Gut Health & Microbiome - Testing
Are microbiome stool tests useful?
Last updated 30 September 2026. How we write this .
Consumer microbiome tests list the microbial DNA in one stool sample, but labs disagree on the same sample, and a 2025 expert consensus found too little evidence to recommend routine testing.
Start here Consumer microbiome tests list the microbial DNA in one stool sample, but labs disagree on the same sample, and a 2025 expert consensus found too little evidence to recommend routine testing.
By the end, you will understand What does a gut microbiome test actually measure? Would two companies give me the same result? Why might my result change from one week to the next? Where this information comes from Claim-cited evidence. Central research claims on this lesson are linked to specific studies or official guidance.
This lesson links the specific studies and guidance used for its central claims.
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For most people, not yet. A consumer stool test can tell you which microbial DNA turned up in one sample, analysed one particular way. It cannot reliably tell you whether your gut is healthy, what is causing your symptoms or what you should eat. In 2025 an international panel of 69 microbiome experts from 18 countries concluded that there is not enough evidence to recommend routine microbiome testing in clinical practice, and discouraged people from ordering tests for themselves without a clinical reason.
What does a gut microbiome test actually measure?
You put a little stool on a swab or into a tube and post it to a lab. The lab extracts the DNA and sequences it. Many tests read a single bacterial gene called 16S, which works like a barcode for groups of bacteria. Others use shotgun metagenomics, which reads all the DNA in the sample and can name organisms in finer detail. Software then matches the reads against a reference database and produces a list of organisms, each with a percentage next to it.
Those percentages are shares, not counts. If one group of bacteria doubles, every other group shrinks on paper even if nothing else changed. That matters more than it sounds. In 2017 Vandeputte and colleagues at KU Leuven counted microbial cells directly alongside sequencing and found up to tenfold differences in total microbial load between healthy people. An apparent trade-off between two common groups, Bacteroides and Prevotella, turned out to be an artefact of working in percentages. And DNA alone cannot show whether a microbe was alive or doing anything.
Would two companies give me the same result?
Probably not, and this has now been tested directly. Stephanie Servetas and colleagues at the US National Institute of Standards and Technology (NIST) prepared a homogenised reference stool, so every tube held the same material. In a study published in 2026 they sent it to seven direct-to-consumer services, three kits each. All 21 reports should have described the same gut. They did not. For identical material, one company reported 20 to 42 species and another about 1,470 to 1,700. Three companies said Clostridioides difficile, a bacterium that can cause serious bowel infection, was present, while four said it was absent. One company rated two of its three kits as healthy and the third as unhealthy, with seven of ten functional categories below average on that kit against one on the others. The differences between providers were on the same scale as the real differences between different people.
Research labs face the same problem when methods are not standardised. The Microbiome Quality Control project, led from the US National Cancer Institute and published in Nature Biotechnology in 2017, sent blinded stool samples and artificial microbial mixtures to 15 laboratories and nine data-analysis teams. DNA extraction, sample handling and software all shifted the results. Diversity estimates for the same specimen varied up to five-fold between labs, and the authors concluded that absolute diversity figures are not comparable from one lab to another. Within a single study the real differences between people still outweighed the noise. Someone comparing their score with a friend's from a different company has no such protection.
Why might my result change from one week to the next?
Your microbiome responds to what is happening in your gut. The Flemish Gut Flora Project, published in Science in 2016, analysed stool from 1,106 Belgian adults and checked the findings in 1,135 Dutch adults. The team found 69 factors linked to differences in microbial make-up. Stool consistency, meaning how firm or loose the sample was, had the largest effect of any single factor. Medicines together explained the most variation. So a sample taken during a loose week, after antibiotics or on a new diet can look quite different from one taken a month later, without your health having changed.
The same study found that several bacteria proposed as markers of disease were also linked to host factors like these. A report that flags a high or low level of a supposed marker may be picking up how your bowels were behaving that week rather than an illness.
What do experts say about gut health scores and diet advice?
The 2025 consensus, led by Porcari and Ianiro and published in The Lancet Gastroenterology and Hepatology, went through features that often appear in consumer reports. The panel voted on each statement, and at least 80% had to agree. Among its conclusions:
Reporting the Firmicutes-to-Bacteroidetes ratio, a popular single number, is discouraged. Broad groupings like this can mislead. There is not enough evidence to include any dysbiosis index. There is no agreed definition of dysbiosis, meaning an unhealthy imbalance of gut microbes, so it cannot yet be used in clinical practice. There is generally not enough information to set strict healthy reference ranges for how much of each species a person should have. Testing providers should not give treatment advice. Any change to a patient's care should come from the clinician who asked for the test. Labs should be accredited and use validated, up-to-date analysis software and databases. Put together, the panel's idea of a useful microbiome test is one ordered by a doctor for a specific question, run by an accredited lab, and read alongside the person's age, diet, medicines and bowel habits. A kit ordered online that comes back with a health score and a list of foods or supplements to change is close to the opposite.
Which stool tests do have a clear use?
Some stool tests are built to answer one defined question. The NHS bowel cancer screening programme posts a faecal immunochemical test, or FIT kit, every two years to people aged 50 to 74 who are registered with a GP. It looks for hidden blood in poo, which can be an early sign of bowel cancer. GPs also use FIT and other stool tests when someone has symptoms. These are not microbiome profiles, and a microbiome report is no substitute for them.
If a gut problem is what makes you curious about testing, see a GP instead. The NHS advises seeing a GP for changes in your poo or how often you go that are not usual for you, blood in your poo, tummy pain, bloating, or losing weight without trying. Ask for an urgent GP appointment or contact NHS 111 if your poo is black or dark red or you have bloody diarrhoea. Call 999 or go to A&E if you are bleeding non-stop from your bottom or passing a lot of blood or large clots.
Will microbiome testing ever be worth doing?
Possibly. Gut microbes are linked with many conditions, and one microbiome treatment is already standard care: faecal microbiota transplant is recommended for C. difficile infection that keeps coming back. The 2025 panel expects testing to find specific clinical uses and called for dedicated studies to build the evidence. What is missing is proof at each step. A test has to give the same answer for the same sample, its findings have to mean something for a defined group of people, and acting on them has to improve health. The NIST study shows consumer tests often fail the first step, and the experts judged the evidence for the other two too thin for routine use. For now, a report describes one sample on one day, not the state of your gut.
This lesson is general education about microbiome testing, not medical advice or a diagnosis. If you have ongoing or worrying gut symptoms, speak to your GP.
Related in this classroom Related in other classrooms The studies behind this page International consensus statement on microbiome testing in clinical practice - The Lancet Gastroenterology & Hepatology, 2025Evaluating the analytical performance of direct-to-consumer gut microbiome testing services - Communications Biology, 2026Assessment of variation in microbial community amplicon sequencing by the Microbiome Quality Control (MBQC) project consortium - Nature Biotechnology, 2017Quantitative microbiome profiling links gut community variation to microbial load - Nature, 2017Population-level analysis of gut microbiome variation - Science, 2016Bowel cancer screening - NHSSymptoms of bowel cancer - NHSFurther reading
Sources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.