Most people doing a bowel test at home have a vague sense that it is “looking for blood.” That is right, but the detail is more interesting than that, and knowing it explains both what the test is good at and where it falls short.
It detects one specific protein
Haemoglobin is the molecule that carries oxygen around your blood and makes it red. It has two parts: a haem group, and a protein called globin.
FIT uses antibodies that recognise human globin specifically. Not blood in general. Not iron. Human globin.
That single design choice produces two consequences that shape the entire test.
Consequence one: you do not need to change your diet
The test FIT replaced, the guaiac faecal occult blood test, detected peroxidase activity, which is present in blood but also in red meat and various vegetables. Patients had to avoid certain foods, avoid vitamin C, and often produced false positives anyway.
Because FIT recognises human globin, none of that applies. There is no dietary preparation for a FIT test. Eat what you like. Steak the night before makes no difference.
Current guidance also states there is no need to stop or adjust medication before testing. If you take anticoagulants or anti-inflammatories, keep taking them, but do mention them so whoever interprets your result has the full picture.
Consequence two: it points at the lower bowel
Globin is a protein, and proteins get broken down by digestive enzymes as they travel through the gut.
Blood entering high up, from the stomach or oesophagus, is largely digested before it reaches the stool. Blood entering lower down, in the colon or rectum, survives the journey and is detectable.
This makes FIT relatively specific to the lower bowel, which is a strength when you are asking about colorectal cancer. But it is a genuine limitation to understand: a normal FIT tells you very little about your stomach or oesophagus. If you are being investigated for iron deficiency anaemia, for example, FIT is one part of the picture and cannot be the whole of it.
What raises a FIT result
The obvious answer is colorectal cancer, and that is what the test is designed around. But several other things raise it:
- Adenomas, particularly larger ones, which are the polyps that can eventually become cancer
- Inflammatory bowel disease, although the test is less sensitive for this than for cancer
- Other bleeding lesions in the colon and rectum
There is also a developing line of research suggesting that raised faecal haemoglobin without an obvious bleeding source may reflect low-grade inflammation of the bowel lining. Studies have found that people with a raised FIT go on to be diagnosed with inflammatory bowel disease at higher rates than those without.
That is worth knowing for one practical reason: it is another argument against explaining a raised result away.
What it is not
FIT is a test for the colon and rectum. It is not a general cancer test.
Several cancers produce symptoms that overlap with bowel cancer, including ovarian, pancreatic, gastro-oesophageal, urological and lung cancer. FIT does not assess any of them. If your symptoms could point in one of those directions, a normal FIT is not the end of the conversation.
The short version
FIT measures human haemoglobin in stool, needs no dietary preparation, and is good at detecting bleeding from the lower bowel. It is not a whole-gut test, not a general cancer test, and not a substitute for being examined when something needs examining.
Within those limits, it is a genuinely excellent test, and better at identifying who needs urgent investigation than any symptom or combination of symptoms.
References
- Monahan KJ, Davies MM, Abulafi M, et al. Faecal immunochemical testing (FIT) in patients with signs or symptoms of suspected colorectal cancer: a joint guideline from the Association of Coloproctology of Great Britain and Ireland (ACPGBI) and the British Society of Gastroenterology (BSG). Gut 2022;71:1939 to 1962. doi:10.1136/gutjnl-2022-327985
- Westwood M, et al. Faecal immunochemical tests to triage patients with lower abdominal symptoms for suspected colorectal cancer referrals in primary care: a systematic review and cost-effectiveness analysis conducted to inform NICE DG30. BMC Medicine 2017;15:189.
- Association between raised faecal haemoglobin and later immune-mediated inflammatory disease: analysis of one million national screening results. BMC Medicine 2022;20:203.
- National Institute for Health and Care Excellence. Quantitative faecal immunochemical testing to guide colorectal cancer pathway referral in primary care. Diagnostics guidance DG56, 2023.
Figures are drawn from the sources above. Framing and commentary are the author’s own. Last reviewed August 2026.