SIBO vs IBS Symptoms: Can You Tell Them Apart?

You cannot tell small intestinal bacterial overgrowth (SIBO) and irritable bowel syndrome (IBS) apart from symptoms. Not by the bloating, not by the burping, not by whether it is diarrhoea or constipation, not by how soon after a meal it starts. Every widely used symptom of one is also a symptom of the other, and in one study of people investigated for suspected overgrowth, total symptom scores were statistically indistinguishable between those who tested positive and those who tested negative on both a duodenal culture and a breath test (Rao 2019).

That is an unsatisfying answer to a question you probably asked because you are three years into an IBS label and it has not helped. So this article does something different from the symptom-comparison tables you have already read. It explains why the overlap is this total, what the breath test actually measures and how often it is wrong in both directions, why two gastroenterologists will give you opposite advice about whether to run one, who is paying for the research underneath that advice, and what is genuinely worth investigating before you spend money chasing a gas reading.

The short answer: the symptom lists are the same list

Every article that promises to distinguish SIBO from IBS by symptom ends up doing one of two things: repeating group averages as if they were individual rules, or inventing a distinction that no study supports.

The group averages are real but weak. In a Chinese study comparing people with IBS, people with a positive breath test and healthy controls, the IBS group actually had the most severe symptoms overall, characterised by intense abdominal pain and frequent diarrhoea, while bacteria enriched in the overgrowth group correlated with constipation (Lu 2024). That is the opposite of the popular claim that overgrowth is the more severe of the two. In the breath test accuracy study, bloating was somewhat more common in people with a positive breath test and gas was more common in people with a positive culture (Erdogan 2015). Small differences, in opposite directions, across different studies.

And then there is the finding that matters most. In a review of the clinical features of overgrowth, the authors point to a study in which mean total symptom scores were the same regardless of whether the patient tested positive or negative on duodenal aspirate and breath testing, with a p value of 0.9 (Rao 2019). Symptoms in that group carried essentially zero information about the test result.

This is the number that reframes the whole question. If close to a third of asymptomatic controls also test positive, a positive result in you tells you much less than it feels like it should. It is a finding shared by people with symptoms and people without them.

Why the overlap is this complete

Both conditions end at the same place: gas in a gut that is sending signals to a brain. They just arrive there by different routes, and the route is invisible from the outside.

The overgrowth route. Bacteria that belong further down the tract sit in the small intestine and ferment carbohydrate early, before it has been absorbed. Fermentation produces hydrogen, and in some people methane or hydrogen sulfide. Gas expands the small bowel, which has a smaller capacity than the colon and is more mechanically sensitive. Symptoms traditionally linked to this are bloating, diarrhoea and abdominal pain or discomfort, with fatty stool in more severe cases (Quigley 2020).

The IBS route. IBS is a disorder of gut-brain interaction. The gut may produce an ordinary amount of gas, but the signalling from gut to brain is amplified, so ordinary distension registers as pain and urgency. That is why the BSG guideline asks clinicians to explain IBS to patients as a gut-brain condition rather than a diagnosis of exclusion (Vasant 2021).

Now put them together. If you have amplified gut-brain signalling and you also have early fermentation, the second makes the first worse and neither can be identified from the sensation. Guidelines treat these as overlapping rather than competing: the AGA's practice update says outright that controversy remains about the role of overgrowth in the pathogenesis of common functional symptoms such as those considered part of IBS (Quigley 2020).

It is also worth knowing how common the headline symptom is in people with no diagnosis at all. In the Rome Foundation Global Epidemiology Study, which surveyed 51,425 people across 26 countries after excluding those with likely organic causes, nearly 18% reported bloating at least weekly, ranging from 11% in East Asia to 20% in Latin America, and women were about twice as likely as men to report it (Ballou 2023). Weekly bloating is not, by itself, a sign of anything specific.

What the numbers actually say about overgrowth in IBS

The honest summary: more people with IBS test positive than controls, the size of that gap depends entirely on which test you use, and with the most commonly sold test the gap can vanish.

Four separate meta-analyses have asked this question and they do not line up neatly.

Read the Ghoshal line again, because it is the crux. The lactulose breath test, which is the substrate that generates by far the most positive results, was the one test that did not distinguish people with IBS from healthy people. The tests that did distinguish them, glucose breath testing and jejunal culture, found much lower prevalence figures. Ford's 2009 analysis says the same thing from the other direction: 54% positive with lactulose, 4% positive with aspirate and culture, in the same population.

That is not a small discrepancy. It is a more than tenfold difference in how many people you would tell they have a bacterial overgrowth, driven purely by which test you happened to be given.

What the breath test actually measures, and how it goes wrong

A breath test measures gas. It does not see bacteria, and it does not know where in your gut the gas came from.

You fast, you drink a measured dose of glucose or lactulose, and your breath is sampled at intervals for two to three hours. The North American consensus set the doses at 75 g for glucose and 10 g for lactulose, and defined a positive result as a rise in hydrogen of at least 20 parts per million from baseline within 90 minutes, or methane of at least 10 parts per million (Rezaie 2017).

Three structural problems sit inside that.

The 90-minute assumption. The window exists because carbohydrate is assumed to take about 90 minutes to reach the colon, where normal fermentation happens. If your transit is faster than average, the sugar reaches your colon early, the normal colonic bacteria ferment it on schedule, and the rise looks like small bowel overgrowth. The BSG guideline cites exactly this as a reason these tests produce false positives in people with IBS (Vasant 2021). Fast transit is common in IBS with diarrhoea, so the test misfires most in the group most likely to be given it.

The substrate trade-off. Lactulose is not absorbed, so it always reaches the colon and always ferments somewhere, which is why it generates so many positives. Glucose is absorbed high in the small bowel, which makes it more specific but means it never reaches an overgrowth further down. Glucose has good specificity and low sensitivity, lactulose the reverse (Rao 2019).

The absent gold standard. Breath tests are usually validated against small bowel aspirate and culture. But a systematic review of 71 papers concluded that there is no validated diagnostic test or gold standard for overgrowth at all, and that not even the culture studies met published quality standards for diagnostic test research (Khoshini 2008). You cannot calculate the accuracy of a test against a reference that is itself unvalidated.

When someone has measured breath testing against culture anyway, the numbers are modest. In 139 patients with unexplained gas, bloating and diarrhoea who had both tests within a week, duodenal culture was positive in 44.6% and the glucose breath test in 27.3%. The two agreed on the diagnosis in 65.5% of cases. Against culture, the breath test had a sensitivity of 42% and a specificity of 84% (Erdogan 2015).

Why two gastroenterologists give you opposite answers

This is the part almost no article explains, and it resolves a lot of confusion: the major guidelines genuinely disagree, and which one your clinician follows usually depends on where they trained.

Notice how narrow the permissive language actually is. The AGA's bloating update says testing may be used in a small subset of at-risk patients, not in everyone who bloats. Somewhere between that sentence and the clinic, it became a first-line test.

On Reddit that gap shows up as a fight. Under a thread by a doctor with IBS-D asking what newly diagnosed patients wish they had been offered, the highest-scoring comment mentioning overgrowth was a flat "SIBO breath test should be the first stop for all folks with IBS", at 43 points, alongside other comments describing the IBS label as a way to get patients out of the door (r/FODMAPS thread). That frustration is legitimate. So is the BSG's reading of the evidence. Both can be true at once, and neither position is proven by the strength of feeling behind it.

Follow the funding

If you are going to weigh these positions yourself, it is worth knowing who paid for the work on each side. This is not a conspiracy claim. It is printed in the papers, in the disclosures, and it is normal practice to declare it.

The North American consensus that set the breath test thresholds states that the consensus meeting was supported in part by Commonwealth Laboratories, a breath testing company. Several authors declare consulting relationships with Commonwealth Laboratories and with Salix Pharmaceuticals, which markets rifaximin, and the document notes that Cedars-Sinai holds licensing agreements with Valeant Pharmaceuticals, Commonwealth Laboratories and Synthetic Biologics (Rezaie 2017).

The review that provides some of the most useful clinical detail in this article was itself supported by Salix Pharmaceuticals, and its senior author declares previous research funding from the same company (Rao 2019). The blood test sometimes marketed as an objective IBS diagnostic, measuring anti-CdtB and anti-vinculin antibodies, was developed in a study that included authors employed by Salix Pharmaceuticals; it reported specificity of 91.6% but sensitivity of only 43.7% for anti-CdtB, meaning it misses more than half of the people it is meant to identify (Pimentel 2015).

None of this makes the science wrong. Industry funds a large share of gastroenterology research, and declarations like these are routine and properly published. But when a field's diagnostic thresholds, its most-used test and its most-used drug all trace back to an overlapping set of commercial relationships, and a guideline produced by an entirely different group reaches the opposite conclusion, the disagreement is worth understanding rather than treating as one side simply being behind the times.

Symptom patterns that shift the odds a little

With all of that caveated, there are patterns that make one explanation slightly more or less likely. Treat these as things to mention to your doctor, not as a self-diagnosis.

If your bowel pattern itself is what you are trying to pin down, our guide to the difference between IBS-D and IBS-C explains the subtype rules and why your memory of your own pattern is often wrong.

Who genuinely has classical overgrowth

There is a version of this condition that is not contested at all, and it looks different from typical IBS.

Classical overgrowth develops when something mechanical or neurological lets bacteria accumulate where they should not. Small intestinal dysmotility is the best documented: in people studied with duodenal aspirate and culture, dysmotility raised the odds of overgrowth roughly threefold, and among people with IBS, 86% of those with overgrowth were considered to have dysmotility compared with 39% of those without (Rao 2019). Other associations in the same review include previous abdominal surgery such as gastrectomy, cholecystectomy or colectomy, small bowel diverticula, pancreatitis, hypothyroidism, diabetes and Parkinson's disease, with reported prevalence ranging enormously from 4% to 79% depending on the population and the test used. In one case-control study, 62% of people who had undergone colectomy tested positive compared with 32% of people with long-standing gut complaints and no colectomy.

The same review notes that up to 13% of healthy individuals test positive on breath testing or aspirate and culture, which is a lower figure than the 29.7% from the case-control meta-analysis but points the same way (Rao 2019, Shah 2020).

The AGA's advice for this group is to focus management on identifying and correcting the underlying cause where possible, correcting nutritional deficiencies, and then antibiotics, and it is explicit that this matters most for people with significant maldigestion and malabsorption (Quigley 2020). If you have had bowel surgery, an established motility disorder, unexplained weight loss or documented nutrient deficiency, you are in a different conversation from someone with fifteen years of bloating and normal bloods.

What is worth ruling out before you chase a gas reading

Several conditions cause exactly the same symptoms, have clearer tests, and change management in a way a breath test usually does not. These are the ones with real numbers behind them.

Coeliac disease. Pooling 14 studies and 4,204 people, biopsy-proven coeliac disease was found in 4.1% of those meeting IBS criteria, more than four times the rate in controls (OR 4.34) (Ford 2009, coeliac). The ACG IBS guideline suggests serologic testing in anyone with IBS and diarrhoea (Lacy 2021). Testing has to happen while you are still eating gluten, which is a reason not to cut it out before asking.

Inflammatory bowel disease. The same guideline suggests checking faecal calprotectin in suspected IBS with diarrhoea (Lacy 2021). It is a stool test, it is cheap, and a raised result changes everything that follows.

Bile acid malabsorption. Across six studies and 908 people with IBS-D who underwent SeHCAT testing, the pooled rate was 28.1% (Slattery 2015). The BSG recommends considering testing in people with IBS-D who have atypical features such as night-time diarrhoea or a previous gallbladder removal (Vasant 2021). It is one of the most upvoted "it was not IBS after all" stories in r/ibs for a reason (r/ibs thread).

Post-infectious IBS. If it started after a bout of gastroenteritis, the prospective data support IBS as an outcome in about one in ten people at twelve months (Klem 2017). It does not give you a different treatment, but it gives you an explanation, and it stops the search.

A pelvic floor or evacuation problem. The AGA suggests anorectal physiology testing when bloating and distension are linked to constipation or difficult evacuation (Moshiree 2023). Antibiotics will not help this and biofeedback might.

None of these require a breath test, and all of them are worth asking about before you pay for one. If you are unsure whether your situation warrants specialist input at all, our guide on whether to see a gastroenterologist for IBS covers what a GP can order without a referral.

If the test is positive, what usually happens next

A positive result is typically followed by an antibiotic course, most often rifaximin, which is poorly absorbed and acts mainly in the gut. It is worth knowing what the evidence for that actually shows before you decide how much weight to place on the result. Every decision here belongs with your doctor.

Pooling 32 studies and 1,331 patients, rifaximin normalised the breath test in 70.8% of people, with adverse events in 4.6%. Among the subset of studies that reported it, symptoms improved or resolved in 67.7% of those whose test normalised. The authors were direct that the quality of the available studies is generally poor and that well designed trials are still needed (Gatta 2017). The Rao review adds that no drug has regulatory approval specifically for this condition in the United States or Europe (Rao 2019).

Recurrence is the part that catches people out. In 80 patients whose breath test had normalised after rifaximin, the test was positive again in 12.6% at three months, 27.5% at six months and 43.7% at nine months, and symptoms returned alongside it (Lauritano 2008). Older age, previous appendectomy and long-term proton pump inhibitor use were associated with recurrence.

Separately, rifaximin has trial evidence in IBS with diarrhoea without any reference to overgrowth. In the two TARGET trials, adequate relief of global IBS symptoms was reported by 40.7% on rifaximin and 31.7% on placebo, and relief of bloating by 40.2% and 30.3%. Both trials were funded by Salix Pharmaceuticals (Pimentel 2011). A nine point difference over placebo is a real effect and also a reminder that most of the response in both arms was not attributable to the drug.

The AGA's reading of this is careful and worth quoting in spirit: although IBS responds to a poorly absorbed antibiotic, whether that response is explained by eradicating overgrowth still needs confirmation in randomised trials (Quigley 2020). And the BSG states that a positive breath test does not predict who will respond (Vasant 2021).

A worked example: two people, identical symptoms

Maya, 29. Bloating that builds through the day, three to five loose stools most mornings, wind, cramping that eases after a bowel movement. It started two months after a bad case of food poisoning on holiday. Bloods are normal, coeliac serology negative, calprotectin normal. She reads that overgrowth causes IBS, buys a lactulose breath test online and it comes back positive.

What that result means in her case: she is one of the roughly 54% of people with IBS who test positive on lactulose, in a population where about 4% test positive on aspirate and culture (Ford 2009). Her loose morning stools point to faster transit, which makes an early hydrogen rise more likely for reasons that have nothing to do with the small bowel (Vasant 2021). Her history of onset after enteritis fits post-infectious IBS, which affects about one in ten people after an episode (Klem 2017). The positive test is not meaningless, but it is weak evidence, and her GP's suggestion of a structured low FODMAP trial with a dietitian rests on better data than the test does.

Tom, 61. The same bloating, the same loose stools, plus stools that are pale and hard to flush, seven kilograms of unintended weight loss over five months, and a small bowel resection twelve years ago. His B12 is low.

Different situation entirely. Previous abdominal surgery is among the recognised predisposing factors, fatty stool and nutrient deficiency are features of the malabsorptive picture, and the AGA frames management around correcting the underlying cause and the nutritional consequences as well as the bacteria (Rao 2019, Quigley 2020). Tom's weight loss needs a doctor promptly regardless of what any breath test says.

Same symptom list. Completely different meaning. That is why the symptom comparison tables cannot work.

What to log for four weeks before you test

If you are going to do a breath test, or argue for one, the most useful thing you can bring is a record that makes your pattern legible. This is also the thing that makes a normal test result useful rather than just disappointing, because it turns the question from "which label" into "what actually changes my day".

Log stool form and frequency daily. Bristol type and a count. This is what tells you whether you are dealing with a diarrhoea-predominant, constipation-predominant or mixed pattern, which is the one variable that does shift the prior a little.

Time the gap between eating and symptoms. The 90-minute assumption inside the breath test is about transit. If your symptoms land routinely within an hour, or routinely the next day, that is worth knowing and worth saying. Our guide to how long after eating IBS symptoms start explains why the gap is so variable.

Record what you ate in enough detail to be re-checkable. Not "curry". The components. Fermentable carbohydrate load is the thing both explanations run on, so you want to be able to look back and see whether the bad days cluster around particular ingredients rather than particular labels.

Note the non-food inputs. Sleep, stress, menstrual cycle, antibiotics, any new medicine, any illness. People in r/SIBO describe symptoms clustering at particular points in the cycle, and discussion there ranges widely over possible mechanisms (r/SIBO thread). Whether or not those mechanisms hold up, a pattern that tracks your cycle is not going to be explained by a gas reading.

Write down what you have already tried and what happened. Including any antibiotic courses and how long the improvement lasted. Given the recurrence figures, "it helped for four months" is clinically informative (Lauritano 2008).

Four weeks is usually enough to see a pattern and short enough that you will actually finish it. If pen and paper works, use pen and paper; our guide to keeping a food diary for IBS covers the mechanics and the traps. Clairop does the same job with less typing: Bristol stool type is one tap, urgency and blood sit underneath as optional detail, and meals can be described out loud. It compares symptoms after meals with and without each food across three delay windows, from within six hours to one to three days, and the method page explains why it waits for at least five meals with and without a food before showing anything.

Myths about SIBO and IBS

Myth: "60 to 70% of IBS is really SIBO." This figure circulates widely and no meta-analysis supports it. The pooled prevalence figures are 35.5% by breath testing, against 29.7% in controls (Shah 2020), and 4% by jejunal aspirate in the earlier analysis (Ford 2009). Prevalence of a positive test is also not the same as causation.

Myth: "A positive breath test proves your IBS diagnosis was wrong." It does not, for two reasons. Around three in ten healthy controls also test positive (Shah 2020), and IBS is diagnosed on symptom criteria that a person with overgrowth can meet at the same time. The AGA treats overlap as the normal state of affairs (Quigley 2020).

Myth: "A negative breath test proves you do not have it." Against culture, the glucose breath test had a sensitivity of 42% (Erdogan 2015). It misses more than it finds.

Myth: "Methane on a test explains my IBS." Methane positivity is genuinely associated with constipation (Kunkel 2011), but it is no more common in people with IBS than in controls overall (Gandhi 2021).

Myth: "IBS is a wastebasket label doctors use when they have not looked." Under Rome IV criteria, IBS is a defined condition with a prevalence of about 4.1% in the Rome Foundation's internet survey and 1.5% in household interviews, notably lower than under the older Rome III criteria (Sperber 2021). The ACG explicitly recommends making it as a positive diagnosis rather than by exclusion (Lacy 2021). A poorly explained diagnosis is a communication failure, not proof the diagnosis is empty.

Myth: "There is a blood test that settles it." The anti-CdtB and anti-vinculin panel had specificity of 91.6% but sensitivity of 43.7% in its validation study, and was less specific at separating IBS from coeliac disease (Pimentel 2015). A test that misses more than half of cases is not a settlement.

Myth: "Probiotics will sort the bloating out." The AGA's bloating update advises that probiotics should not be used to treat abdominal bloating and distension (Moshiree 2023).

Myth: "If I just find the root cause, this resolves." A widely upvoted post in r/SIBO pushes back on this from inside the community, arguing that the sub is full of single-cause claims that do not replicate, that both terms are umbrellas covering very different situations, and that the shared experience there may be a shared test result rather than a shared disease (r/SIBO thread). That is a fair reading of the evidence as well as of the forum.

When to see a doctor promptly

Symptom chasing is not the right response to any of the following. See a doctor promptly, rather than ordering a test or starting a diet, if you have:

• Blood in your stool, or black tarry stools
• Unintended weight loss
• Fever, or night sweats
• Symptoms that wake you from sleep
• Pale, greasy or hard-to-flush stools, which can indicate fat malabsorption
• Symptoms that started for the first time after the age of 50
• Feeling unusually tired or breathless, which can indicate anaemia
• A family history of bowel cancer, coeliac disease or inflammatory bowel disease
• A new and persistent change in bowel habit lasting more than six weeks

Seek urgent same-day help for severe abdominal pain, persistent vomiting, a swollen tender abdomen, or being unable to keep fluids down.

Also see a doctor before you act on a test you bought yourself. Home breath test kits produce a result with no clinician attached to interpret it, and the thresholds they apply vary. A result is a starting point for a conversation, not an instruction.

The honest bottom line

The question "is it SIBO or IBS?" is nearly always a proxy for a better question: "is there something treatable here that nobody has looked for?" That question deserves a serious answer, and it usually is not answered by a breath test.

What the evidence supports is roughly this. The two conditions overlap so heavily that symptoms cannot separate them. Breath testing is positive in a large minority of healthy people, its result depends on which sugar and which threshold your lab uses, and it does not predict who will respond to treatment. A genuinely distinct malabsorptive picture exists, and it comes with weight loss, fatty stool or nutrient deficiency, usually in someone with a structural or motility problem. And the conditions most worth excluding first, coeliac disease, inflammatory bowel disease and bile acid malabsorption, all have clearer tests than anything in this field.

If your current diagnosis is not helping, the productive move is usually not a new label. It is a clear record of your own pattern, a short list of the specific tests that have not yet been done, and a doctor willing to go through them with you. For more on the subtypes and how they change what gets ordered, browse our IBS types guides.