As many as one in every 5,000 people who undergo major surgery in the United States may become infected with the AIDS virus from tainted blood that slips through the screening process, a new computer analysis estimates.

"It's alarming," said Dr. Allan M. Salzberg, chief of medical service at the Veterans Administration Medical Center in Miles City, Mont., who performed the analysis.In areas where AIDS is more common, the risk may be as high as one out of every 500 to 1,000 people who require large amounts of donated blood, according to the analysis.

Salzberg said he computed the risk for being infected with the AIDS virus from donated blood using a computer model he developed of the AIDS epidemic. He released his findings Wednesday in a letter in The New England Journal of Medicine.

The risk, Salzberg said, stems from the fact that tests used to screen donated blood for the AIDS virus fail to pick up all infected blood because people do not produce antibodies to the deadly virus for at least several weeks after they have been infected.

Salzberg's model computed that up to 7 percent of people carrying the AIDS virus will be in the "window" period before antibodies to the virus can be detected with the most common screening test.

Based on that finding and the risk of becoming infected after receiving infected blood, Salzberg estimated that as many as one in every 5,000 to one in every 10,000 people who need a large amount of blood due to major surgery would become infected with the virus nationwide. In parts of the country like New York, where AIDS is more common, the risk is about 10 times as high, Salzberg said.

Based on the findings, Salzberg urged the Food and Drug Administration to make available to blood centers tests that can detect the presence of the AIDS virus earlier.

He also recommended that everyone who receives a transfusion be tested three to six months later to see if they have been infected with the virus and to try to limit the spread of the disease.