While gut tests can rule out some possible causes of your symptoms, none of the tests mentioned in this article can confirm that your problem is "just" irritable bowel syndrome (IBS). Quick overview:
- Worth it, with limits: Measuring calprotectin is a well-validated method for ruling out inflammatory bowel disease (IBD). A normal result is very reassuring. An elevated level requires further testing, as it does not constitute a diagnosis on its own.
- Use with caution: The small intestinal bacterial overgrowth (SIBO) breath test is widely used, but the results depend heavily on how quickly the test sugar passes through your intestines. Repeated tests in healthy individuals have yielded varying results.
- Not a standalone answer: Zonulin (commercial tests likely measure other proteins), alpha-1-antitrypsin, secretory IgA, β-defensin, EPX, and cytokines in stool. These may be part of a more comprehensive, medically supervised evaluation, but a single value in a self-paid test report rarely provides a definitive answer.
- Before you spend any money, ask yourself: What question is this test supposed to answer? Would a single marker answer that question for less money than the complete package? Who will help me interpret the results?
When I first mentioned my digestive problems to my family doctor—a very dedicated and kind person—he ran a few tests but still sent me on my way with the vague statement, “It’s probably irritable bowel syndrome; unfortunately, there’s not much we can do about it. Maybe try some peppermint.”
Many people with gut symptoms hear some version of "It is probably just irritable bowel syndrome" once their first tests come back normal. It is the word "just" that stings because it feels like the end of the conversation. That's it, live with it. Or, even worse, the problem is all in your head.
Irritable bowel syndrome (IBS) is a real condition with real symptoms, but it describes what you feel, not why you feel it. This article explores what common gut tests can and cannot reveal, so that a normal result serves as a starting point for further questions rather than the end of the investigation.
An important clarification up front
Most of the tests we are presenting today measure a specific biochemical marker. This could be inflammatory activity, bacterial gas production, or the permeability of the intestinal mucosa. This distinguishes them from pure microbiome analyses, which “only” measure which bacteria are actually present.
A biochemical marker is not necessarily synonymous with high quality. Some of these markers are among the most thoroughly researched tools in gastroenterology, while others—though heavily promoted in functional medicine—have not yet been conclusively validated scientifically.
Hardly any other field of medicine is currently changing as rapidly as gut research. What was considered established fact just a few years ago is, in some cases, already being revised today. On the one hand, this makes the field incredibly exciting; on the other hand, it also means that, as someone affected by this issue, you should take a particularly critical look before investing money in a test.
A quick note on my part: I research all of this to the best of my knowledge and belief, and my findings reflect the current state of research as best as I can assess it at the time of writing. However, even I make mistakes, and what is considered fact today may be outdated tomorrow. If you notice anything, please feel free to comment.
Step 1: ruling out inflammatory bowel disease with fecal calprotectin
The calprotectin test is a highly reliable method for ruling out inflammatory bowel disease (IBD) and is often performed routinely by a primary care physician when patients experience intestinal symptoms.
Calprotectin is a calcium-binding protein produced by immune cells such as neutrophils and monocytes when inflammation is present in the gut. This makes it a reliable marker for distinguishing classic chronic inflammatory bowel diseases, such as Crohn’s disease or ulcerative colitis, from other conditions with similar symptoms, such as the irritable bowel syndrome (IBS).
Crohn’s disease and ulcerative colitis are chronic inflammatory bowel diseases characterized by detectable biochemical or structural changes in the tissue, changes that are measurable and visible under a microscope. For this reason, these conditions are classified as organic diseases.
Irritable bowel syndrome is associated with similar or nearly identical symptoms. However, during a colonoscopy, the intestinal tissue usually appears completely normal. For this reason, the term “functional disorder” is used. “Functional,” however, does not mean that the pain and symptoms are imaginary or even psychosomatic. The symptoms are very real. Unfortunately, patients are often still classified into this category.
When should you take the test?
Measuring calprotectin is always helpful in cases of severe intestinal symptoms to rule out IBD. A recent meta-analysis involving nearly 2,000 patients [1] confirms the accuracy of the test, with a sensitivity of 85.8% and a specificity of 91.7%. IBD is relatively rare: assuming a prevalence of 1%, the negative predictive value is 99.8%. A negative result can therefore reassure you with a very high degree of certainty, because it rules out IBD.
On the other hand, a positive result does not automatically mean that IBD is present. The positive predictive value is significantly lower, at only 10%, primarily due to the low prevalence. For this reason, further medical evaluation is usually necessary, such as a colonoscopy.
In simple terms: if the concepts of sensitivity and specificity give you a headache, imagine 10,000 people, 100 of whom actually have IBD (1%). Here is what the test does with them:
- 86 of the 100 people with IBD test positive (sensitivity 85.8%).
- 14 of them are missed and receive a false-negative result.
- 9,078 of the 9,900 people without IBD test negative (specificity 91.7%).
- 822 of them (about 8.3%) still test positive, although they do not have IBD at all.
In total, 908 people test positive (86 + 822). Of these, only 86 (about 10%) actually have IBD. That is why a positive result is a reason for further evaluation, not a diagnosis.
What if the result is normal?
Then the test has done its job: inflammatory bowel disease is very unlikely. It does not explain your symptoms, though. Irritable bowel syndrome is diagnosed from your symptoms once other causes have been looked at, so a normal calprotectin neither confirms nor excludes it. A normal result is a good reason to talk with your doctor about which other causes fit your situation.
Health Disclaimer
This article addresses a health topic. It is important to have your symptoms examined and treated by a qualified healthcare professional. This article cannot replace, and does not intend to replace, professional medical care and advice.
Step 2: SIBO breath tests for small intestinal bacterial overgrowth
Most of you have probably come across the term SIBO, or small intestinal bacterial overgrowth, at some point. That is because this condition can manifest in many different ways, with a whole range of symptoms and numerous causes. Especially when something is already a bit off in the digestive tract, it often comes on top of that—like the proverbial icing on the cake—only nowhere near as pleasant.
In small intestinal bacterial overgrowth (SIBO), bacteria normally found in the large intestine colonize the small intestine, and this abnormal colonization triggers a number of symptoms. The majority of cultivable intestinal bacteria are actually found in the large intestine (> 10¹⁰ CFU/g; CFU = colony-forming units), whereas the small intestine, by comparison, resembles a bacterial desert (< 10³ CFU/g). These misplaced bacteria compete in the small intestine for the nutrients we consume—nutrients that are actually meant to be processed by our cells—and they produce substances that damage the epithelium and, on top of that, inhibit fat digestion.
We can easily imagine the consequences: micronutrient deficiencies, fatigue, headaches, difficulty concentrating, and reduced stress resilience. The most obvious problems, however, arise from gas production. Gas in the rectum can escape relatively easily, but if it is produced further up in the small intestine instead, it is no longer so simple. This leads to a bloated abdomen, a feeling of fullness, abdominal pain, and flatulence.
How the breath test works
It is these gases that make analysis with breath tests possible. The test measures the concentration of hydrogen and methane in exhaled breath after ingesting a defined amount of the test substance lactulose (less commonly glucose).
Lactulose is a synthetic sugar that cannot be metabolized by the body’s own enzymes. However, lactulose is broken down by bacteria in the gut. Normally, lactulose should reach the large intestine unchanged and only be broken down by bacteria there. This metabolic process primarily produces hydrogen, but also methane.
Although the test is recommended for people with irritable bowel syndrome according to the current guidelines of the American College of Gastroenterology, there is an important caveat: the recommendation itself has been classified as having a very low level of evidence. Similarly, one can look at a recent study from 2025 [2]. The breath test was repeated twice, two weeks apart, in 40 healthy adults. The results were unsatisfactory. The test could yield a negative result the first time and suddenly a positive result the second time for the same person, even under consistent health conditions. The authors therefore expressly urge caution in interpreting the results and call for more standardized testing protocols.
Another meta-analysis [3] involving 3,000 people with irritable bowel syndrome and 3,000 healthy control subjects reached a similarly sobering conclusion. Depending on the testing method, the prevalence of SIBO among those with irritable bowel syndrome ranged from 4 to 60 percent; however, up to 34 percent of the healthy subjects also tested positive. Although the association between irritable bowel syndrome and SIBO was statistically significant, the large overlap demonstrates how little a single test result can inform an individual diagnosis.
Lactulose is mostly used as the test medium. Glucose is considered more specific and produces fewer false-positive results, but it only detects activity in the upper section of the small intestine because it is absorbed there. Lactulose reaches the entire small intestine, although it is more prone to false-positive results. As soon as lactulose reaches the large intestine, the bacteria residing there metabolize it rapidly. Whether the test results reflect bacteria from the small intestine or the large intestine therefore depends largely on the individual’s transit time. The earlier the rise is observed, the more likely it is caused by bacteria in the small intestine. In one study [4] the participants also had to swallow a liquid radioactive tracer, similar to what is used in a scintigraphy. This allowed researchers to observe almost in real time when the lactulose reached the large intestine while simultaneously measuring the exhaled gases. Unfortunately, this setup is not very practical for the average person.
When should you take the test?
A SIBO test makes the most sense when one of the possible causes is obvious. The most common causes include impaired bowel motility (for example, due to surgery or injury, endometriosis, etc.), low stomach acid, anatomical problems, or previous gastrointestinal infections.
If symptoms such as bloating, a feeling of fullness, or diarrhea persist, it may well be worth looking into SIBO.
Step 3: the heavily advertised markers Zonulin, secretory IgA, and more
None of the following markers is a stand-alone answer to Do I have leaky gut? or Is it just IBS? If you ever looked at any of those order forms for gut tests, you know what I mean. A seemingly endless list of fancy names and recommendations. Most are neatly organized into practical categories like irritable bowel, silent inflammation, or leaky gut. Kind of implying a definite diagnosis after doing the testing. As a layperson, you are tempted to order the whole package right away, until you see the price and wonder whether you could leave out one or two items.
Most of these tests can be ordered without a doctor, so it is definitely worth having a closer look.
Zonulin
Zonulin is a protein produced naturally by the body that regulates the permeability of the tight junctions in the intestinal mucosa. It temporarily opens this barrier, which, when dysregulated, is associated with inflammation and autoimmune diseases such as celiac disease.
That sounds like an ideal marker, but reality is more complicated, for two reasons.
Many tests likely measure the wrong protein
Several different studies have evaluated the accuracy of commercial zonulin tests using mass spectrometry and found something surprising [5; 6]. Instead of actual zonulin, the assays apparently often detect other proteins such as properdin or the complement factor C3, which are two completely different proteins of the immune system. So what is listed as zonulin on your test results is, from a scientific standpoint, a poorly defined term that could potentially encompass several different proteins.
Some of the proteins identified instead are part of the innate immune system, which helps fight pathogens but is not actually directly related to the intestinal barrier. Elevated C3 levels generally indicate increased immune system activation. They can occur in a wide variety of conditions, including infections, autoimmune diseases, cardiovascular diseases, insulin resistance, obesity, asthma, and diabetes. Properdin, on the other hand, is the only known positive activator of the complement pathway; it stabilizes an enzyme complex that enhances the immune response. Properdin is released, among other sources, by neutrophils. It plays a role in bacterial defense, as well as in inflammatory and autoimmune diseases.
A low zonulin level is not always a cause for relief
Here is an interesting point: If the intestinal mucosa is already damaged, it can lead to lower—rather than higher—zonulin levels. This is simply because the cells that normally produce zonulin are damaged. One provider of zonulin tests therefore recommends using the marker only in combination with alpha-1-antitrypsin. This is because the intestinal wall could be damaged even if the zonulin level is normal.
When should you take the test?
After so much criticism, here is the crucial question: Is there any situation at all in which measuring zonulin makes sense? To be honest, as a standalone test on which you want to base a decision, I would say No. Currently, it is not possible to meaningfully interpret either an excessively high or an excessively low level.
At most, I could imagine measuring the zonulin level in combination with other parameters such as alpha-1-antitrypsin or secretory IgA, but more on that in a moment.
Alpha-1 Antitrypsin
Alpha-1 antitrypsin has already been mentioned as a partner for zonulin. Originally, however, it was intended for something completely different. Alpha-1 antitrypsin is a protein produced by the liver that normally remains in the blood. However, if it is found in the stool, this indicates increased protein loss through the intestinal mucosa, medically known as “protein-losing enteropathy”. It was not primarily developed as a marker for leaky gut, but it can be used for this purpose.
Another point to keep in mind: in cases of diarrhea, the test result may appear normal due to dilution of the sample, even if there is actually inflammation in the intestine. A false-negative result is therefore possible.
When should you take the test?
In its originally established role, alpha-1 antitrypsin makes perfect sense. In cases of unexplained protein loss—for example, with fluid retention (edema) or low blood protein levels that cannot be explained by liver or kidney function tests—this test is a useful part of the diagnostic workup.
As a standalone marker for leaky gut—as it is often marketed in many self-pay packages—I would not necessarily recommend it. This is especially true if diarrhea is present, as it can lead to the dilution effect described above. It can certainly be included as a supplement to other markers, but it should not be relied upon as the sole answer to the question, “Do I have leaky gut?”.
Secretory IgA
Secretory IgA is an antibody released directly into the intestinal mucosa by our gut-associated lymphoid tissue (GALT). Unlike zonulin or alpha-1-antitrypsin, it does not concern the structural permeability of the intestinal wall, but rather the immunological defense capacity directly at the surface of the mucosa.
In practice the level of secretory IgA depends heavily on the current state of inflammation [7]. If the intestinal mucosa is healthy, the value should be neither elevated nor reduced. In cases of dysbiosis or inflammation, one would expect an elevated value as a sign of an intact, responsive immune system. If this increase does not occur despite inflammation, it indicates a compromised immune system. The inflammation is still present, but the immune system is not responding as expected. Whether a specific value is therefore “good” or “bad” cannot be assessed without considering the broader clinical context.
When should you take the test?
If the question is whether the immune defense in the intestinal mucosa is functioning properly or not, this test makes the most sense as part of a broader, medically supervised evaluation—not as an isolated numerical value on a self-pay test report.
β-Defensin
β-defensin belongs to a family of endogenous antimicrobial peptides that, in addition to defending against pathogens, play a variety of roles in inflammatory processes [8].
Several studies [9, 10] show that elevated β-defensin-2 levels can be found in both ulcerative colitis and irritable bowel syndrome [8].
The marker is therefore not meant to tell the two apart. It is known as a marker of intestinal immune balance (homeostasis), with the same interpretation logic as sIgA: if no rise occurs despite detected inflammation, laboratories read this as a reduced immune response.
When should you take the test?
β-defensin is rarely offered on its own, but is often included in a larger panel along with secretory IgA, zonulin, and others. Since β-defensin essentially answers the same question as secretory IgA, you can actually skip one of the two parameters. Compare the price of the complete package with the prices of the individual parameters, and choose parameters that answer a very specific question. For β-defensin, the question would be the same as for secretory IgA: Is my intestinal mucosal immune defense functioning properly or not?
EPX
Eosinophil X (EPX) is a protein released by eosinophils, immune cells that play a key role in defending against parasites and in type I allergies (immediate-type reactions). Some studies show an increase in cases of Crohn’s disease or ulcerative colitis [11], so it broadly indicates inflammatory activity in the gut, without telling where that activity comes from.
In one very specific clinical context including infants with suspected non-IgE-mediated food allergy (proctocolitis), a study involving confirmed oral challenge demonstrated good diagnostic accuracy when EPX was combined with calprotectin and TNF-alpha [12]. However, this is a very narrowly defined clinical context that cannot be applied to the general question “Do I have a food intolerance?”, the very question for which the test is often advertised in self-pay packages.
When should you take the test?
In cases of concrete suspicion of a parasitic infection or in very specific pediatric situations [12], where there is actually useful evidence. I would generally refrain from using it as a general test for food intolerances in adults; the overlap with other causes is simply too great for that.
Stool cytokine profile
Cytokine analysis in stool is relatively new. The introductory webinar offered by the lab convinced me so much that I decided to pay the rather high price of the test. It sounded so logical and impressive that I concluded I could rule out a type IV allergy if the cytokines were normal. Unfortunately, it is definitely not that simple, and I probably could have saved myself the money.
The cytokine profile is intended to detect inflammatory changes in the gut. It works reliably for chronic inflammatory bowel diseases. It is also explicitly marketed for irritable bowel syndrome symptoms, and this is precisely where it becomes questionable. A recent study [13] shows that cytokines in stool are generally below the detection limit of conventional ELISA tests, the very method used in commercial cytokine profiles. The researchers therefore had to develop a completely new, ultra-sensitive method to be able to measure them reliably at all. Overall, though, it is an exciting topic, which is why I might add an extra article to the series. If you are interested, just let me know in the comments.