A reassuring bowel-test result should answer a question, not end a conversation. Faecal immunochemical testing, or FIT, helps clinicians decide who needs investigation for possible bowel cancer. But when bowel symptoms continue, a low result cannot settle the matter on its own. The distinction is explicit in NICE’s assessment of FIT and follow-up: ongoing unexplained symptoms still need a plan.
What FIT measures, and what it cannot see
FIT looks for small quantities of blood in a stool sample, including amounts too small to see. It is a test of a biological signal, rather than a photograph of the bowel. NHS England’s patient instructions describe that purpose and explain how to collect and return the sample. The immediate question is whether the sample contains detectable blood, not whether a particular disease has been proved.
That distinction matters in both directions. A positive result does not establish cancer. A negative result does not provide a complete explanation for symptoms. FIT is most useful when its finding leads to an appropriate next step, rather than becoming a stand-alone health certificate.
Royal United Hospitals Bath’s colorectal service explains that a positive result can reflect bleeding associated with haemorrhoids, diverticulosis, inflammation or polyps, as well as cancer. These possibilities require interpretation; they are not interchangeable diagnoses. Knowing that a benign cause is possible should soften the assumption that a positive result means cancer, without cancelling the reason for investigation.
Screening and symptoms are different questions
A FIT kit can arrive as part of routine screening, before symptoms have prompted a consultation. It can also be requested during the assessment of bowel symptoms. Those settings may look similar from the kitchen table, but the clinical question differs. Screening seeks signs of disease in people who have not presented with symptoms; a symptom-led assessment is trying to explain a problem already noticed.
The NHS explanation of bowel cancer screening makes the limitation clear: even a screening result requiring no further tests does not always exclude bowel cancer. Symptoms that cause concern still warrant a GP discussion. A recent screening letter should therefore be mentioned during an appointment, rather than used as a reason not to make one.
Equally, a request for further tests is not a diagnosis. In the screening pathway, the NHS describes a specialist-nurse discussion and usually a colonoscopy after blood is found. The useful question is which pathway the result belongs to, and what that service has advised next.
The accuracy figure needs its denominator
In a 2022 systematic review in The Lancet Regional Health – Europe, Richard Booth and colleagues analysed 31 studies involving 79,566 symptomatic patients. At a threshold of 10 micrograms of haemoglobin per gram of faeces, pooled sensitivity for colorectal cancer in the “all symptoms” analysis of 35,945 patients was 91.0%, with a 95% confidence interval of 88.9–92.7%. Specificity was 75.2%.
Sensitivity describes the proportion of people with cancer whose test reaches the positive threshold. It does not mean that someone with a negative result has a 9% chance of cancer. That personal probability also depends on how common cancer is in the population being tested and the person’s clinical circumstances. Confusing those denominators can turn useful reassurance into unnecessary fear.
The same review found differences in performance across symptom groups and thresholds. Its findings support FIT as a way to guide investigation, with an acknowledged possibility of missed cancers. They do not establish a universal percentage that applies to every reader, every laboratory, or every reason for testing.
Specificity asks the complementary question: among people without colorectal cancer, how many have a result below the positive threshold? The BJS meta-analysis shows why that matters when considering lower thresholds. Detecting a larger share of cancers also brings more people without cancer into further assessment. Neither sensitivity nor specificity alone describes the whole pathway. The decision still needs the symptoms, the testing setting, and an agreed response to a result that does not explain the problem.
A low result still needs a follow-up plan
A separate meta-analysis in the British Journal of Surgery included 15 prospective cohorts and 28,832 symptomatic patients. At the commonly used threshold of 10 micrograms per gram, pooled sensitivity was 88.7%. Lower thresholds increased sensitivity but reduced specificity: more cancers were detected, while more people without cancer had positive results.
This is the trade-off behind a referral threshold. Moving the boundary changes who proceeds to investigation. It does not create a biological dividing line between a healthy bowel and an unhealthy one. Nor does the existence of false negatives make FIT worthless. It makes the surrounding assessment essential.
NICE’s committee discussion identifies follow-up options for people with negative results and continuing unexplained symptoms, including further assessment or referral, review in primary care, and sometimes another FIT. The appropriate route depends on clinical concern and local arrangements. A useful plan names who will review the symptoms and what should trigger earlier contact. “Negative” is a laboratory finding; it is not, by itself, that plan.
Repeating FIT is a clinical decision
Could a second sample improve detection? A 2023 study in the British Journal of Surgery compared sequential cohorts offered one or two FIT samples before investigation. Among the 2,637 people completing both tests and investigation in the double-testing cohort, sensitivity for colorectal cancer was 96.6%: 85 of the 88 cancers in that group were detected by at least one positive FIT. Some paired results disagreed, illustrating that a single sample does not capture every relevant signal.
But this was a comparison of cohorts, not a randomised trial proving that everybody with a low result should collect another sample. Completing two tests and investigation also selected a subset of the people initially included. The result supports further consideration of testing strategies, not an instruction to repeat tests independently.
Repeating a test should have a defined purpose and an agreed response to the result. For a reader, asking whether reassessment, another test or referral is appropriate is more useful than trying to assemble a personal testing schedule. A second number should not become a reason to postpone assessment of symptoms that remain unexplained.
What this means in practice
- Keep the result and the accompanying advice together. At a review, clarify whether the test was routine screening or part of a symptom assessment.
- Follow the collection instructions supplied with the kit and ask the requesting service for help if collecting or returning a sample is difficult. NHS stool-sample guidance explains why the instructions and return arrangements matter.
- If symptoms continue after a low result, contact the clinician and ask what follow-up is planned. A short record of changes and dates can help structure that conversation.
- Ask what a proposed repeat test would change. Further investigation and repeat sampling answer different questions; the choice belongs within the clinical assessment.
- Use urgent services for warning signs. NHS advice on bowel symptoms recommends an urgent GP appointment or NHS 111 for black or dark-red stool or bloody diarrhoea, and 999 or A&E for non-stop rectal bleeding, a lot of blood, or large clots.
What we don’t know
There is no single best follow-up strategy established for every FIT-negative patient with continuing symptoms. NICE noted the lack of comparative evidence on different safety-netting approaches. Accuracy studies cannot resolve that service-design question simply by reporting a sensitivity figure.
There are also limits to translating pooled research into an individual’s next step. Study populations, testing thresholds and investigation arrangements vary. The repeat-testing study is promising evidence about detection, not proof of better long-term outcomes for every testing strategy. None of these results makes FIT a general measure of gut health or biological ageing.
FIT can make bowel investigation more informed. Its value is strongest when the result is connected to symptoms, clinical judgement, and a clear next step.
Photo: Clay Elliot on Pexels. Illustrative home bathroom; not a photograph of a FIT kit.
