I still remember reading the early WSJ reporting by John Carreyrou that was questioning the validity of the blood tests. People would get a Theranos test for something and it would say they are fine. Then they would suffer a health emergency and need to be hospitalized where they would get another non-Theranos blood test that would show they were decidedly not fine. The variance between the two tests was quite significant in some cases for metrics where the acceptable margin of error was quite small.
It would later be discovered of course that Theranos' touted devices could not do what she claimed. And the company would often simply use traditional devices for testing.
An early skeptic of Theranos was epidemiologist John Ioannidis, who has unfortunately beclowned himself on the coronavirus pandemic. He wrote a JAMA article questioning the lack of peer-review of their work.