Gut symptoms are common and often hard to describe. Loose stools for weeks, a change in bowel habit, a burning stomach after meals. Most of the time the cause is benign, but the right test can help a professional decide what to do next. Several of the most useful gut tests are done on a small stool sample at home. This guide explains four of them, what each one looks for and when it is usually used.
Faecal calprotectin: a marker of gut inflammation
Calprotectin is a protein found in white blood cells. When the lining of the bowel is inflamed, white blood cells move into it and calprotectin passes into the stool. LML describes the test as a way to look for inflammation in the gastrointestinal tract, with raised levels suggesting active inflammation in the intestines, as can happen in inflammatory bowel disease (IBD) such as Crohn's disease and ulcerative colitis.
How it is used
Its main role is to help tell apart two groups of conditions that can feel alike: IBD, where there is inflammation, and irritable bowel syndrome (IBS), where there usually is not. NICE recommends faecal calprotectin as an option to support clinicians in this distinction for adults with recent-onset lower gut symptoms who are being considered for specialist assessment, when cancer is not suspected.
The evidence has nuances. A systematic review in BMJ Open found that, at a threshold of 50 µg/g, sensitivity for IBD was generally high, but specificity was more variable, and accuracy appeared lower in primary care than in hospital settings (Freeman et al., 2019). In plain terms, a low result is reassuring, and a raised result is a reason for a closer look rather than an answer in itself.
FIT: the faecal immunochemical test
The faecal immunochemical test looks for tiny amounts of human blood in stool, often too small to see. LML describes faecal occult blood as a possible sign of bleeding in the digestive tract, which may come from causes such as ulcers, polyps or, less commonly, bowel cancer.
Two different uses
- Screening without symptoms. The NHS bowel cancer screening programme uses FIT as a home test. The NHS currently offers it to people aged 50 to 74 every 2 years, and the kit arrives automatically for those registered with a GP. If you are eligible, that is the route to use.
- Helping prioritise people with symptoms. A joint guideline from the Association of Coloproctology of Great Britain and Ireland and the British Society of Gastroenterology describes FIT as having high sensitivity for bowel cancer, and as an objective measure that helps decide who needs investigation with the highest priority while identifying a truly lower-risk group (Monahan et al., 2022).
A positive FIT does not mean cancer. It means blood was found and a professional should consider further investigation. A negative FIT in someone with ongoing symptoms should still be discussed with a professional.
Calprotectin and FIT together
LML offers a combined profile that measures both, because each answers a different question: calprotectin looks at inflammation, FIT looks at bleeding.
H. pylori: a common stomach bacterium
Helicobacter pylori is a bacterium that can live in the stomach lining. The NHS lists H. pylori infection as one of the main causes of stomach ulcers, alongside regular use of anti-inflammatory painkillers. LML describes the infection as associated with gastritis and peptic ulcers, and with a higher long-term risk of stomach cancer.
Two ways to test
- Stool antigen test. LML describes it as a check for active infection. NICE lists the stool antigen test, alongside the breath test, as a recommended way to test for H. pylori.
- Blood antibodies (IgG). LML explains that antibodies indicate a previous or current infection. Because antibodies can stay after the infection has gone, they cannot show whether treatment has worked. NICE accepts blood tests where their performance has been locally validated.
A Cochrane review comparing non-invasive tests found that breath tests were the most accurate, and that serology and stool antigen tests were somewhat less accurate (Best et al., 2018). That review also excluded many people who had recently taken acid-reducing medicines or antibiotics, which leads to an important practical point.
Before an H. pylori stool test
NICE advises leaving a 2-week gap after taking proton pump inhibitors (PPIs), such as omeprazole or lansoprazole, before a breath or stool antigen test, as they can affect the result. Recent antibiotics may also affect it. Do not stop a prescribed medicine without speaking to the professional who prescribed it. To check whether treatment has worked, NICE recommends a breath test.
The Gastrointestinal Panel: looking for infection
When diarrhoea is the main symptom, especially after travel or an outbreak, infection may be the question. LML's Gastrointestinal Panel uses PCR to look for more than 20 bacteria, viruses and parasites in one stool sample, including Campylobacter, Salmonella, Giardia, norovirus and several types of E. coli. It answers a very different question from calprotectin or FIT.
