If your result comes back above the threshold, the word your mind reaches for is cancer. That is understandable, and it is usually wrong.
Most people with a raised result do not have bowel cancer. It is worth knowing what else can produce one, partly for reassurance and partly because some of the alternatives matter in their own right.
What the test is actually measuring
A stool test measures blood in your sample, in quantities far too small to see. Anything that bleeds into the lower bowel can raise it.
That includes:
Polyps. Small growths on the bowel lining. Most are harmless, but a proportion can become cancerous over years, which is why finding and removing them is worthwhile. In one large study, lowering the screening threshold in early adopter sites turned up nearly 500 additional high-risk polyps alongside 60 extra cancers.
Inflammatory bowel disease. Crohn’s disease and ulcerative colitis both inflame the bowel lining and can cause it to bleed. Interestingly, research has found that people with a raised result go on to be diagnosed with inflammatory bowel disease at higher rates than those without, even when no bleeding source is found at the time.
Diverticular disease. Small pouches in the bowel wall, very common with age, which occasionally bleed.
Ulcers and other lesions in the colon or rectum.
Where the test is weaker
This is the part that is less often explained, and it matters.
A stool test is very good at detecting bowel cancer. It is less reliable for other bowel conditions, including inflammatory bowel disease and larger polyps. So while a raised result can point towards them, a normal result does not rule them out.
If inflammatory bowel disease is a genuine possibility, a different test called calprotectin is the better tool. It measures inflammation rather than blood. In practice, doctors sometimes request both at the same time, and national guidance acknowledges this happens in real-world primary care.
What it says nothing about
Two important blind spots.
The upper gut. Blood from the stomach or oesophagus is broken down by digestive enzymes before it reaches your stool. A normal result tells you very little about anything above the small bowel. This matters particularly if you are being investigated for iron deficiency.
Other cancers entirely. Several cancers produce symptoms that overlap with bowel cancer, including ovarian, pancreatic, gastro-oesophageal, urological and lung. A stool test assesses none of them. If your symptoms could point in one of those directions, a normal result does not settle the question.
Why this is a reason to talk to someone, not to relax
There is a version of this article that ends “so a raised result probably isn’t cancer, don’t worry.” That would be the wrong conclusion.
The right one is that a raised result has several possible explanations, most of them less serious than cancer, and the only way to know which applies to you is to investigate rather than guess. Several of the alternatives are worth finding in their own right. Polyps removed today are cancers that never happen. Inflammatory bowel disease diagnosed early is far easier to control.
The one thing not to do is pick the most comfortable explanation and stop there.
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 ACPGBI and BSG. Gut 2022;71:1939 to 1962.
- National Institute for Health and Care Excellence. Quantitative faecal immunochemical testing to guide colorectal cancer pathway referral in primary care. Diagnostics guidance DG56, 2023.
- NHS England. NHS to detect and prevent thousands more bowel cancers with more sensitive screening, January 2026.
- Positive faecal immunochemical test and subsequent onset of inflammatory bowel disease: nationwide propensity score-matched study.
Figures are drawn from the sources above. Framing and commentary are the author’s own. Last reviewed August 2026.