The study said that Cologuard was an "inefficient screening option" for colorectal cancer given its cost as compared to other CRC screening methods.
Of the 27 companies in the index, 12 firms' share prices retreated last month, 14 companies' stocks increased, and one firm's share value was flat.
Financial analysts seemed convinced of the long-term advantages of the deal but noted that investors may remain skeptical in the near term.
The company's total revenues were $114.1 million compared with $95.6 million in the same quarter last year, beating analysts' expectations.
Exact said it will pay $72 per share for Genomic Health for a total transaction price of $2.8 billion in cash and stock.
Exact Sciences and the Mayo Clinic are working to develop a blood test for pancreatic cancer that might be able to diagnose patients with early-stage disease.
During its earnings call this week, the liquid biopsy firm said it is planning a 10,000-patient trial to test its Lunar assay for colorectal cancer screening.
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.
The firm recently submitted an application to the US Food and Drug Administration to expand Cologuard's label to the 45 to 49 age group.