The ratings agency also assigned a Negative Rating Watch to the firm's notes offering, noting it will resolve this notation as more details of the offering become available.
The rating agency said it expects that the company will use the proceeds from the offering to finance its $4.2 billion acquisition of FEI.
Fitch has given the two-year senior notes offering a BBB rating with a Stable outlook.
The 10-year notes were rated BBB by Fitch, which also declared the ratings outlook Stable.
The firm plans to use the proceeds, along with cash on hand, to redeem its outstanding $1 billion senior notes that mature August 15.
The rating agency said it expects the company to use the proceeds to redeem the $1 billion principal amount of its 2.25 percent senior notes due in August.
The ratings agency has affirmed BBB ratings for Thermo Fisher's long term issuer default rating, senior notes, and bank revolving credit facility.
Fitch upgraded PerkinElmer's issuer default rating to BBB and cited the firm's improved operating profit and the potential for M&A.
Fitch noted Quest's leadership position in the clinical lab space and its operational improvement, but said that the company's debt leverage is elevated relative to its rating.
Fitch said the offering will temporarily raise Quest's debt leverage in 2015, but it anticipates Quest's total debt balances to be reduced by the end of the year.
In PNAS this week: carbapenem resistance in Enterobacteriaceae, selection against educational attainment-linked variants, and more.
Technology Review points out that a new US presidential science advisor hasn't been selected.
Researchers find that blood tests might be able to help determine severity of a concussion, Wired reports.
President-elect Donald Trump considers other candidates for director of the National Institutes of Health, Nature News reports.