WASHINGTON — Blood banks look at the nation's Hispanic population boom and see the potential to save thousands of lives: the possibility of more so-called universal blood donors.

Latinos are more likely than the average American to have Type O blood, the type blood banks value most because it can be used by so many patients.

That potential so far is mostly untapped. Now blood banks are beginning major efforts to boost Latino donations, amid concern that the blood supply could face a serious imbalance if population shifts bring more demand for Type O transfusions without a parallel increase in giving.

"It's a great opportunity we have," said Dr. Alexander Indrikovs, blood bank director at the University of Texas Medical Branch in Galveston, who outlined the issue at a recent meeting of leading blood bankers.

On the other hand, he added, "It's obvious that if we continue to target only the traditional blood donation group, which has been mostly Caucasian, the imbalance is going to grow larger."

Faced with a tight supply and an aging donor pool, blood banks have long struggled to increase the nation's overall stocks. While 60 percent of the U.S. population is eligible to donate blood, just 5 percent does.

More than 34,000 pints of blood are needed every day, and demand is rising annually. Spot shortages that force hospitals to postpone elective surgeries occur in various cities every year, usually around holidays.

There are four major blood groups: A, B, AB and O. A and B are antigens, essentially markers on blood cells that the immune system can target. Blood labeled "O" lacks those antigens. Blood also is designated as either Rh-positive or Rh-negative.

And while blood banks want donors of all types, Type O-positive blood is especially valuable. It is the most common blood type, which means it's most in demand — and if you have Type O blood, only a Type O transfusion will work for you. If you were accidentally given another type, your immune system would spot the antigens and attack.

However, people with other blood types can receive Type O blood in an emergency. That's why Type O's are called universal donors and why blood banks make a concerted effort to recruit them, both O-positives and the fairly rare O-negatives.

About 45 percent of the general U.S. population has Type O blood, a proportion that varies by race and ethnicity. Estimates suggest roughly 37 percent of whites are O-positive, and 47 percent of blacks.

Blood banks have long thought Hispanics had even higher Type O rates. Indrikovs surveyed major blood banks in parts of Latin America to try to nail that down — and found O-positive donors ranging from 54 percent in Venezuela to 62 percent in Guatemala and 71 percent in Mexico.

"We have legally millions of people from Latin America living in the U.S. We need to get them on board to be regular blood donors," said Indrikovs, himself a native of the Dominican Republic.

To target them, blood banks are turning to Spanish-language media, reaching out to new donors.