Even as the region's blood supply has improved slightly, the availability of two particularly important types of blood for many District residents has fallen to serious levels.

"We are in critical need of O and B," said Tracy M. Laubach, director of marketing and communications for the regional Red Cross.

Type O negative is considered a universal type that can be received by almost everyone. Black people are more likely to have Type B than other types of blood, which makes its shortage in the District, with its high percentage of African Americans, especially troublesome.

Supplies of other types of blood are starting to rise above the critical level, but for O and B, Laubach said, "we're shipping it out as fast as we can bring it in."

So far, that seems to be fast enough, at least for Howard University Hospital, on Georgia Avenue NW. "We're fine. We get our blood from the Red Cross. Occasionally we have blood drives, but not recently," said Ann Chisholm, marketing and public relations director at Howard University Hospital.

Although there has been a slight improvement in the region's blood supply over the last two weeks, both the Greater Chesapeake and Potomac Region of the American Red Cross and Inova Blood Donor Services, which are the two major blood suppliers in the Washington region, have issued warnings and appeals for donations.

Inova Blood Donor Services now offers each donor a $10 gasoline card as a thank-you gift. The giveaway will continue through the end of the month.

Officials of the groups said summer is always hard on the blood supply, as many potential donors go on vacation. But this year has been particularly difficult, Laubach said. She said that a yearly drop-off that occurs just before the Fourth of July holiday hit earlier this year and that unlike in past years, giving has remained slow.

"It's put our inventory at a level that is just not safe," she said. "People are just busy. They're on vacation, they've got family in town and schools are out of session. A huge portion of our supply comes from high schools and colleges."

The regional American Red Cross group, one of 35 across the country, supplies blood to 65 hospitals across Maryland, Washington, Northern Virginia and parts of Pennsylvania, including 10 trauma centers. Inova Blood Donor Services gathers blood for an additional 15 hospitals, two with trauma centers, including Northern Virginia's Inova Health System hospitals and clinics.

Laubach said the Red Cross depends on 1,100 to 1,300 donors every day, but has been getting about 900. Inova needs 200 donors a day but has been getting only about 60, said Linda Wilson, the service's donor recruitment manager.

"For each day you fall a couple hundred short, and then a couple hundred short the next day, it really adds up over time," Laubach said.

The low supply is endangering both groups' ability to meet their hospitals' routine blood requirements for trauma and surgical patients as well as premature babies and others who might need transfusions, officials said.

The shortage also means, they said, that the region is ill-prepared for a terrorist attack such as the subway and bus bombings in London on July 7 that killed more than 50 people. They said it takes about 48 hours to screen, package and label donated blood, meaning that hospitals cannot rely on instant donations to fulfill immediate blood needs during an emergency.

"The blood needs to be there before the need," Inova's Wilson said. Compounding the problem is that blood banks nationwide are experiencing the same shortages, she said, so there is no one to whom the Washington region can appeal for more blood -- other than local residents.

Wilson said she hoped the gas-card incentive would entice first-time donors as well as past donors. Though it is safe to give blood as often as once every eight weeks, the average donor gives 1.6 times a year. Less than 5 percent of eligible donors give at all.

Both organizations strongly prefer that donors make an appointment before giving blood, but they can accommodate walk-ins. More information about giving blood, including contact information for donor centers, can be found by calling the American Red Cross at 800-GIVELIFE or Inova Blood Donor Services at 866-BLOODSAVES.

Staff writer Carrie Donovan contributed to this report.