Cross-sectional study of cancer diagnostic delay rates associated with a population-based screening trial evaluating a cell-free DNA multicancer early detection test

Clinical trials evaluating population-based screening tests or other interventions likely to affect care delivery in real-world settings often do not consider spillover effects, such as whether the intervention rollout affects access to limited health care services. The NHS-Galleri trial evaluated in England evaluated a cell-free DNA-based multicancer early detection (MCED) test. A cross-sectional study examined whether regional participation in NHS-Galleri was associated with changes in cancer diagnostic delay rates. In this difference-in differences analysis, diagnostic delay rates for suspected head and neck, lung, and upper gastrointestinal cancers increased by 3.4 percentage points in the 8 cancer alliance regions in England participating in NHS-Galleri compared with 13 nonparticipating regions during the first 6 months of the trial. These differences persisted through 12 months, after which they were no longer statistically significant. The authors conclude that trials evaluating population-based MCED screening likely to affect demand for limited health care resources may be associated with system-level spillover effects.