WASHINGTON - The blood bank in Las Vegas pulled in 7,700 extra donations last year without having to recruit extra donors - at a time when other parts of the country had shortages.

The trick: technology that lets one person give the amount of red blood cells normally gotten from two people, without feeling woozy afterward.

Call it double-blood donation, a small but growing trend that blood bankers hope will boost the nation´s tight supply - a supply many warn is likely to get even tighter with new restrictions on who can donate.

"We saw the writing on the wall because of the shrinking donor base and said, ´We have to do this now,´" says Jan Dalby of United Blood Services, Las Vegas, where double-blood collections are steadily rising.

Typical donations haven´t changed much in decades: A nurse sticks a needle in your arm and fills a bag with whole blood. The blood is shipped to a lab to be separated into different components - red blood cells, platelets and plasma - used for different treatments.

Red blood cells make up the typical transfusion; several thousand gallons a day are used. Ensuring that supply is a challenge. Only about 5 percent of people eligible to donate blood do so, and increasingly strict safety rules limit who can donate. Meanwhile, demand for blood is rising and periodic shortages force hospitals to cancel elective surgeries.

You can´t donate more than about a pint of whole blood at a time. But the new trend: Hook donors to a machine that separates blood on the spot, keeping the red cells and putting the rest back into their bodies right away. Because only part of the blood is taken, banks can collect twice the red cells - totaling about a pint - from a single donor who doesn´t feel the loss.

Removing one compound from blood is called apheresis, and it has long been used in different ways in medicine. Massachusetts-based Haemonetics Inc. pioneered using apheresis to double the red cells collected from regular blood donors.

It does take longer, about 35-45 minutes hooked up to a needle instead of 15. Not everybody is eligible - you must weigh about 20 pounds more than the minimum required to donate blood the regular way.

Donors can give lots of blood with fewer visits. You can donate blood the regular way once every two months - although the average donor does so only twice a year - or give a double red cell donation once every four months.

"I´ve been doing this close to 40 years, giving blood, and it doesn´t feel different," says Robert Drymalski, 71, of Las Vegas, who began donating the new way a year ago. "The only thing you feel is when they push the needle in your arm."

It´s attracting first-time donors, too, Dalby says. When Las Vegas bloodmobiles started carrying the double-blood equipment, they drew 38 percent more red cells during the city´s high-school blood drives, as some teen-agers who once shrugged at donation asked to try out the cool machines.

Double donating is not available everywhere yet. Haemonetics so far has supplied the equipment to five of the American Red Cross´ 36 blood regions; United Blood Services, the largest non-Red Cross blood banker, is spreading it beyond Las Vegas to the 18 states it serves, and smaller blood banks are starting to offer it.

A competitor, Gambro BCT, also sells double-red cell machinery, and other companies are exploring it.

"This technology holds promise to further enhance our nation´s available blood supply," says Dr. Jerry Squires, the Red Cross´ medical director. It "will be especially important for donors whose busy schedules keep them from donating as often as they would like."

Blood banks also could concentrate on persuading people with Type O blood, so-called universal donors, to do a double red cell donation, while the platelets or plasma may be more needed from people with other blood types.