The story says blood-testing firm Theranos – headed by 31-year-old Elizabeth Holmes, who is reputed to be the world’s youngest self-made billionaire – doesn’t use its own technology for many of its tests. The story also raises questions about the accuracy of the testing technology.

Holmes has achieved rock-star status since Fortune put her on the cover in June of last year. Her company is valued at $9 billion – she owns roughly half – and her board includes the likes of former Secretaries of State Henry Kissinger and George Schultz. She has touted her technology as being able to process “a full range of blood tests from a few drops of blood.”

The Journal story was written by John Carreyrou, who is an experienced investigative reporter specializing in medical issues. I have no independent information on the story, but can vouch for the quality of Carreyrou’s work.

Holmes declined comment to the Journal, but overnight tweeted a link to her web site defending her testing technology. Her high-powered lawyer, David Boies, told the Journal the transition to doing all tests with the company’s new technology was “a journey.”

As the economy has slowed, speculation about the fate of unicorns – the roughly 130 venture-backed start ups with valuations exceeding $1 billion – has grown. Fortune’s Erin Griffith reported recently that many venture capitalists keep “dying unicorn” lists, although they won’t reveal which companies are on them.

There’s a good chance Theranos now joins that herd.

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