 Shares of Guardant Health Inc. tanked 40 and shares of Exact Sciences Corp. rallied more than 25 in the extended session Thursday after Guardant announced results from a study using its blood test to screen for colorectal cancer that didnt appear to surpass Exact Sciences stool-sample screening test. Guardant said that the results pave the way for first potential FDA-approved and Medicare-reimbursed blood test for colorectal cancer screening, and that it plans to ask for U.S. Food and Drug Administration approval in early 2023. 