FIT test vs colonoscopy

The two tests do different jobs. FIT is a screening or triage tool: a simple measurement that sorts people into those who need a bowel examination and those who probably do not. Colonoscopy is a definitive examination: it shows the bowel lining, takes biopsies and removes polyps.

Because colonoscopy involves preparation, sedation or pain relief, a small risk of complications and limited capacity, using FIT first means colonoscopy is focused on those most likely to benefit.

FIT is convenient, non-invasive and acceptable to many people, which improves participation in screening. It is more specific for human blood than older tests and does not require dietary restrictions. It is quantitative, so a threshold is set to balance missing disease against over-referral.

Its weakness is that bleeding can be intermittent, so polyps and early cancers can be missed on a single sample. Not all polyps bleed, and benign causes can produce a positive result. It is not a replacement for investigating persistent or alarming symptoms.

Colonoscopy is the reference test: it examines the whole colon, finds polyps that FIT may miss, and allows removal of polyps, which can prevent cancer from developing. It also finds other causes of symptoms such as inflammatory bowel disease and diverticular disease.

It requires bowel cleansing, sedation or pain relief, time away from usual activities, and carries small risks of bleeding and perforation. It is also an invasive test that some people find daunting, and capacity limits can lead to waits.

The NHS bowel screening programme in England uses FIT as its first test, inviting eligible people at regular intervals and offering colonoscopy to those above the threshold. The age range has been lowered in recent years, so check the NHS screening pages for current eligibility.

For symptoms, NICE NG12 identifies features that warrant an urgent colonoscopy pathway, while NICE DG56 supports FIT for people with lower-risk symptoms, with a threshold set to select who needs further investigation. People with rectal bleeding that is persistent or with anaemia generally follow symptom-based pathways rather than relying on a single negative FIT.

• No symptoms and invited for screening: complete the FIT kit and return it as instructed.

• Persistent change in bowel habit, blood in the stool, or unexplained weight loss: see a GP, who will decide between FIT and an urgent pathway.

• Positive FIT: expect a conversation about colonoscopy; do not wait for symptoms to appear.

• Negative FIT but ongoing symptoms: tell your GP; a negative result does not close the question.

• Strong family history or previous polyps: surveillance colonoscopy follows specialist guidance rather than FIT alone.

A positive FIT is not a cancer diagnosis; many positives turn out to be benign. A negative FIT is not a guarantee that the bowel is healthy, particularly if symptoms continue.

It is also a misconception that colonoscopy is always needed instead of FIT. For lower-risk people, FIT-first spares many people an invasive test.