Key points
- A positive faecal occult blood test does not mean cancer: in Spanish screening programmes, around 5 in every 100 positive tests turn out to be cancer.4
- What it does mean is that you need a colonoscopy. It is the only way to find where the blood is coming from and, if there are polyps, to remove them there and then.1,2
- Repeating the test does not help: a second negative result rules nothing out.2,8
- The colonoscopy should be done within the following weeks. Waiting more than 9-10 months was linked to more cancers and to later-stage disease.7
- Piles, anticoagulants and aspirin rarely explain a positive result, and are no reason to skip the colonoscopy.5,6
- If the colonoscopy is normal, the next check is usually in 10 years, not in two.1
This article summarises the evidence and clinical guidelines so you can understand your situation. It does not replace an individual assessment.
Getting a letter or a text from the screening programme saying your faecal occult blood test is positive is understandably worrying: most people link the test with bowel cancer. But the result means something more specific, and far less alarming: there is a small amount of blood in your stool that you cannot see, and its source needs to be found.
In this article I explain what the test actually measures, what the real odds behind a positive result are, what to do now and what happens after the colonoscopy.
What the test measures
The test used in Spanish screening programmes is the faecal immunochemical test (FIT). It detects human haemoglobin, the protein in blood, in a small stool sample collected at home. The programme invites people aged 50 to 69 every two years.1
It has several advantages over older tests: it only reacts to human blood, so no special diet is needed beforehand, and a single sample is enough.2 It is a very useful test: it picks up around 79% of bowel cancers, and in 94 out of 100 people without cancer it comes back negative.3
The test does not look for cancer directly. It looks for blood, and blood can come from a cancer but much more often comes from a polyp or a benign lesion. That is why a positive result is not a diagnosis: it is a signal that the bowel needs to be looked at.
What a positive result means
Data from Spanish screening programmes help put the result in context. In the first year of the Aragon programme, for example, the test was positive in about 11% of people aged 60 to 69, and colonoscopy found a cancer in just over 5 in every 100 positive tests. In almost 4 in 10, it found high- or intermediate-risk adenomas, which are the polyps most worth removing.4
| What colonoscopy may find | What it means |
|---|---|
| Polyps (adenomas) | The commonest finding. They are removed during the same colonoscopy, which stops them from ever becoming cancer1 |
| Bowel cancer | Around 5 in every 100 positive tests.4 When found through screening, it is usually at an earlier stage than when it causes symptoms |
| Piles, fissure, diverticula or inflammation | They can bleed, but explain only a small share of positive results5 |
| Normal colonoscopy | Happens in some cases. It does not mean the test was wrong: bleeding can be tiny and intermittent |
Finding a polyp is good news
Most bowel cancers start as a benign polyp that takes years to turn malignant. If the colonoscopy finds and removes it, the cancer never develops. The aim of screening is not only to find cancers early but to prevent them. A large Spanish trial, COLONPREV, showed that inviting people to do this test every two years reduces bowel cancer deaths as much as inviting them to have a colonoscopy straight away.9
Why repeating it does not help
It is the most common temptation: repeating the test in the hope that it comes back negative, so the colonoscopy can be avoided. Guidelines are clear that this should not be done.2,8
Polyps and tumours do not bleed all the time. A second negative test may simply fall on a day without bleeding, and it does not cancel out the first result. A single positive test is enough to need a colonoscopy.2
Piles, periods and medicines
Many people assume the positive result "must be the piles". Studies say otherwise: in one study of more than 2,800 people who had a colonoscopy, haemorrhoids as the only finding explained a very small fraction of positive tests.5 Known piles are no reason to skip the colonoscopy.
| Situation | What you should know |
|---|---|
| Known piles | Rarely the only cause of a positive test. The colonoscopy is still needed5 |
| Anticoagulants, aspirin or anti-inflammatories | They do not meaningfully reduce the reliability of the test, and you should not stop them to do it6 |
| Period or blood in the urine | Can contaminate the sample. Collect it outside those days |
| Food (red meat, vegetables) | No effect: the test only detects human haemoglobin2 |
| Kits bought at a pharmacy | Not all of them are immunochemical or validated like the programme test. If positive, still get it checked |
How long you can wait
It is not an emergency, but it is not something to leave for a year either. The recommendation is to have the colonoscopy within the following weeks, preferably in under two months.8
The best data on delays come from a study of more than 70,000 people with a positive test: having the colonoscopy within the first few months made no difference, but from 10 months onwards the risk of cancer rose and, above all, cancers were more likely to be found at a later stage.7
If the programme calls you, go
The screening programme has its own pathway: your GP requests the colonoscopy at your local hospital. If the waiting time is reasonable, that is an excellent option. If it drags on, or if you did the test privately outside the programme, the important thing is not to let it slide.
What the colonoscopy involves
Colonoscopy lets the doctor see the whole bowel from the inside using a flexible tube with a camera. What worries most people is the bowel preparation: the day before, you follow a low-fibre diet and drink a laxative that clears the bowel. It is the most unpleasant part of the process, and doing it well is what makes sure no lesion is missed.
The procedure itself is usually done under sedation, so you do not feel it and most people remember nothing. It takes 20 to 40 minutes. If a polyp is found, it is removed there and then and sent for analysis; if there is a lesion that cannot be removed, biopsies are taken.
Before the test, tell us if you take anticoagulants or antiplatelet drugs, because they sometimes need adjusting so polyps can be removed safely. Never stop them on your own.
What happens afterwards
The next step depends on what the colonoscopy finds:1
| Result | Next step |
|---|---|
| Normal, complete colonoscopy | No need to repeat the stool test every two years. The next check is usually in 10 years1 |
| Polyps removed | The interval depends on how many there were, their size and the lab result. Many people need no special follow-up |
| Incomplete colonoscopy or poor preparation | It needs repeating or completing, because a lesion could have been missed |
| Cancer | Further tests are arranged and you are referred to a specialist unit. Found through screening, it is usually at a more treatable stage |
I explain in more detail how often to have a check depending on the polyps found in At what age should you have your first colonoscopy?
Common myths
"If the test is positive, I have cancer."
In Spanish programmes, around 5 in every 100 positive tests turn out to be cancer. The commonest finding is polyps, which are removed during the same procedure.4
"I'll repeat it and, if it's negative, I can relax."
A second negative test rules nothing out, because bleeding comes and goes. A single positive test is enough to need a colonoscopy.2
"It's bound to be my piles."
Piles as the only cause explain very few positive tests. They are no reason to skip the colonoscopy.5
"I should have been on a diet before the test."
The modern test only detects human blood. Meat or vegetables do not affect the result.2
"I have no symptoms, so it can wait."
Screening exists precisely to find lesions before they cause symptoms. The colonoscopy should be done within the following weeks.7,8
"If the colonoscopy is normal, the test was wrong."
The bleeding can be tiny and have harmless causes. A normal colonoscopy is a good result, not a mistake.
Red flags: don't wait
See a doctor promptly if…
Screening is for people without symptoms. If you have any of the following, do not wait for the programme letter, and do not be reassured by a negative test:
- Visible blood in your stool, or black stools
- A lasting change in bowel habit over several weeks
- Weight loss you cannot explain
- Anaemia or low iron without a clear cause
- Persistent abdominal pain or a feeling of incomplete emptying
With symptoms, colonoscopy is diagnostic rather than screening, and it is done at any age.
How I approach it in clinic
The first thing is to put the result in context, because almost everyone arrives fearing the worst. I review your personal and family history, any symptoms and the medicines you take, especially anticoagulants and antiplatelet drugs.
Then we schedule a colonoscopy under sedation at the hospital itself, with written bowel-preparation instructions, because a clean bowel is what makes the test reliable. If there are polyps, I remove them during the procedure, and when the lab result comes back I explain what they were and when your next check is due.
If you are already in the programme pathway with a reasonable waiting time, I will tell you so: what matters is not where the colonoscopy is done, but that it is done on time and done well.
Has your faecal occult blood test come back positive?
We review your case and, if needed, schedule a colonoscopy under sedation without waiting lists.
Frequently asked questions
Does a positive test mean I have cancer?
No. The vast majority of positive results are not caused by cancer. In Spanish screening programmes, around 5 in every 100 positive tests turn out to be cancer; the rest are polyps, other benign findings or a normal colonoscopy.4
Can I repeat the test to confirm it?
It does not help. Bleeding from a polyp or a tumour comes and goes, so a second negative test rules nothing out. After a positive result, the next step is a colonoscopy.2
Could it have been my piles?
It is possible, but uncommon. In studies with colonoscopy, haemorrhoids as the only cause explain a very small share of positive results. Having piles is not a reason to skip the colonoscopy.5
How long can I wait before the colonoscopy?
I take anticoagulants or aspirin. Is the result reliable?
Yes. These drugs do not meaningfully reduce the reliability of the test, and you should not stop them to do it. Do mention them before the colonoscopy, in case a polyp needs removing.6
If my colonoscopy is normal, do I go back to testing every two years?
No. After a complete, good-quality colonoscopy with no findings, someone at average risk usually does not need another check for 10 years.1
References
- Cubiella J, et al. Clinical practice guideline. Diagnosis and prevention of colorectal cancer. 2018 update. Gastroenterol Hepatol. 2018. PubMed
- Robertson DJ, et al. Recommendations on fecal immunochemical testing to screen for colorectal neoplasia: a consensus statement by the US Multi-Society Task Force on Colorectal Cancer. Gastroenterology. 2017. PubMed
- Lee JK, et al. Accuracy of fecal immunochemical tests for colorectal cancer: systematic review and meta-analysis. Ann Intern Med. 2014. DOI
- Solé Llop ME, et al. Programa de cribado poblacional de cáncer colorrectal en Aragón. Primeros resultados. Gac Sanit. 2018. Full text
- van Turenhout ST, et al. Hemorrhoids detected at colonoscopy: an infrequent cause of false-positive fecal immunochemical test results. Gastrointest Endosc. 2012. DOI
- Nieuwenburg SAV, et al. Effect of anticoagulants and NSAIDs on accuracy of faecal immunochemical tests (FITs) in colorectal cancer screening: a systematic review and meta-analysis. Gut. 2019. Gut
- Corley DA, et al. Association between time to colonoscopy after a positive fecal test result and risk of colorectal cancer and cancer stage at diagnosis. JAMA. 2017. PubMed
- Forbes N, Hilsden RJ, Heitman SJ. Fecal immunochemical testing for colorectal cancer screening. CMAJ. 2020. PubMed
- Castells A, et al. Effect of invitation to colonoscopy versus faecal immunochemical test screening on colorectal cancer mortality (COLONPREV): a pragmatic, randomised, controlled, non-inferiority trial. Lancet. 2025. The Lancet
Links go to PubMed or the original publication and open in a new tab.
Related reading
Dr Fernando Casals Seoane
Consultant gastroenterologist with a PhD in Medicine from the Autonomous University of Madrid. He has worked at La Princesa University Hospital since 2009 and sees private patients at Ruber Juan Bravo Hospital. Certified by the European Board of Gastroenterology and Hepatology and honorary lecturer at the UAM. Principal investigator in international clinical trials on acute pancreatitis, including WATERFALL (NEJM, 2022).
Transparency: this article is not sponsored and does not promote any product or brand. All sources are linked.