Blood supplies across the region have dipped to critically low levels, donor services reported last week, the result of an annual summer slowdown in giving that is more severe this year.

Officials for both the Greater Chesapeake and Potomac Region of the American Red Cross and Inova Blood Donor Services said blood reserves have dropped well below the one-day supply mark.

Both groups, which are the major blood suppliers in the Washington region, are putting out the warning and appealing for donations. Inova Blood Donor Services has been offering each donor a $10 gas card as a thank-you gift since Thursday. 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, said Tracy M. Laubach, director of marketing and communications for the regional Red Cross. 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 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 the 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.

A gas-card giveaway July 4 resulted in record giving for the Inova service. On that day alone, donors gave 197 units, when in the past the group had never received more than 82 units on July 4. But the increased supply, Wilson said, was used up within days.

The cards, which are redeemable at Shell stations, seemed a particularly good gift, since gas prices are rising, she said.

"People came because we gave them an extra reason to put that on their list of things to do," Wilson said. "We try to watch the news and key in with thank-you gifts to something that's relevant, something that will help them."

Both organizations strongly prefer that donors make an appointment before giving blood but 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.

Inova Blood Donor Services is giving $10 gas cards to donors until the end of July.