Gut Health and Stool Testing: A Practical Guide to Choosing the Right Test

Digestive symptoms such as diarrhoea, bloating, abdominal discomfort and changes in bowel habits can have many possible causes. Stool testing may provide useful information, but the right test depends on what you are trying to investigate.

Some tests look for infection, inflammation or hidden blood. Others examine parasites, markers associated with colorectal cancer, or the composition of the gut microbiome.

There is no single test that provides a complete assessment of digestive health. A larger testing panel is not automatically a better one. The most useful test is usually the one selected to answer a specific health question.

Depending on the test, a stool sample may be used to look for:

Each type of test provides different information. For example, a test for intestinal inflammation cannot identify which bacteria make up the microbiome. In the same way, a microbiome profile does not replace targeted testing for infection, bleeding or inflammatory bowel disease.

A general stool examination may use microscopy to look for parasites, eggs, cysts and other findings within the sample. A stool concentration method processes the specimen to make parasites present in small numbers easier to detect.

These tests may be considered when symptoms or exposure history suggest a possible intestinal parasite. Examples include persistent diarrhoea or illness following relevant travel, contaminated food or unsafe water exposure.

One negative result does not always exclude a parasitic infection. Parasites may not be present in every sample, and some require specific antigen or molecular tests that are not included in a routine stool examination.

More than one specimen collected on separate days may be recommended when there is a strong suspicion of infection. The appropriate number and type of samples depend on the parasite being investigated.

Laboratories can investigate infectious diarrhoea using microscopy, antigen detection, toxin testing, culture or molecular methods such as PCR. The method used depends on the organism being considered.

Most brief, uncomplicated episodes of diarrhoea do not require laboratory testing. Testing becomes more relevant when symptoms are severe or persistent, when stool contains blood or mucus, after certain travel or food exposures, or in people with weakened immune systems.

Results still need to be considered alongside symptoms and exposure history. Some microorganisms can be carried without causing illness. Molecular tests may also detect genetic material from organisms that are no longer viable.

Clostridioides difficile, commonly called C. difficile or C. diff, can cause diarrhoea and inflammation of the colon. Testing may be considered when someone develops otherwise unexplained diarrhoea, particularly after antibiotic treatment or healthcare exposure.

Testing is generally intended for people with active diarrhoea and is usually performed on an unformed stool sample. This matters because some people carry C. difficile without having an active infection.

The meaning of a positive or negative result also depends on the laboratory method used. Toxin tests and molecular tests detect different features of the organism and may have different limitations. Results should therefore be interpreted together with the person’s symptoms and medical history.

Testing people without diarrhoea, or repeating a test simply to show that treatment has worked, can produce misleading results.

Helicobacter pylori is a bacterium that infects the stomach lining. It is associated with gastritis, peptic ulcers and an increased risk of certain stomach cancers.

A stool antigen test looks for proteins associated with an active H. pylori infection. It may be used during the initial investigation of appropriate symptoms or after treatment to confirm that the infection has been eradicated.

Medication and timing can affect the result. Proton pump inhibitors, antibiotics and bismuth-containing medicines may reduce the amount of detectable bacteria and contribute to a false-negative result.

Current clinical guidance recommends waiting at least four weeks after completing antibiotics before performing a test of cure. Proton pump inhibitors are generally paused for at least two weeks beforehand. Do not stop prescribed medication without first discussing it with a healthcare professional.

Calprotectin is a protein released by certain white blood cells during inflammation. Measuring it in stool can help assess whether inflammation is present in the intestines.

The test is often used when investigating persistent digestive symptoms. It may also be used to monitor people with established inflammatory bowel disease.

A low result makes significant active intestinal inflammation less likely. An elevated result suggests that further assessment may be needed.

Calprotectin is not specific to one condition. Levels can rise because of inflammatory bowel disease, intestinal infection and several other causes. The test cannot diagnose Crohn’s disease, ulcerative colitis or colorectal cancer on its own.

An abnormal result may need to be followed by additional tests, imaging or endoscopy.

A faecal immunochemical test, commonly called FIT, checks for small amounts of human blood that may not be visible in the stool.

FIT is widely used for colorectal cancer screening. Blood may come from colorectal cancer or a precancerous polyp, but it can also have other causes. A positive result does not mean that cancer is present.

An abnormal FIT result should be followed by further assessment, usually colonoscopy, to identify the source of the bleeding. A negative result cannot rule out every colorectal condition.

Screening intervals and the most appropriate screening method depend on age, personal risk, family history and local recommendations.

FIT is intended for screening people without warning symptoms. Visible blood or black, tarry stool requires medical assessment and should not be managed as routine screening.

Stool DNA tests look for molecular changes that may be associated with colorectal cancer or advanced precancerous growths.

The term “stool DNA test” covers several different types of assay. Some combine multiple DNA markers with testing for hidden blood. Others examine a particular methylation marker, such as SDC2.

These tests are not interchangeable. Their performance, thresholds, recommended screening intervals and acceptance within clinical guidelines may differ.

Research into SDC2 and other methylation markers has produced promising results, but performance varies between assays and study populations. Stool DNA tests are screening tools and do not provide a definitive diagnosis.

A positive result should be followed by further investigation, usually colonoscopy. A negative result cannot guarantee that colorectal cancer or an advanced polyp is absent.

The gut microbiome is the community of bacteria and other microorganisms living in the digestive tract.

Microbiome profiling typically analyses microbial genetic material in a stool sample. Depending on the method used, the report may describe the organisms detected, their relative abundance and measures such as microbial diversity.

This provides a descriptive snapshot of the submitted sample. It is not the same as testing for a specific infection, intestinal inflammation or hidden blood.

There is currently no single, universally accepted definition of a healthy microbiome. Microbial composition varies considerably between people and can change with diet, medication, antibiotic exposure, illness, travel and time. Sample collection, storage and laboratory methods can also affect the result.

For these reasons, routine microbiome profiling currently has limited established clinical value. An international expert consensus found insufficient evidence to recommend microbiome testing for widespread routine use in clinical practice.

A microbiome result should not be used on its own to diagnose dysbiosis, irritable bowel syndrome, inflammatory bowel disease, food intolerance or another medical condition. It should also not be the sole basis for choosing antibiotics, probiotics, supplements or restrictive diets.

Microbiome profiling may be of interest to people who want to explore their microbial composition. However, the findings should be interpreted cautiously and within the wider clinical picture.

The most suitable test depends on your symptoms, health history and reason for testing.

People with ongoing symptoms may require blood tests, imaging, endoscopy or other investigations in addition to a stool test. In some cases, a different type of investigation may be more appropriate from the outset.

Collection instructions vary between tests. Some require a fresh sample, while others use a container with a preservative or stabilising solution.

Always follow the instructions supplied with the test kit. In general:

Incorrect collection, storage or timing can affect the reliability of the result.

Stool testing should not delay medical care when symptoms may indicate a more serious problem.

Seek prompt medical advice if you have:

Earlier assessment may also be appropriate during pregnancy, after recent antibiotic treatment, or for older adults, young children and people with weakened immune systems.

There is no single comprehensive test for gut health. A useful stool test is one selected to answer a particular question.

Targeted testing can help identify infection, inflammation, hidden bleeding or other findings that may require follow-up. Microbiome profiling provides a different type of information and should be understood as descriptive rather than diagnostic.

If you are unsure which test is appropriate, discussing your symptoms, medical history, medication use and recent exposures with a healthcare professional can help guide the next step.