Objective We evaluated whether people who had not completed a faecal immunochemical test (FIT) for colorectal cancer (CRC) screening would complete a blood-based testing option if offered one during health encounters. Blood-based screening tests for CRC could add to the total number of people screened for CRC by providing another testing alternative. Design Study participants were patients aged 45–75 years at a large, integrated health system who were offered but did not complete an FIT in the prior 3–9 months and were scheduled for a clinical encounter. Individuals were randomised (1:1) to be offered a commercially available CRC blood test (Shield, Guardant Health) versus usual care. We compared 3-month CRC screening proportions in the two groups. Results We randomised 2026 patients; 2004 remained eligible following postrandomisation exclusions (1003 to usual care and 1001 to blood draw offer; mean age: 60, 62% female, 80% non-Hispanic white). Of the 1001 allocated to the blood test g