 percent at downtown Doctors Hospital, which will be forced to close by late 1980 unless it can overturn a District health planning officials' refusal to approve a new location.

Most hospital authorities think a general-purpose hospital should be at least 80 at 85 percent filled, on the average, to give economical care.

Wilson said a city with to many complex hospitals should be expected to hae a higher inflation rate, as well as high costs in the first place. But American Hospital Association officials in Chicago said Washington also seems to be an outstanding victim of rising unit costs caused by less use.

In Chicago, which has several big university medical centers,the average cost of care rose just 11.4 percent in the year that ended Sept. 30 -- from $228.61 to $254.67 a day, nearly $30 under the Washington figure.

Johns Hopkins University Hospital inBaltimore increased total spending by only 8.2 percent in the year that ended June 30.

Hospitals nationwide managed to hold their spending to a 12.8 percent projected annual increase from January through September 1978, compared to a 15.9 per cent increase in the same montns of 1977.

He Center administration Thursday asked the nation's hospitals to hold spending next year to a far tighter 9.7 percent average increase.