Stool tests (also called faecal tests) analyse a sample of faeces to detect infection, bleeding in the gastrointestinal tract, inflammation and other conditions. They are non-invasive and can provide important information about gut health that blood tests alone cannot.
Stool tests are used for a surprisingly wide range of conditions — from a simple stomach bug to colorectal cancer screening. Here's a guide to the most common types and what the results mean.
Faecal occult blood test (FOBT)
The FOBT looks for tiny amounts of blood in the stool that aren't visible to the naked eye. Blood in the stool can come from any part of the gastrointestinal tract — a stomach ulcer, polyp, haemorrhoid or, in some cases, colorectal cancer.
A positive FOBT result means blood was detected — not that cancer is present. Most positive results are caused by benign conditions such as haemorrhoids or a diet high in red meat. However, a positive result requires investigation, usually a colonoscopy, to find the source.
FOBT is used as a population-level colorectal cancer screening tool in many countries, recommended for adults over 50 every one to two years.
Faecal immunochemical test (FIT)
The FIT is a more modern version of the FOBT that specifically detects human blood — not the haem from meat or vegetables. It requires only a single sample and is more accurate at identifying colorectal cancer and adenomas (pre-cancerous polyps). Most national bowel screening programmes now use FIT rather than FOBT.
H. pylori stool antigen test
Helicobacter pylori (H. pylori) is a bacterial infection that lives in the stomach lining and is the leading cause of peptic ulcers. A stool antigen test detects H. pylori proteins in the stool with high accuracy and is the recommended non-invasive method for both diagnosis and confirming eradication after treatment.
You need to stop proton-pump inhibitors (like omeprazole) at least two weeks before the test, as they can suppress H. pylori and give a false-negative result.
Stool culture
A stool culture is used to identify bacteria causing gastrointestinal infections — Salmonella, Campylobacter, E. coli O157, Shigella and others. It's typically ordered when diarrhoea is severe, prolonged (more than three days), bloody or occurs during or after international travel.
The lab grows the bacteria from the sample and, if one is found, tests which antibiotics will kill it. Results take two to four days.
Faecal calprotectin
Calprotectin is a protein released by white blood cells when the gut is inflamed. Elevated faecal calprotectin suggests active inflammation in the bowel — it's used to distinguish irritable bowel syndrome (IBS) from inflammatory bowel disease (IBD) like Crohn's disease or ulcerative colitis, and to monitor IBD activity.
How to collect the sample correctly
- Use the collection kit provided by the lab — avoid contaminating the sample with urine or toilet water
- For FOBT/FIT, follow the specific dietary instructions — some require avoiding red meat or vitamin C for 72 hours
- Refrigerate the sample if it can't be returned to the lab within two hours
- Note any recent antibiotics, anti-diarrhoeals or PPIs — these can affect results