Jean-Louis Gassée, former head of Apple engineering in the late ’80s, wrote about his personal experience using the technology of Theranos, a multibillion dollar health care startup that aims to revolutionize blood testing. Last week a major controversy arose over the company’s claims.

Gassée has a condition that requires him to get his blood checked often. He was attracted to Theranos because it markets itself as an inexpensive and painless alternative to regular blood tests. After visiting the hematology lab at Stanford University, Gassée went to a Walgreens WAG store to try out the Theranos method. The results he received were significantly different than what he saw at Stanford.

Gassée continued his experiment by returning to both locations the next day to test his blood. The Stanford results had changed, but not significantly so. The Theranos results, on the other hand, were strikingly different from both the Stanford results and the previous day’s Theranos results.

Curious about the reliability of this technology, Gassée wrote a letter to Theranos founder Elizabeth Holmes, stating his findings and saying, “I find the price and convenience of Theranos services attractive, but I worry about the reliability of the important HCT number,” referencing the “hematocrit” number, which measures the volume percentage of red blood cells in the blood. He asked about the company’s methodology, standards, and quality controls, and gave Holmes a chance to respond before publishing an account of his experience. He never received a response, he said.

Theranos is currently under fire following a Wall Street Journal article in which four former employees pose allegations against the company, including that it was not returning accurate results to patients. Theranos has since responded, calling the claims “factually and scientifically erroneous and grounded in baseless assertions by inexperienced and disgruntled former employees.”

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