May 14 (Reuters) - Activist investor Jana Partners built a new stake in U.S. diagnostics company QuidelOrtho during the first quarter, sources familiar with the hedge fund's position told Reuters.

It is unclear whether the hedge fund will seek changes at the company, which makes the popular QuickVue COVID-19 test for home use, and it could not be determined how large Jana's stake is.

A representative for Jana declined to comment and a representative for QuidelOrtho could not be immediately reached for comment. By building the position however, the hedge fund is signaling its interest in the company and that it has already committed significant resources to what could become an activist campaign.

The stake could be revealed publicly when Jana makes its 13F filing, which requires fund managers to detail their ownership in publicly traded U.S. companies for a given three-month period.

QuidelOrtho currently has a market valuation of roughly $2.8 billion and its stock price has tumbled more than 50% in the last 52 weeks. Since January the stock price has dropped 44%.

The San Diego-headquartered company fired Chief Executive Officer Douglas Bryant, who had led the company for roughly 15 years, in February after fourth quarter earnings and revenue fell, in part because COVID testing had dropped off. This month it appointed Brian Blaser as president and CEO.

Jana has a 23-year track record of investing in companies, including Frontier Communications, which recently launched a formal strategic review following pressure and Freshpet, which has generated a return of approximately 250% in the two years since Jana began engaging with the company.

Before that, Jana pushed for changes at companies such as Whole Foods Market, which sold itself to Inc (AMZN.O).

(Reporting by Svea Herbst-Bayliss; Editing by Josie Kao)