Most American men over 50 get an annual blood test called a PSA -- prostate-specific antigen -- that is thought to indicate whether they may have prostate cancer. On a scale of zero to 10, doctors had thought that the level four or below was safe. Now researchers report in the New England Journal of Medicine that even men with a PSA below four may have a substantial risk of cancer.

The research was part of a $73 million study funded by the National Cancer Institute. It enrolled almost 19,000 men over 55. An analysis found that men with a PSA level formally considered safe still had a 15-percent risk of prostate cancer.

Study co-author Dr. Ian Thompson, of the University of Texas in San Antonio, said the most worrisome finding was that aggressive, or "high-grade," cancers were found at all levels of PSA; the researchers were unable to find a level of PSA below which there was no risk of prostate cancer. NPR's Richard Knox reports on the implications of the study.

Copyright 2022 NPR. To see more, visit https://www.npr.org.