Trends in the full blood count blood test and colorectal cancer detection: a longitudinal, case-control study of UK primary care patient data.

Colorectal cancer is a common type of cancer in the UK. It is the second most common cause of cancer-related death in the UK. Chances of surviving depend heavily on the tumour stage at diagnosis, which represents how much the tumour has developed. If diagnosed and treated at the earliest stage, where the tumour is confined to the colon, nine in 10 patients are expected to be alive five years later. If diagnosed at the latest stage, when the cancer has spread outside the colon, this drops to one in 10 surviving. The majority of UK patients with colorectal cancer are diagnosed with late-stage tumours, so are likely to die. Detecting and treating the cancer earlier can save lives. There is a blood test called the Full Blood Count, which is commonly ordered by doctors for many reasons. This test includes many blood levels, such as haemoglobin, which carries oxygen around the body. Growing tumours cause subtle changes in the blood levels over time, but it is unclear what these changes are and if they could help find cancer in the early stages. In our study, we looked at blood tests from almost one million patients in the UK, including around 17,000 with colorectal cancer. We checked how blood levels change over 10 years before diagnosis. We found that in the few years before patients are diagnosed, patients usually had blood levels that rapidly started increasing or declining (depending on the blood level) and this was often not seen in patients without colorectal cancer. Our study highlights that using trends over time in blood test results may be useful to identify colorectal cancer. Such trends could facilitate earlier detection because they were present for years before diagnosis. That would improve the chances of successful treatment and chances of survival.