Cancer loyalty card study-2 (CLOCS-2): protocol for an observational case-control study focusing on the patient interval in cancer diagnosis

Introduction Some cancers are diagnosed late, making them harder to treat. People with an undiagnosed cancer may use over-the-counter medications to manage non-specific cancer-related symptoms that often mimic other more common, easily treatable conditions.

Results from the original Cancer Loyalty Card Study (CLOCS) suggest there may be an increase in purchases of pain and indigestion medication 8 - 9 months before an ovarian cancer diagnosis. We aim to validate the CLOCS findings by exploring whether a significant change in medication purchases could be an indication for early signs of the following cancer types: oesophageal, stomach (gastric), colorectal (bowel), pancreatic, liver, bladder, endometrial, uterine sarcoma, ovarian and vulval, using data collected through store loyalty cards.

Methods and analysis Using a retrospective case-control design, we aim to recruit 1450 participants with one of the cancers of interest (cases) and 1450 participants without cancer (controls) in the UK who (or whose household members) hold a loyalty card with at least one participating high street retailer. We will use pre-existing loyalty card data to compare past purchase patterns of cases with those of controls.