Sarah Kliff and Artist Bhatia:

Some said the blood screenings that look for the rarest conditions are good for little more than bolstering testing companies’ bottom lines.

“It’s a little like running mammograms on kids,” said Mary Norton, an obstetrician and geneticist at the University of California, San Francisco. “The chance of breast cancer is so low, so why are you doing it? I think it’s purely a marketing thing.”

There are few restrictions on what test makers can offer. The Food and Drug Administration often requires evaluations of how frequently other consequential medical tests are right and whether shortfalls are clearly explained to patients and doctors. But the F.D.A. does not regulate this type of test.

Alberto Gutierrez, the former director of the F.D.A. office that oversees many medical tests, reviewed marketing materials from three testing companies and described them as “problematic.”

“I think the information they provide is misleading,” he said.