Theranos, the embattled blood-testing company, announced on Wednesday that it would close its laboratory operations, shutter its wellness centers and lay off about 340 employees, around 40 percent of its work force.

In a letter posted to the company’s website, Elizabeth Holmes, the company’s founder and chief executive, said the company would now focus on an initiative to create miniature medical testing machines.

In making the announcement, Ms. Holmes, who founded Theranos as Silicon Valley’s answer to conventional blood testing, completed the pivot she first proposed this past August. Addressing the annual meeting of the American Association for Clinical Chemistry, Ms. Holmes chose not to defend the company against widespread skepticism over its testing methods, which had promised to use a finger prick of blood at a fraction of the costs of traditional tests. She instead said that the company would develop miniature laboratories, or miniLabs, small devices that could be placed on a table in a doctor’s office and be used to perform numerous medical tests.

On Wednesday, Ms. Holmes said Theranos would “return our undivided attention to our miniLab platform,” which she described as the production of “miniaturized, automated laboratories capable of small-volume sample testing.”