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Theranos founder Elizabeth Holmes leaves federal court, in San Jose, Calif., on Nov. 22.BRITTANY HOSEA-SMALL/Reuters

Theranos founder Elizabeth Holmes testified in her fraud trial on Tuesday that she did not intend to misrepresent the work the now-defunct blood testing startup did with Pfizer Inc. by adding the drugmaker’s logo to a report Theranos had prepared.

Ms. Holmes took the stand for a third day to defend herself from accusations of lying about Theranos, which had touted technology that could run diagnostic tests faster and more accurately than traditional lab testing with a drop of blood from a finger prick.

Prosecutors have suggested Ms. Holmes misrepresented its work with pharmaceutical companies Pfizer and Schering-Plough by adding the company logos in an attempt to pass off Theranos’s conclusions as theirs.

Ms. Holmes admitted that she added the logos to the reports just before sending them to pharmacy operator Walgreens, which was discussing a partnership with Theranos in 2010, to convey the drugmakers’ involvement in promising studies using Theranos technology.

“I wish I had done it differently,” Ms. Holmes said.

Holmes also testified that she did not conceal the addition from Pfizer, showing jurors an e-mail where the report with the logo was sent to individuals at Pfizer in 2014.

Ms. Holmes, 37, previously spent about 2 1/2 hours on the stand at the courthouse in San Jose, Calif., discussing Theranos’s technology and its positive performance in early studies.

Once valued at US$9-billion, Theranos collapsed after the Wall Street Journal published a series of articles starting in 2015 that suggested its devices were flawed and inaccurate.

Ms. Holmes’s decision to testify is risky as it exposes her to a potentially tough cross-examination by prosecutors.

The trial has shone a spotlight on Silicon Valley start-ups, which often attract high valuations based on promises of future success rather than actual revenue and profit.

At the beginning of the trial, Ms. Holmes’s lawyer told the jury that Ms. Holmes was a hardworking entrepreneur and that the failure of her business was not a crime.

Over the two-month trial, jurors have heard testimony from more than two dozen prosecution witnesses, including patients and investors whom prosecutors say Ms. Holmes deceived.

Ms. Holmes has pleaded not guilty to nine wire fraud counts and two conspiracy counts.

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