A patient with a bleeding stomach ulcer was being prepared for surgery at Georgetown University Hospital. At the last minute the operation was postponed. There was no blood for a needed transfusion.

"We're one step away from having to cancel a patient," was the chilling way Dr. Fred Darr, assistant medical director of the Washington Regional Blood Center of the Red Cross, described the blood shortage yesterday in making an urgent appeal for volunteers.

That was before the fatal air crash near National Airport, which put even a further strain on the supply. The center broadcast an emergency appeal, which brought about 200 donors to the downtown center, 2025 E St. NW, despite the snowstorm. The center, which normally closes at 4 p.m., stayed open until after 8 o'clock.

The shortage is in Type O blood, the so-called universal type that can be accepted by virtually everybody. Jim McLeod, a blood bank spokesman, said donations have fallen to only 40 percent of need.

McLeod noted one statistic the Red Cross would like to change: Only 2 percent of the people, about 200,000, supply all the blood used in a District-Maryland-Virginia region with a population of 4 million.

Potential donors were asked to call 857-3400 for a Red Cross bloodmobile location or to call the Red Cross donor center at 857-3553.