SIBO Breath Testing vs Gut Microbiome Testing: What Is the Difference?
- Ola Krawczyk, Clinical Nutritionist
- 2 days ago
- 8 min read
ATMS Accredited Member | SIBO-Certified Nutritionist | Microba Certified Nutritionist
Published: 26 August 2026 | Last reviewed: 27 August 2026
If you experience persistent bloating, excessive gas, abdominal discomfort, constipation or diarrhoea, you may be wondering whether a SIBO breath test or gut microbiome test would provide the most useful information.
Although both tests investigate digestive health, they measure different things and should not be used interchangeably.
A SIBO breath test measures gases produced after drinking a test solution and is used to investigate suspected bacterial overgrowth—or methanogen overgrowth—in the small intestine.
A gut microbiome test generally analyses a stool sample to provide information about microorganisms and microbial functions represented mainly in the large intestine.
In some circumstances, the tests may complement each other. However, neither test should be selected or interpreted without considering your symptoms, health history, medications, diet and previous investigations.
Clinical Nutritionist’s Perspective: “A SIBO breath test and a gut microbiome test answer different clinical questions. If SIBO is suspected, a stool microbiome test should not be used to confirm or exclude it. Testing is most useful when it is selected for a clear reason and interpreted alongside the person’s symptoms and health history.” — Ola Krawczyk, Clinical Nutritionist, ATMS Member, SIBO-Certified Practitioner and Microba Certified Practitioner
Quick comparison
SIBO breath test | Gut microbiome test | |
Sample collected | Breath samples | Stool sample |
Main area assessed | Fermentation associated with the small intestine | Microorganisms and functions represented mainly in the large intestine |
What it measures | Hydrogen and methane; some tests may also measure hydrogen sulphide | Microbial DNA, composition, diversity and—in more comprehensive tests—functional potential or selected gastrointestinal markers |
Primary purpose | To support investigation of suspected SIBO or intestinal methanogen overgrowth | To explore the broader gut microbial ecosystem |
Can it diagnose SIBO? | It can support a SIBO assessment, although it has limitations | No |
Can it identify every cause of digestive symptoms? | No | No |
Preparation required | Yes—preparation is essential for test accuracy | Usually less extensive, but the laboratory’s instructions must be followed |
Interpretation | Should be considered with symptoms, risk factors and preparation quality | Should be considered with symptoms, diet, medications and clinical history |
What is SIBO?
Small intestinal bacterial overgrowth, commonly called SIBO, refers to an excessive number of bacteria or an altered bacterial population in the small intestine.
The small intestine normally contains microorganisms, but considerably fewer than the large intestine. When bacterial fermentation occurs in an inappropriate location or amount, it may contribute to symptoms such as:
Bloating or visible abdominal distension
Excessive gas or belching
Abdominal discomfort
Diarrhoea, constipation or alternating bowel habits
Nausea
Food intolerance
Nutrient deficiencies or unintended weight loss in more significant cases
These symptoms are not specific to SIBO. They may also occur with irritable bowel syndrome, coeliac disease, constipation, carbohydrate malabsorption, inflammatory bowel disease and other gastrointestinal conditions. Symptoms alone therefore cannot confirm SIBO.
How does a SIBO breath test work?
During a SIBO breath test, you drink a measured amount of glucose or lactulose. Breath samples are then collected at scheduled intervals, commonly over two to three hours.
Human cells do not produce hydrogen or methane. These gases are generated when microorganisms ferment carbohydrates. Some of the gas enters the bloodstream, travels to the lungs and can then be measured in the breath.
What do the gases mean?
Hydrogen
An early rise in hydrogen may support a diagnosis of hydrogen-predominant SIBO when it occurs within the relevant testing period.
The North American Consensus defines a positive hydrogen result as an increase of at least 20 parts per million above baseline within 90 minutes after consuming glucose or lactulose. Interpretation can still vary between laboratories and clinical guidelines.
Methane
Methane is produced by archaea rather than bacteria. For this reason, a methane-positive result is more accurately described as intestinal methanogen overgrowth, or IMO, rather than methane SIBO.
A methane level of at least 10 parts per million at any point during testing is generally considered methane-positive. Methane production is frequently associated with constipation and slower intestinal transit.
Hydrogen sulphide
Some newer tests also measure hydrogen sulphide. This may provide additional information in certain cases, particularly when diarrhoea or very low hydrogen readings are present. However, hydrogen sulphide testing is not yet available through every laboratory, and its interpretation is still evolving.
Glucose or lactulose: which breath test is better?
Both substrates have advantages and limitations.
Glucose is absorbed relatively early in the small intestine. It may therefore be more specific for overgrowth in the upper part of the small intestine but could miss overgrowth located further down.
Lactulose is not absorbed and travels throughout the small intestine before reaching the colon. It may detect fermentation occurring further along the digestive tract, but rapid intestinal transit can make it difficult to distinguish small-intestinal fermentation from normal fermentation in the colon.
A systematic review and meta-analysis reported:
Glucose breath testing: pooled sensitivity of 54.5% and specificity of 83.2%
Lactulose breath testing: pooled sensitivity of 42% and specificity of 70.6%
These figures highlight an important point: breath testing can contribute useful information, but it is not a perfect stand-alone test. A negative result does not always exclude SIBO, and a positive result must be interpreted in its clinical context. Systematic review and meta-analysis
Why preparation matters
Breath-test results can be affected by recent antibiotics, laxatives, promotility medications, probiotics, smoking, exercise, food choices and inadequate fasting.
Preparation requirements vary, but may include:
Avoiding antibiotics for approximately four weeks, where medically appropriate
Discussing probiotics, laxatives and promotility medications with the referring practitioner
Following the laboratory’s specified preparation diet the day before testing
Fasting for the required period
Avoiding smoking, vigorous exercise and sleeping during the test
Collecting every sample at the correct time
Never discontinue a prescribed medication without consulting the doctor or practitioner responsible for your care.
What is gut microbiome testing?
Gut microbiome testing generally uses a stool sample to analyse microbial DNA.
Depending on the laboratory and type of test, the report may include information about:
Microbial composition
Relative abundance of detected organisms
Microbial diversity and richness
Potential production of short-chain fatty acids such as butyrate
Potential methane, hydrogen sulphide or trimethylamine-related functions
Selected gastrointestinal markers
Particular pathogens, if pathogen screening is specifically included
Not every microbiome test examines the same organisms, genes, functions or clinical markers. The technology, reference database and reporting methods can differ considerably between providers.
What can a microbiome test tell you?
A carefully selected microbiome test may help build a broader picture of the gut ecosystem and may be useful for guiding discussions about:
Dietary variety and plant intake
Fibre and prebiotic tolerance
Microbial diversity
Fermentation patterns
Short-chain-fatty-acid production potential
The possible effects of diet, medications and lifestyle
Areas that may warrant further medical or nutritional investigation
However, a microbiome report is a snapshot taken at one point in time. Results may be influenced by recent diet, antibiotics, supplements, illness, bowel transit and sample-handling methods.
Importantly, the presence or absence of an individual organism does not automatically mean that it is causing symptoms.
Can a stool microbiome test diagnose SIBO?
No. A stool microbiome test cannot confirm or exclude SIBO.
Stool primarily represents microbial material from the large intestine. SIBO concerns microbial overgrowth or altered microbial populations within the small intestine. These are different environments.
A microbiome report might reveal patterns that help guide broader nutrition discussions, but it cannot determine whether bacterial fermentation is occurring too early in the small intestine.
This is one of the most important distinctions between the two tests.
Are microbiome tests clinically validated?
Microbiome science is developing rapidly, but the clinical interpretation of many commercial microbiome results remains limited.
A major international consensus published in The Lancet Gastroenterology & Hepatology in 2025 involved 69 clinicians and microbiome researchers from 18 countries. The panel concluded that there is currently insufficient evidence to recommend microbiome testing routinely across clinical practice.
The consensus did not suggest that microbiome testing has no potential. Rather, it highlighted the need for appropriate test selection, standardised methods, transparent reporting and interpretation within the person’s clinical context. International consensus statement on microbiome testing
This is why microbiome testing should not be presented as a definitive diagnosis, a complete explanation for every symptom or a list of foods that everybody must eliminate.
Which test might be more appropriate?
A SIBO breath test may be considered when:
Bloating or distension becomes worse after eating
There is persistent excessive gas or belching
Diarrhoea, constipation or mixed bowel habits are present
Symptoms or risk factors raise clinical suspicion of SIBO or IMO
A practitioner wants to assess hydrogen and methane production
Similar gastrointestinal conditions have been appropriately considered
A gut microbiome test may be considered when:
A broader assessment of the large-intestinal microbial ecosystem is wanted
You would like to explore microbial diversity and functional potential
Symptoms, diet and health history justify a more detailed investigation
The results will be interpreted by a suitably trained practitioner
The findings are likely to influence a safe and realistic nutrition strategy
Both tests may sometimes be useful when:
There are persistent digestive symptoms and the practitioner is asking two separate questions:
Is excessive or misplaced fermentation occurring in the small intestine?
What broader information can be gathered about the colonic microbiome and gastrointestinal environment?
Ordering both tests automatically is not always necessary. Testing should have a defined purpose and should be capable of influencing the next step in care.
Can either test identify parasites or pathogens?
A standard SIBO breath test does not identify specific bacteria, parasites or gastrointestinal pathogens.
Some comprehensive stool tests include pathogen screening, while others provide only microbiome composition and functional information. Always check what the selected test actually includes.
New or severe diarrhoea, blood in the stool, fever, persistent vomiting, unexplained weight loss, anaemia, severe abdominal pain or night-time symptoms require medical assessment. A general microbiome profile should not delay appropriate diagnostic testing.
Does a positive test tell you which treatment you need?
Not by itself.
A positive breath test does not explain why overgrowth developed. Possible contributing factors may include altered intestinal motility, chronic constipation, previous gastrointestinal surgery, anatomical changes, particular medications or an underlying digestive condition.
Similarly, a microbiome result should not lead automatically to restrictive diets or large supplement protocols.
A useful interpretation considers:
Your symptoms and their timing
Constipation or diarrhoea patterns
Medical and surgical history
Medications and supplements
Dietary intake and food tolerance
Relevant pathology or medical investigations
Possible drivers of recurrence
Whether the proposed intervention is safe and proportionate
The bottom line
SIBO breath testing and gut microbiome testing are not competing versions of the same test.
A SIBO breath test measures hydrogen and methane produced following a carbohydrate substrate and may support the investigation of SIBO or intestinal methanogen overgrowth.
A gut microbiome test analyses stool to provide broader information about organisms and microbial functions represented mainly in the large intestine. It may offer useful insights in selected circumstances, but it cannot diagnose or rule out SIBO.
The best test is not necessarily the most comprehensive or expensive one. It is the test that addresses a clear clinical question and is interpreted carefully alongside your symptoms, medical history, diet and other relevant findings.
Need help choosing the right test?
If you are experiencing persistent bloating, gas, constipation, diarrhoea or food-related digestive symptoms, I can help you review your history and determine whether SIBO breath testing, gut microbiome testing or another referral pathway may be appropriate.
I offer personalised online nutrition consultations across Australia, including nutrition support for digestive symptoms, SIBO, thyroid conditions, perimenopause and microbiome-related concerns.
This article is for educational purposes only and does not replace medical diagnosis, treatment or advice. Speak with your doctor about persistent, worsening or concerning gastrointestinal symptoms.
Author
Ola Krawczyk is a Polish-Australian Clinical Nutritionist, accredited ATMS member, SIBO-Certified Practitioner and Microba Certified Practitioner specialising in gut health, SIBO, thyroid support and perimenopause nutrition.
Her professional approach is also informed by her personal experience with SIBO, hormonal health challenges and thyroid cancer followed by thyroid removal. Ola provides warm, personalised and evidence-informed online nutrition consultations across Australia through Clinical Nutrition by Ola.
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