Quick answer: The faecal immunochemical test (FIT) detects tiny amounts of human blood in a stool sample that you cannot see. It is used in bowel cancer screening programmes and, increasingly, by doctors to decide who needs a colonoscopy when someone has bowel symptoms. A positive (abnormal) result does not mean you have cancer: bleeding can also come from polyps, haemorrhoids, fissures or inflammation, and most people with a positive result do not have cancer. It does mean you need further investigation without delay. A negative result is reassuring but does not rule everything out, so symptoms that persist should always be reviewed by your doctor.
FIT is one of the stool tests people bring to sessions most often, sometimes from a screening letter, sometimes requested by their GP for a change in bowel habit. It is a simple test with an important job, and it is easy to misread in both directions. It sits alongside other stool markers such as faecal calprotectin, which looks at inflammation rather than blood.
What is a FIT test and what does it measure?
FIT uses antibodies that recognise human haemoglobin, the protein in red blood cells. Because it reacts to human blood specifically, there is usually no need to change your diet beforehand, unlike older guaiac-based tests. You collect a very small sample of stool at home with a stick and return it to the lab or pharmacy. Many labs report a number (micrograms of haemoglobin per gram of stool) rather than just positive or negative.
What does a positive FIT result mean?
A positive result means blood was found above the lab's threshold. Possible sources include haemorrhoids, small tears (fissures), polyps, inflammation of the bowel such as colitis, and, less commonly, bowel cancer. The test cannot tell which one, so the usual next step is a referral for a colonoscopy or another examination. Please do not delay this, even if you feel well or think it is "just piles".
When to see your doctor first
- Any positive FIT result: follow your doctor's referral without delay
- Visible blood in or on the stool, or black, tarry stools
- Unintentional weight loss
- Iron-deficiency anaemia or unexplained tiredness
- A new, persistent change in bowel habit, especially after 50
- A lump in the abdomen or persistent abdominal pain
- A family history of bowel cancer or inflammatory bowel disease
Can a negative FIT test be wrong?
FIT is good at picking up bowel cancer in people with symptoms, which is why it is used to help prioritise who needs a colonoscopy. But no test is perfect. Some cancers and many polyps bleed only now and then, so a single sample can miss them, and studies have looked at whether repeating the test improves detection. A negative result with ongoing symptoms, especially visible bleeding, weight loss or anaemia, still needs to be discussed with your doctor.
Why would a doctor request a FIT test for bowel symptoms?
In the UK and parts of Europe, FIT is now widely used when people see their GP with a change in bowel habit, unexplained abdominal pain, weight loss or anaemia. It helps doctors decide how urgently someone needs further tests. Research in people with "low-risk" symptoms has supported this approach. A FIT result is always interpreted together with your symptoms, blood tests and history.
Can diet or supplements affect a FIT result?
Because FIT targets human haemoglobin, foods such as red meat or beetroot do not usually affect it. Bleeding from haemorrhoids, a fissure or menstrual blood contaminating the sample can cause a positive result, so it is best to avoid collecting during a period. Medications that increase bleeding, such as anticoagulants or aspirin, may also play a role; never stop these without talking to your doctor.
How does nutritional therapy fit in?
FIT is a medical test and its follow-up belongs with your doctor. Once serious causes have been ruled out, many people are still left with symptoms such as constipation, haemorrhoids, bloating or irregular stools. That is where looking at fibre, fluids, transit, the microbiome and daily habits may help support your gut. If you have also noticed mucus, you may find mucus in stool helpful.
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Book your free 15-minute call →Frequently asked questions
Is a FIT test the same as a colonoscopy?
No. FIT is a simple stool test that looks for hidden blood. A colonoscopy looks directly at the lining of the bowel and can remove polyps. A positive FIT usually leads to a colonoscopy.
Can haemorrhoids cause a positive FIT?
Yes, they can. But you cannot assume that haemorrhoids are the cause, so a positive result should always be investigated.
Do I need to avoid red meat before a FIT test?
Usually not, because FIT detects human blood specifically. Always follow the instructions that come with your kit.
Is FIT the same as a calprotectin test?
No. FIT looks for blood, while calprotectin is a marker of inflammation in the gut. Doctors sometimes use both to decide on next steps.
How often is FIT done in screening?
It depends on the country. Many European programmes offer FIT every two years from around age 50. Ask your doctor about the programme where you live.
References
- Stonestreet J, Chandrapalan S, Woolley D, et al. Systematic review and meta-analysis: diagnostic accuracy of faecal immunochemical testing for haemoglobin (FIT) in detecting colorectal cancer for both symptomatic and screening population. Acta Gastroenterol Belg. 2019. PubMed: PMID 31314191
- Khan AA, Klimovskij M, Harshen R. Accuracy of faecal immunochemical testing in patients with symptomatic colorectal cancer. BJS Open. 2020. PubMed: PMID 32949085
- Ball AJ, Aziz I, Parker S, et al. Fecal Immunochemical Testing in Patients With Low-Risk Symptoms of Colorectal Cancer: A Diagnostic Accuracy Study. J Natl Compr Canc Netw. 2022. PubMed: PMID 36075395
- Gerrard AD, Garau R, Xu W, et al. Repeat Faecal Immunochemical Testing for Colorectal Cancer Detection in Symptomatic and Screening Patients: A Systematic Review and Meta-Analysis. Cancers (Basel). 2024. PubMed: PMID 39335170
This article is for educational purposes and is not medical advice. Nutritional therapy does not replace medical care. Always work with your doctor for any persistent or new symptoms and before making changes to your care, medication or supplements.