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 group, 924 were recruited following chart-review exclusions; 548 (59.3%) were reached via phone, of which 280 (51.1%) scheduled an appointment with the research team. CRC screening proportions were 17.5 percentage points higher in the blood test group versus usual care (30.5% vs 13.0%; OR 2.94, 95% CI 2.34 to 3.70; p<0.001).

Conclusion Among adults who had declined prior CRC screening, the offer of a blood-based screening test boosted CRC screening by 17.5 percentage points over usual care. Further research is needed on how to balance the favourable adherence with lower advanced adenoma detection compared with other available tests.

Trial registration number NCT05987709.