D avid Agus’s first encounter with Elizabeth Holmes at a health conference came as a shock. The Holmes he met in the green room was personable and pleasant. The Holmes he interviewed on stage was altogether different.

“The accent changed: this deep voice, very serious,” Agus, a medical oncologist , recalls by phone from Los Angeles. “It was as if I had met a person and then interviewed a separate person. That transition from the green room to the stage – I had never experienced something like that before.”

Holmes is not like you or me. She was still a teenager when she founded Theranos, touting technology capable of performing medical tests using only a few drops of blood. She cultivated a distinctive persona: blond hair, an unblinking gaze, a husky voice and a black turtleneck worn in homage to Apple co-founder Steve Jobs .

Holmes mesmerised investors and board members including Rupert Murdoch, Larry Ellison, Carlos Slim and former secretaries of state Henry Kissinger and George Shultz.…