IntroductionColorectal cancer is the fourth most common type of cancer and the second most common cause of cancer-related deaths in the UK. The full blood count (FBC) is a blood test that may play a role in early detection of the disease. Previous studies have aimed to identify how levels of individual components, such as haemoglobin, can be used to assist the diagnosis. We aim to systematically review studies to identify whether components of the FBC are risk factors for diagnosis of colorectal cancer, critically appraise the methods used to assess the association and assess performance of the components.Methods and analysisThe MEDLINE (via OVID), EMBASE (via OVID), CINAHL (via EBSCOhost) and Web of Science databases will be searched to identify studies reporting the association between the levels of at least one FBC component and the risk of a future diagnosis of colorectal cancer in undiagnosed individuals. Clincialtrials.gov and the WHO registry will be searched to identify relevant ongoing research. Search terms will include relevant Medical Subject Headings and Emtree headings, and free-text terms relating to FBC, colorectal cancer and diagnosis. No date or language restrictions will be applied. Two reviewers will independently identify the studies for inclusion and perform data extraction. Time intervals between the blood tests and diagnosis will form the subgroups for analysis.Ethics and disseminationThere is no direct patient involvement and only published articles will be reviewed; no ethical approval is required. Results from this review will set a foundation for intended future work on developing a new risk score for early detection of colorectal cancer, derived using FBC data. This systematic review will also provide guidance on the analysis of time to diagnosis. The model will be freely available to UK primary care practices.PROSPERO registration numberCRD42019134400.