Competing interests: The BMJ has judged that there are no disqualifying financial ties to commercial companies. The authors declare the following other interests: SHB receives funding from Cancer Research UK (CanTest collaborative, [C8640/A23385]) for doctoral research on cancer diagnosis and serves on the executive committee of the Fabian Society, which is a think tank affiliated to the Labour Party. A full statement of interests for SHB is available at https://medicinehealth.leeds.ac.uk/medicine/staff/1211/dr-stephen-bradley. MEB carries out epidemiological and health services research to support earlier diagnosis of cancer but is not involved with research into biomarker tests. He has no other interests to declare. The BMJ policy on financial interests is here: https://www.bmj.com/sites/default/files/attachments/resources/2016/03/16-current-bmj-education-coi-form.pdf.