“The F.D.A. should not have permitted this out-of-date approach to be used for medical purposes,” Dr. King said. “Misleading, falsely reassuring results from their incomplete testing can cost women’s lives.”

How was the study conducted?

The study, which was presented at the American College of Medical Genetics and Genomics annual meeting and has not been peer reviewed, was built around more than 100,000 patients who underwent breast cancer risk testing with Invitae, a diagnostic company.

Despite the lack of peer review and Invitae’s potential role as a business competitor, genetic medicine experts not affiliated with Invitae said in interviews that they found the work to be credible, particularly as the company’s findings echo other, smaller studies.

23andMe’s test focuses on BRCA1 and BRCA2, genes involved in suppressing growth of abnormal cells. Specifically, it looks for three notorious genetic variants, known as founder mutations. Invitae’s analysts expanded their search to include thousands of other variants.

Dr. Susan Klugman, vice president for clinical genetics at the American College of Medical Genetics and Genomics, likened it to a broader spell-check. Whereas 23andMe looks for errors in a few paragraphs, the Invitae analysts used more advanced genetic technology to search through 25 chapters. (Dr. Klugman was not involved in the Invitae study.)

In about 5,000 subjects, analysts identified at least one variant known to significantly increase an individual’s risk of breast and ovarian cancer. Among the Ashkenazi Jews in the positive group, 81 percent had one of the three founder mutations, suggesting that 23andMe’s test could be helpful for them. Among the rest, 94 percent carried variants that would have failed to be detected by 23andMe.

Why create a test that misses so many people?

One reason is purely technical: To return to the literary metaphor, 23andMe isn’t set up to scan entire genetic books the way some labs are. So even if the company wanted to look for other variants, that would not be possible without changing its approach, said Dr. Robert C. Green, a professor at Harvard Medical School.