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.
This year's Breakthrough Prize winners include a pair that developed a therapy for spinal muscular atrophy.
The New York Times reports on how white supremacists misconstrue genetic research, concerning many geneticists.
Researchers find that people's genetics influence their success at university, but that it is not the only factor.
In Nature this week: approach to identify genetic variants that affect trait variability, application of read clouds to microbiome samples, and more.