How Low Can FIT Go?

Professor Callum G. Fraser Explores the Need for Low Faecal Haemoglobin Concentration Estimates

Faecal immunochemical tests for haemoglobin (FIT) are now widely used in asymptomatic screening for bowel cancer and in assessment of patients presenting with lower bowel symptoms. FIT might also be of value in other clinical settings such as surveillance of patients with previous bowel disease. Quantitative FIT provide estimates of the faecal haemoglobin concentration (f-Hb) for use in all of these.

It is now well documented that the f-Hb is related to the severity of bowel disease. Thus, lowering the f-Hb cutoff used to trigger clinical action confers better detection of bowel cancer, adenoma and other serious bowel pathology. The clinical sensitivity increases, but the positivity rises, the demand for colonoscopy increases, the number of false positive results rises and the positive predictive value falls.

Since a number of users of FIT in assessment of symptomatic patients see cancer detection as the most important role, there is considerable interest in using very low f-Hb as cut-offs. In addition, those wishing to use FIT as a rule-out test to reassure those patients who are unlikely to have significant bowel disease, are also interested in using very low f-Hb cut-offs, so as to minimise the chances of missing disease. So, the question for both approaches, is “how low can FIT go?”