If you have had a bowel screening kit through the post and the letter came back normal, you were probably relieved and then didn’t think about it again. That is a reasonable response. But there is something about that letter almost nobody is told, and it matters if you later develop symptoms.
There is only one test, but two different ways of reading it
The kit that arrives through your letterbox from the NHS screening programme and the kit your GP gives you when you go in with symptoms are the same test. Same technology, same laboratory analysers, same sample. What differs is the number at which the result is called positive.
The test produces a figure: the concentration of human haemoglobin in your stool, measured in micrograms of haemoglobin per gram of faeces, usually written as µg Hb/g. Everything then depends on where the line is drawn.
Screening thresholds across the UK:
| Nation | Positive at |
|---|---|
| Scotland | 80 µg Hb/g |
| England | 120 µg Hb/g, reducing to 80 by 2028 |
| Wales | 80 µg Hb/g |
| Northern Ireland | 120 µg Hb/g |
The threshold used when someone has symptoms is 10 µg Hb/g, and that figure is the same across the whole UK.
So in England today, a result of 100 would be reported to you as normal by the screening programme. That same figure, in someone attending their GP with a change in bowel habit, would trigger an urgent referral.
Why the thresholds are so far apart
It is tempting to conclude that the screening threshold is simply wrong. It isn’t, and it is worth understanding why.
Screening is offered to an entire population of people who feel perfectly well. Lower the threshold and you detect more cancers, but you also send far more people for colonoscopy, the overwhelming majority of whom have nothing wrong. Colonoscopy carries its own small risks, and there are only so many slots. A national programme has to set a line that balances detection against capacity and against the harm of over-investigating healthy people.
That is a defensible decision for a population. It is a different question for an individual sitting in front of a doctor with a specific concern.
Why this matters if you develop symptoms
Here is the part that should change how you think about your screening letter.
A meta-analysis pooling data from nearly 30,000 patients with symptoms compared how the test performs at different thresholds:
- At 10 µg Hb/g, the symptomatic threshold, it detected 88.7% of colorectal cancers
- At 2 µg Hb/g, it detected 96.8%, but at the cost of far more people being investigated unnecessarily
- At 100 and 150 µg Hb/g, screening-style thresholds, it detected 68.1% and 66.3%
Read that last line again. Applied to people with symptoms, a screening threshold misses about a third of cancers.
This is why national guidance is unambiguous on the point. NICE states directly that FIT should be offered to someone with relevant symptoms even if they have already had a negative result through the NHS screening programme.
What to do with this
If you feel well and you are within the screening age range, take the free NHS kit when it arrives. It is a good test doing exactly the job it was designed for.
But if you have developed symptoms since that letter arrived, do not let it reassure you. A normal screening result answers the question “was there a lot of blood in this sample?” It does not answer “is there something going on now?”
If you are unsure which situation you are in, that is exactly the sort of thing worth spending fifteen minutes discussing with a doctor.
References
- Saw KS, Liu C, Xu W, Varghese C, Parry S, Bissett I. Faecal immunochemical test to triage patients with possible colorectal cancer symptoms: meta-analysis. British Journal of Surgery 2022;109(2):182 to 190. doi:10.1093/bjs/znab411
- National Institute for Health and Care Excellence. Quantitative faecal immunochemical testing to guide colorectal cancer pathway referral in primary care. Diagnostics guidance DG56, 2023.
- NHS England. Announcement on reducing the bowel cancer screening FIT threshold, January 2026; and The National Cancer Plan for England, 2026.
- Monahan KJ, Davies MM, Abulafi M, et al. Faecal immunochemical testing (FIT) in patients with signs or symptoms of suspected colorectal cancer: a joint guideline from the Association of Coloproctology of Great Britain and Ireland (ACPGBI) and the British Society of Gastroenterology (BSG). Gut 2022;71:1939 to 1962. doi:10.1136/gutjnl-2022-327985
Figures are drawn from the sources above. Framing and commentary are the author’s own. Last reviewed August 2026.