ORLANDO, Fla. – A common blood-scanning test for prostate cancer, an annual medical ritual for millions of older men, can safely be done less frequently for the majority who have low readings, a major study concludes.

The federally funded study concludes the risk is so low for these men that checking once every two years, or even every five years, is enough to find cancer in time.

About half of all men over age 50 now get a regular PSA test, which scans the blood for a protein that goes up as prostate cancer develops.

Researchers said that if their new guidelines are followed, the less-frequent testing would cut the number of PSA tests each year in half, saving between $500 million and $1 billion.

Dr. E. David Crawford of the University of Colorado, who directed the analysis, presented the results Monday at a meeting in Orlando of the American Society of Clinical Oncology.

“When we first thought about screening, we thought it should be done every year,” he said. “We had no data to suggest otherwise. Now we have data that lets us step back.”

Doctors generally recommend further testing if men’s PSA levels rise above 4 nanograms per milliliter of blood. The new study concludes:

. If a man’s initial PSA reading is between zero and 1, he can wait five years before having another test.

. If the reading is between 1 and 2, he can wait two years before another test.

. Those whose readings are between 2 and 4 should continue to have annual tests.

The study was designed primarily to ask a bigger question: Does early detection of prostate cancer save lives? The answer is not known yet.