Positive FIT test result? Most aren’t cancer — here’s what to expect

A positive FIT test result is one of the most worrying letters or text messages a person can receive. The instinct, understandably, is to assume the worst. But before that thought takes root, there’s an important fact that most people don’t know until after they’ve already had several sleepless nights: the majority of positive FIT results are not caused by cancer.

In UK primary care, only around 1 in 14 positive FIT results — roughly 7% — turn out to be bowel cancer when investigated further. The other 93% have a different, usually far less serious cause. Even within national bowel cancer screening, where positive results are rarer to begin with, most lead to colonoscopies that find polyps or nothing of concern, not cancer.

What a positive FIT test result actually means

A FIT — faecal immunochemical test — looks for tiny traces of blood in a stool sample. The “positive” label simply means the test has detected blood at or above a defined threshold.

That’s all it means. It does not mean cancer has been found. It does not mean the bowel is bleeding heavily. It means something, somewhere along the lower digestive tract, has released a small amount of blood into a stool sample on the day the test was taken — and a sensitive laboratory analyser picked it up.

The amount of blood matters too. FIT results are reported quantitatively, in micrograms of haemoglobin per gram of faeces (µg Hb/g). A result just above the threshold is treated very differently from a result several times higher, because the likelihood of a serious cause rises with the amount of blood detected. Your GP or specialist will look at the actual number, not just the positive/negative label.

What percentage of positive FIT tests are cancer?

The honest answer is: it depends on which FIT you took and which threshold was used.

For people having a FIT because they’ve gone to their GP with bowel-related symptoms, the threshold for a positive result is 10 µg Hb/g — a sensitive setting designed to catch as much disease as possible. At this threshold, a UK study of nearly 4,000 primary care patients published in the British Journal of Cancer found that around 7% of positive FITs were associated with a colorectal cancer diagnosis within a year. That’s roughly 1 in 14. The same study confirmed that the test is excellent at ruling cancer out — a negative result at this threshold has a negative predictive value of more than 99%.

For people taking a FIT through national bowel cancer screening, the picture is different. Until 2026, the screening threshold in England was 120 µg Hb/g — a much higher setting because the population being tested has no symptoms. From early 2026, NHS England began phasing in a lower screening threshold of 80 µg Hb/g — bringing England in line with Scotland and Wales, where 80 µg Hb/g is already the standard. The change is being rolled out across early-adopter sites first, with national coverage expected by 2028. Once fully implemented, NHS England estimates the lower threshold will detect around 600 more bowel cancers and identify 2,000 more high-risk polyps each year.

In both contexts, the same headline holds: a positive FIT raises the possibility of bowel disease enough that further investigation is sensible, but most positive results have a non-cancer cause.

Why else might a FIT test come back positive?

Several things can release small amounts of blood into the bowel. The most common non-cancer causes of a positive FIT are:

• Haemorrhoids (piles). Internal piles can bleed enough to be picked up by a sensitive FIT, often without the person noticing visible blood. This is one of the single most common explanations for an unexpected positive result.
• Bowel polyps. These small growths in the lining of the bowel are extremely common, particularly with age. Most are benign. A few types can develop into cancer over many years, which is exactly why a positive FIT followed by colonoscopy and polyp removal is one of the most effective ways to prevent bowel cancer rather than just diagnose it.
• Anal fissures. Small tears in the lining of the anal canal — often related to constipation or hard stools — bleed easily and frequently cause positive FIT results.
• Diverticular disease. Small pouches in the wall of the bowel, which become more common with age, can occasionally bleed.
• Inflammatory bowel disease (IBD). Conditions such as Crohn’s disease and ulcerative colitis cause inflammation and bleeding in the bowel and will often produce a positive FIT.
• Recent menstruation. Blood from a menstrual period can contaminate a stool sample. This is a common preventable cause of false positives, which is why most kits advise avoiding the test during a period.
• Recent NSAID use. Anti-inflammatory medications such as ibuprofen, naproxen and aspirin can irritate the lining of the gut and cause small amounts of bleeding.
• Bleeding from elsewhere in the digestive tract. Although FIT is designed specifically to detect blood from the lower bowel (it doesn’t react to the digested blood from the stomach or upper gut), occasional false positives can come from other sources.

The list is long because the bowel is sensitive and small amounts of blood are common. The same length of list is also why a positive FIT on its own can’t tell you what’s going on — only a colonoscopy can do that.

What happens after a positive FIT result

If your FIT was arranged through your GP because you had symptoms, the result will go back to your GP, who will refer you on the urgent suspected cancer pathway. Under current NHS guidance, an urgent suspected cancer referral should lead to a specialist appointment within two weeks, with the aim of confirming or ruling out cancer within 28 days under the Faster Diagnosis Standard. The next investigation is almost always a colonoscopy.

A colonoscopy is a procedure in which a thin, flexible camera is passed through the bowel to look directly at the lining. It’s done with sedation, takes around 30 to 45 minutes, and allows the specialist to see exactly what’s there — polyps, inflammation, fissures, or, much less commonly, cancer. Polyps can usually be removed during the same procedure. The most uncomfortable part for most people is the bowel preparation the day before, which involves taking a strong laxative.

Colonoscopy is the gold-standard test for investigating a positive FIT. It’s the step that turns “something’s there” into a clear answer.

How the FIT test works

FIT detects human haemoglobin — the protein that carries oxygen in red blood cells — using antibodies that bind specifically to it. Because it’s specific to human blood, FIT is not affected by what you’ve eaten (unlike older guaiac-based tests, which could give false positives from animal blood in the diet) and you don’t need to fast or follow a special diet before doing it.

The test itself is a small home kit. You collect a tiny sample from a single bowel motion using a probe, post it back in the prepaid envelope, and a UKAS-accredited laboratory measures the amount of haemoglobin in the sample to several decimal places. The result is reported as a quantitative number in µg Hb/g, with positive or negative determined by the threshold being used.

Hydration is helpful for an easier sample, but no other preparation is needed. Most kits advise avoiding the test during a menstrual period or shortly after, and to be aware that recent NSAID use can affect the result.

How accurate is the FIT test?

FIT is one of the better screening tools in widespread use, but no test is perfect.

In symptomatic primary care, FIT at the 10 µg/g threshold has a sensitivity for bowel cancer of around 84% — meaning it correctly identifies the large majority of cancers, but a small proportion are missed. Specificity is around 85%, meaning most healthy people get a correctly negative result, but some false positives occur. The negative predictive value — the chance that a negative FIT genuinely means no cancer — is over 99%.

That last figure is the one most patients care about: a negative FIT is reassuring. But it isn’t a guarantee. A previous negative FIT does not rule out the possibility of bowel cancer developing later, and it absolutely does not override new symptoms. If symptoms appear, they need investigating regardless of any earlier FIT result.

Why test privately?

There are several reasons people choose to take FIT testing into their own hands. You may want to test on your own timing rather than wait for a screening invitation. You may want a clinical consultation alongside your result rather than just a number returned in the post. You may want FIT alongside other cancer-related blood markers in a single appointment. Or you may simply want to be proactive about your bowel health rather than wait until symptoms develop.

Private FIT testing uses the same kit and the same kind of UKAS-accredited laboratory analysis as you would expect from any clinically rigorous provider. The difference is access, timing and the wider context the test sits in.

At Optimum Health Screening, FIT is included as part of both our Men’s Cancer Marker Health Check and Women’s Cancer Marker Health Check. Both checks sit alongside a wider panel of cancer-related blood markers and a clinical consultation, so the FIT result is interpreted in the context of your overall picture rather than handed back as a single number with no follow-up.

For men in particular, a FIT alongside a PSA test is one of the more straightforward ways to take stock of two of the most common cancer risks at the same appointment.

You can book a Men’s Cancer Marker Health Check or a Women’s Cancer Marker Health Check online.

When to see your GP regardless of your FIT result

A FIT result — positive or negative — does not replace a clinical assessment of symptoms. The following symptoms should always be discussed with your GP, even if you’ve had a recent negative FIT:

• Bleeding from the bottom or blood in your stool
• A persistent change in your bowel habit lasting more than a few weeks
• Unexplained weight loss
• Persistent abdominal pain or a lump in the abdomen
• Unexplained tiredness, particularly with paleness or breathlessness
• Iron-deficiency anaemia picked up on a routine blood test

These symptoms have many possible causes, most of them not cancer. But they are reasons to be assessed properly by a clinician, not reasons to take another home test and hope.

If you’ve had a positive FIT and are waiting for your colonoscopy appointment, your GP and the specialist team are the right people to contact if anything changes — particularly heavy bleeding, severe pain, or a sharp deterioration in how you feel.

faqs:

• question: What percentage of positive FIT tests are cancer? answer: Around 7% — roughly 1 in 14 — of positive FIT tests done at the symptomatic threshold of 10 µg Hb/g are found to be cancer when investigated further in UK primary care. At higher screening thresholds, the proportion is different, but in both contexts the majority of positive results are not cancer.
• question: What is the most common cause of a positive FIT test? answer: Haemorrhoids (piles) and bowel polyps are among the most common causes of a positive FIT result. Anal fissures, diverticular disease, inflammatory bowel disease, recent menstruation and recent use of anti-inflammatory medications can all also cause small amounts of blood to be detected. Only a colonoscopy can identify the exact cause.
• question: Can haemorrhoids cause a positive FIT test? answer: Yes. Internal haemorrhoids can release small amounts of blood into the bowel that are easily picked up by a sensitive FIT, even when there is no visible bleeding. Haemorrhoids are one of the most common non-cancer explanations for an unexpected positive FIT result.
• question: Should I worry about a positive FIT result? answer: A positive FIT should be taken seriously, but it should not be assumed to mean cancer. The majority of positive FITs have non-cancer causes, including haemorrhoids, polyps and benign inflammation. The right response is to follow the next step — usually a colonoscopy — promptly, and to contact your GP or specialist team if any new symptoms appear in the meantime.
• question: How long do FIT test results take? answer: Symptomatic FITs requested by a GP typically come back within around two weeks of the laboratory receiving the sample. Private FIT testing through a clinic such as Optimum Health Screening typically returns results within a few working days of the laboratory receiving the sample, with results explained in the context of any other tests done at the same appointment.