A stool test checks a small sample of faeces for infection, parasites, hidden blood, digestion problems and inflammation. It is simple, painless and often the fastest way to find the cause of diarrhoea, stomach pain or unexplained anemia. This guide explains the two most common stool tests, the routine stool examination and the fecal occult blood test (FOBT), how to collect a sample correctly, and what the results mean.
Quick answer: A routine stool examination looks at colour, consistency, mucus, blood, pus cells, and eggs or cysts of worms and parasites under the microscope. A fecal occult blood test looks for blood too small to see. The modern immunochemical version (FIT) needs no diet changes and is preferred for bowel cancer screening. Collect the sample in a clean, dry, screw-top container without urine or water, and get it to the lab within about 1 to 2 hours. A positive occult blood test does not mean cancer, but it does need follow-up, usually with a colonoscopy.
Key takeaways
- ✅ A good sample matters: no urine, toilet water or toilet paper in the container.
- ✅ Parasites can be missed in one sample; your doctor may ask for three samples on different days.
- ✅ FIT is more accurate than the older guaiac test and is not affected by food.
- ✅ Hidden blood in the stool is a common cause of iron deficiency anemia in adults.
When is a stool test done?
- Diarrhoea lasting more than a few days, or with blood or mucus
- Abdominal pain, bloating or suspected worm infestation, especially in children
- Unexplained iron deficiency anemia
- Screening for bowel (colorectal) cancer
- Suspected malabsorption, with pale, greasy, foul-smelling stools
Routine stool examination: what is checked
| Part of the test | Normal | What an abnormal result may suggest |
|---|
| Colour | Brown | Black and tarry: bleeding high in the gut, or iron tablets. Pale or clay: bile blockage. Red: bleeding low in the gut |
| Consistency | Formed | Watery in infections; greasy in malabsorption |
| Mucus and visible blood | Absent | Dysentery, inflammatory bowel disease, piles or fissures |
| Pus cells and red cells | None or occasional | Bacterial infection or bowel inflammation |
| Ova, cysts and parasites | Not seen | Roundworm, hookworm, Giardia, Entamoeba histolytica and others |
| Fat globules, undigested food | Few | Malabsorption or pancreatic problems |
| Reducing substances, pH (mainly in children) | Negative | Lactose or sugar intolerance |
Other stool tests your doctor may add include stool culture for bacteria, H. pylori stool antigen, C. difficile toxin after antibiotics, and fecal calprotectin, which helps tell inflammatory bowel disease from irritable bowel syndrome.
Fecal occult blood test (FOBT)
"Occult" means hidden. This test detects small amounts of blood that cannot be seen. There are two types:
| Immunochemical test (FIT / iFOBT) | Guaiac test (gFOBT) |
|---|
| How it works | Antibodies detect human hemoglobin | A chemical reaction with the heme in blood |
| Diet restrictions | None | Avoid red meat, some raw vegetables, vitamin C and NSAIDs for several days before |
| Samples | Usually one | Usually three, from separate stools |
| Best for | Bowel cancer screening and lower-gut bleeding | Older method, still used in some labs |
Causes of a positive result include piles, fissures, polyps, ulcers, inflammatory bowel disease, hookworm infestation and bowel cancer. Bleeding gums or menstrual blood contaminating the sample can also cause a false positive. A positive test should always be followed up, usually with a colonoscopy, rather than simply repeated.
How to collect a stool sample
- Get a clean, dry, screw-top container from the lab, and label it with your name, date and time.
- Pass urine first so it does not mix with the stool.
- Catch the stool on clean paper, a disposable plate or a plastic sheet placed over the toilet, not from the toilet water.
- Use the scoop to collect a walnut-sized amount (or about 5 ml if watery), taking any blood or mucus if present.
- Close the lid tightly, wash your hands, and take the sample to the lab within 1 to 2 hours. For FIT kits, follow the kit instructions.
Do not wait for a stool test if you have heavy bleeding from the back passage, black tarry stools with dizziness, severe abdominal pain, or diarrhoea with signs of dehydration. Seek medical care straight away.
Frequently asked questions
Can I collect the stool sample at night and give it in the morning?
For parasites and culture, a fresh sample is best. If there is a delay, ask the lab; some provide containers with preservative, and refrigeration (not freezing) may be allowed for some tests.
Do iron tablets affect the test?
Iron turns stool black but does not cause a positive FIT. It can interfere with some older guaiac tests, so tell the lab what medicines you take.
Does a negative occult blood test rule out cancer?
No test is perfect. If you have ongoing symptoms such as a change in bowel habit, weight loss or anemia, see a doctor even if the test is negative.
Related reading: Anemia: symptoms, causes and treatment.
Last updated: September 2026. General health information only, not a diagnosis. Please follow your lab's instructions and discuss results with a qualified doctor.
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