Overall, the share prices for 18 of the index's 27 companies fell month over month. Exact Sciences was the only firm whose share price improved in the double digits.
UBS initiated coverage of Quest with a Neutral raing and a $90 price target on its shares. It gave LabCorp a Buy rating and a $169 price target.
Just five of the 28 firms in the 360Dx Index saw their share prices rise last month. Veracyte led the gainers, while Accelerate Dx had the sharpest decline.
Quest is expected to see incremental volume growth from the UnitedHealthcare contract and from a potential rise in M&A activity with outreach labs.
Overall, 20 of the 25 firms in the index saw their share prices grow in May, including seven whose stocks rose more than 20 percent compared to April.
The investment bank previously had a Market Perform rating for the company, and it increased the price target on T2 Bio's stock to $12 from $5 per share.
The investment bank cited disappointing first quarter financial results, a slow uptake to the business, and competitive pressures for the downgrade.
The 360Dx Index was down 3 percent in February, reflecting similar losses in the broader markets. GenMark Diagnostics and Opko Health were the biggest decliners.
The investment bank gave the company's stock an Outperform rating with a target price of $90 per share.
The stock prices of 17 of the 25 firms in the 360Dx Index fell month over month with Enzo Biochem and Bio-Rad leading decliners.