Faecal immunochemical testing is a screening tool for people without symptoms. Its negative predictive value is calculated in that population, and it does not transfer to a patient sitting in front of you with rectal bleeding, iron deficiency or a change in bowel habit.
The error is common because the test is easy to order and the result is reassuring to receive. A patient with alarm symptoms and a negative faecal test still needs a colonoscopy, and the test should not have been sent. Where the test is used to prioritise a symptomatic waiting list - which some services do deliberately - the negative result changes the urgency, never the decision to scope.
The same distinction applies to the follow-up of a positive screening test. A positive result is not a diagnosis and not a reason to start treatment; it is a reason for colonoscopy, and the interval between the two is where screening programmes lose their benefit.
- Do not use a faecal immunochemical test to rule out cancer in a patient with alarm symptoms
- Rectal bleeding, iron deficiency anaemia or persistent change in bowel habit means colonoscopy
- A positive screening test is a referral, not a diagnosis - book the colonoscopy, do not repeat the test
- Track the interval between a positive test and colonoscopy; that delay erodes the whole programme's benefit
- Record whether the test was ordered for screening or for triage - they carry different meanings
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