Why screening exists at all

Bowel cancer is among the most common cancers in the UK, with around 44,000 new cases a year. It is also one of the most survivable when it is found early, and one of the least survivable when it is not. That gap is the entire argument for screening.

Most bowel cancers begin as polyps, small growths on the bowel wall that are harmless for years before some of them turn malignant. A polyp found and removed is a cancer that never happens. That is why screening aims at people who feel completely well: by the time symptoms appear, the disease is usually further along.

Illustration of the large bowel over a person's abdomen — Most bowel cancers begin as a polyp, and most polyps take years to become cancer. That gap is what screening exploits.

What the NHS offers

The NHS Bowel Cancer Screening Programme sends a test kit through the post to eligible adults every two years. The age range has been widening, and now begins at 50 in most of the UK, having previously started at 60. Around four million people are invited each year.

The kit is a faecal immunochemical test, or FIT. You collect a small stool sample and return it, and the laboratory measures human haemoglobin. If the level is above a threshold, you are offered a colonoscopy. The programme is free, well run and worth taking part in. If a kit arrives, use it.

What FIT does and does not do

FIT detects blood. That works because bowel tumours and large polyps often bleed, and the bleeding is usually invisible to the eye long before it becomes obvious.

The limitation is that bleeding comes and goes. A tumour may bleed one week and not the next, and a single sample captures only the day it was taken. Blood from higher in the bowel is also partly broken down before it reaches the stool, which makes it harder to detect. So a normal FIT is reassuring rather than conclusive, and the programme's two-year interval exists partly to compensate for that.

What stool DNA testing adds

Cells shed from a tumour carry the tumour's genetic changes with them into the stool, whether or not it is bleeding. Testing the stool for those changes is a different signal from testing for blood, and it does not depend on bleeding happening on the day you test.

The mutations most commonly looked for in colorectal cancer are in the KRAS and BRAF genes. Some tests also measure the overall amount of human DNA present, which tends to rise when bowel cells are shed abnormally quickly. Combining these with a blood test in the same sample is the approach ColoAlert takes.

This is not a magic solution. Stool DNA tests still miss some cancers, still produce false positives, and still cannot diagnose anything on their own. What they offer is a second independent way for a tumour to reveal itself.

Testing for blood captures a tumour only on a day it happens to bleed. DNA is shed either way, which is the argument for combining both in one sample.

Colonoscopy remains the reference

Every stool test, NHS or private, is a filter rather than an answer. A colonoscopy lets a doctor look directly at the bowel lining, take biopsies and remove polyps in the same appointment. It is the only procedure that can both diagnose and treat.

That is why an abnormal stool test leads to a colonoscopy rather than to a diagnosis. And it is why a normal stool test should not stop you seeing a GP if something changes.

Couple in their fifties — Most people screened are in their fifties and sixties and feel completely well.

A man in his late forties — Bowel cancer rates begin climbing from the mid-forties, before NHS invitations start.

The gap between 45 and 50

Rates of bowel cancer in younger adults have been rising internationally, and the reason is not well understood. Several countries have lowered their screening age to 45 in response. The UK programme currently begins at 50 in most nations, which leaves a five-year window where the risk is real but no invitation arrives.

That window is where private screening has the clearest argument. It is also worth considering if you have a family history, or if you would rather screen more often than every two years.

Person holding their abdomen, with the large bowel illustrated — Symptoms need a GP appointment, not a home screening test.

A person in a bathroom holding toilet paper — Bleeding, or anything that has changed and stayed changed, needs a GP appointment rather than a screening test.

When screening is the wrong answer

If you have symptoms, screening is not what you need. A screening test is designed to find disease in people who feel well; using one to investigate a symptom introduces delay, and a normal result can be falsely reassuring at exactly the wrong moment.

Bleeding, a persistent change in bowel habit, unexplained weight loss, recurrent abdominal pain and new unexplained tiredness all warrant a GP appointment. The NHS urgent referral route exists for this and is faster than anything you can buy.

If you are weighing it up, our two-minute suitability check walks through the same questions a GP would ask and tells you whether to test, wait, or get seen.

If you	Then
Have symptoms	See your GP. Do not use a screening test.
Have an NHS kit	Use it. It is free and effective.
Are 45–49 with no symptoms	Not yet invited by the NHS. Private screening is reasonable.
Have a family history	Speak to your GP about earlier or more frequent surveillance.
Want to screen between invitations	Private screening is reasonable alongside the NHS programme.

Bowel cancer screening, explained plainly.

If you have symptoms, see your GP instead