Being told you are anaemic, and that your iron stores are low, is unsettling in a particular way. It is rarely presented as an emergency, but it is clearly not nothing, and the investigation that follows can feel vague.
Here is where bowel testing fits into that picture, and, just as importantly, where it does not.
Why anaemia leads to bowel investigation
Iron deficiency anaemia in an adult usually means iron is being lost faster than it is replaced. The commonest route for that loss is the gut, through slow bleeding that is far too gradual to see. Over months, a small, invisible daily loss depletes iron stores.
Because that bleeding can come from a lesion in the bowel, iron deficiency anaemia is treated as a reason to investigate the gastrointestinal tract rather than simply to prescribe iron tablets and move on.
Does FIT work in this situation?
Yes. Early on there was caution about this, and clinicians were advised that FIT might be less accurate in people with iron deficiency anaemia and that results should be treated carefully.
Larger studies have since settled the question. At the 10 µg Hb/g threshold used in England, Wales and Northern Ireland, FIT has been found to have a sensitivity of around 96.7% in this group. That is a strong performance, and guidance now supports using FIT as part of the assessment.
The limitation that matters most here
This is the part to hold onto, because it is where FIT can mislead if it is over-interpreted.
FIT cannot rule out a problem in the upper gut.
The test recognises the globin protein in haemoglobin, and globin is broken down by digestive enzymes as it travels. Blood entering from the stomach or oesophagus is largely digested before it reaches the stool. Blood entering in the colon or rectum survives and is detected.
So a normal FIT is genuine reassurance about the lower bowel and says very little about anything higher up. Since upper gastrointestinal causes of iron deficiency are common, a normal FIT does not complete the investigation.
What a full work-up usually involves
FIT is one component. Depending on your age, sex, symptoms and blood results, assessment of iron deficiency anaemia may also include:
- Blood tests to confirm iron deficiency and look for other causes
- Coeliac screening, since coeliac disease impairs iron absorption
- Consideration of upper gastrointestinal endoscopy
- In women who menstruate, an assessment of whether menstrual loss explains the picture, though this should not automatically close the question
- Dietary assessment
What to do
If a blood test has shown iron deficiency anaemia, please see a doctor about the anaemia itself. A FIT test is a useful part of that assessment and a raised result would change things significantly, but it is not a way of dealing with anaemia on your own.
If you are already under investigation and want your own FIT figure to bring to those appointments, that is a reasonable thing to do, and knowing your actual number is more useful to whoever is looking after you than a positive or negative result would be.
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
- National Institute for Health and Care Excellence. Quantitative faecal immunochemical testing to guide colorectal cancer pathway referral in primary care. Diagnostics guidance DG56, 2023.
- Saw KS, Liu C, Xu W, Varghese C, Parry S, Bissett I. Faecal immunochemical test to triage patients with possible colorectal cancer symptoms: meta-analysis. British Journal of Surgery 2022;109(2):182 to 190. doi:10.1093/bjs/znab411
Figures are drawn from the sources above. Framing and commentary are the author’s own. Last reviewed August 2026.