Fecal occult blood (FIT)
A test on a stool sample that looks for tiny amounts of blood that cannot be seen with the eye. The modern version, the faecal immunochemical test (FIT), specifically detects human blood from the lower bowel, and is a key screening test for bowel (colorectal) cancer and its precursors.
What it measures
This test is performed on a small stool sample collected at home and looks for hidden ('occult') blood — amounts too small to change the colour of the stool. Cancers and pre-cancerous polyps in the bowel, as well as other conditions, can bleed a little intermittently, and this test aims to catch that. The modern method is the faecal immunochemical test (FIT), which uses an antibody to detect human haemoglobin specifically. FIT is more accurate than the old guaiac chemical test: it does not react to blood or peroxidases in food (so diet does not need to change), it targets bleeding from the lower bowel where colorectal cancer arises, and it gives a numerical result that can be set against a threshold. Its main role is screening: many national bowel-cancer programmes send FIT kits to people in a target age range, and a positive result leads to a colonoscopy to look for the source. FIT is also used to help decide who with bowel symptoms needs urgent investigation. A single negative test does not completely exclude cancer, which is why screening is repeated at intervals and why persistent symptoms are always taken seriously.
What a high value can mean
- A positive result means hidden blood was detected — it needs investigation, usually a colonoscopy.
- Colorectal cancer or a pre-cancerous polyp — the reason screening exists.
- Other bleeding sources — such as inflammation (colitis), diverticular disease, or haemorrhoids.
- A positive test is not a diagnosis of cancer — most positives turn out to be a benign cause, but all need to be looked into.
What a low value can mean
- A negative result is reassuring — no hidden blood was detected in this sample.
- It does not completely exclude cancer — bleeding can be intermittent, so screening is repeated and persistent symptoms are still investigated.
When to discuss with a doctor
Fecal occult blood testing with FIT is above all a screening tool: if you are invited to a national bowel-cancer screening programme, taking part is one of the most effective ways to catch bowel cancer early or prevent it by finding polyps. A positive result does not mean you have cancer — most positives have a benign cause — but it always needs to be followed up, usually with a colonoscopy, and this should not be delayed. FIT is also used to help triage bowel symptoms. Importantly, a negative test does not override worrying symptoms such as a change in bowel habit, blood in the stool, or unexplained weight loss, which still need medical assessment. Mediora.AI can highlight a positive result and show it alongside anaemia markers that may accompany bowel bleeding; acting on a positive FIT, and on persistent symptoms, is done with your doctor.