If LabCorp was to acquire PPD, the transaction's value would exceed the $5.7bn for which it purchased Covance in 2015.
PPD offers contract research services from inhalation products to biologics, for a wide range of indications, with headquarters in Wilmington, North Carolina.
The firm has more than 18,500 employees worldwide and is owned by Carlyle Group LP and Hellman & Friedman LLC, which made the company private in 2011 for $3.9bn.
LabCorp is a North Carolina, US-based clinical lab network with a $14.3bn (€13.3bn) market cap. Last month, it acquired Mount Sinai’s clinical outreach laboratories, adding seven patient service centers to its network.