A normal result: what it rules out, and what it does not

Most articles about bowel testing are written to reassure. This one is written to be accurate, which is a slightly different thing.

A normal FIT result is good news. It is also not a guarantee, and understanding the gap between those two statements is the difference between using the test well and being falsely reassured by it.

The number that matters

Around 1 in 10 people with colorectal cancer will have a FIT result below the threshold.

Put more precisely: pooled data from nearly 30,000 patients with symptoms found that at the 10 µg Hb/g threshold used in England, Wales and Northern Ireland, the test detected 88.7% of colorectal cancers. It missed 11.3%.

That is a good test. It is better at identifying who needs urgent investigation than any individual symptom or any combination of symptoms. But it is not perfect, and no one should present it as though it were.

What the residual risk actually looks like

For someone who has symptoms and a normal FIT, the remaining chance of colorectal cancer is in the region of 0.1 to 0.3%.

For context, the threshold at which national guidance recommends urgent referral is a 3% risk. So a normal result moves someone well below the line at which the system would normally investigate urgently.

That is genuinely reassuring. It is not the same as zero.

Why lowering the threshold is not a simple fix

If 10 misses 11.3%, why not test at 2, which misses only 3.2%?

Because specificity collapses. At the 10 threshold, specificity is around 80.5%. At 2, it falls to about 65.6%. In practice that means a very large number of additional people sent for colonoscopy, nearly all of whom have nothing wrong, each carrying a small procedural risk, and all competing for the same limited endoscopy capacity.

NICE reviewed the alternatives, including testing two separate samples, and concluded that a single sample at a single threshold remains the most effective approach on the current evidence.

The three limitations worth knowing

It does not assess the upper gut. Blood from the stomach or oesophagus is broken down before it reaches the stool. If you are being investigated for iron deficiency anaemia, FIT is one part of the work-up and cannot be the whole of it.

It is less sensitive for other bowel conditions. Inflammatory bowel disease and larger polyps do raise FIT, but less reliably than cancer does. A normal result does not exclude them.

It is not a general cancer test. Ovarian, pancreatic, gastro-oesophageal, urological and lung cancers can all produce symptoms that overlap with bowel cancer. FIT assesses none of them.

What to do if your result is normal but you still feel unwell

This is the important part, and it is the reason we include a consultation with every result rather than emailing you a number.

If your symptoms settle, a normal result is a reasonable place to stop. If they don’t, a normal result is a reason to keep looking, not a reason to stop looking. That might mean blood tests, a calprotectin test, a different referral pathway, or a repeat FIT after an interval.

Two negative FIT results together bring the residual risk below 0.04%, which is why a repeat is sometimes the right next step when symptoms persist.

The principle behind all of it is simple: keep re-evaluating until either the symptoms resolve or you have an explanation for them. A test result is not an explanation.