Stool Tests and SIBO: What They Can and Can't Show

In short: a stool test cannot diagnose small intestinal bacterial overgrowth. Stool reflects the colon, while SIBO is diagnosed with a breath test or by culturing fluid from the small intestine². Some stool tests are still useful when you have SIBO-like symptoms, because they can rule out other conditions: calprotectin for inflammatory bowel disease, elastase for pancreatic insufficiency, and tests for infections such as Giardia⁷. Commercial microbiome panels are a different matter: a 2025 international consensus found insufficient evidence to recommend them for routine clinical use³.

People with digestive symptoms are often offered comprehensive stool panels such as the GI-MAP, GI-Effects or GI360. This guide explains what stool tests can and cannot tell you if you suspect SIBO, and which ones doctors actually use.

Why a stool test cannot diagnose SIBO

SIBO means too many bacteria in the small intestine. Stool is formed in the colon, which naturally holds trillions of bacteria, so a stool sample mostly reflects the colon. The US guideline recommends diagnosing SIBO with a breath test or with a culture of small-intestinal fluid taken during an endoscopy². A US expert review also found insufficient evidence to use stool inflammation markers such as calprotectin to detect SIBO¹.

Stool tests doctors use for SIBO-like symptoms

Chronic bloating and diarrhea have many causes besides SIBO. A 2025 review of stool testing noted that, for people with diarrhea-predominant IBS symptoms and no alarm signs, guidelines recommend testing for Giardia, calprotectin, blood in the stool (fecal immunochemical test) and bile acid diarrhea⁷.

Calprotectin: ruling out inflammatory bowel disease

Calprotectin is a protein released by white blood cells into the gut when the bowel lining is inflamed. A 2015 meta-analysis found that in adults with IBS symptoms, a calprotectin level of 40 μg/g or lower meant a 1% or lower chance of having inflammatory bowel disease⁶. A high level calls for further investigation, usually a colonoscopy. A normal level does not rule SIBO in or out¹.

Fecal elastase: checking the pancreas

Elastase is a digestive enzyme made by the pancreas that passes through the gut largely intact. Low stool elastase points to exocrine pancreatic insufficiency, in which the pancreas does not make enough digestive enzymes. It is generally diagnosed below 200 μg/g, and pancreatic function is assumed to be normal above 500 μg/g⁹. In a 2023 study of people with levels between 200 and 500 μg/g, 71% of those who tried pancreatic enzyme supplements reported improvement⁹.

Watery stool dilutes the sample and can give a falsely low result, so elastase should not be measured on liquid stool⁸. This matters for SIBO, which often causes diarrhea. Pancreatic insufficiency is also a recognized risk factor for SIBO, because pancreatic enzymes help keep bacterial numbers down¹¹.

Infections and bile acid diarrhea

Stool tests can detect parasites such as Giardia, which can cause symptoms similar to SIBO. Bile acid diarrhea, in which bile acids reach the colon and cause watery diarrhea, can also be tested for with specific stool tests⁷.

Methanobrevibacter smithii in stool

Some panels report Methanobrevibacter smithii, the main methane-producing microbe in the human gut. Stool levels do track breath methane. In a 2012 study of 19 people with IBS, those with methane on their breath test had much higher stool counts of M. smithii, and stool levels correlated well with how much methane they breathed out⁴. A 2022 study found the same link in a larger group, with higher breath methane going with more Methanobrevibacter in stool and with constipation⁵.

This makes a stool M. smithii result a reasonable clue to methane overgrowth (IMO). But IMOIntestinal methanogen overgrowth (IMO) Overgrowth of methane-producing archaea, mainly Methanobrevibacter smithii. It is diagnosed by a breath methane level of 10 ppm or more and is linked to constipation. is defined by a breath methane level of 10 ppmParts per million (ppm) The unit breath-test laboratories use to report the concentration of hydrogen, methane and hydrogen sulfide in a breath sample. or more², and there is no validated stool cut-off, so a breath test is still needed to diagnose it.

Comprehensive microbiome panels

Commercial panels use DNA methods such as PCR or sequencing to list dozens of bacteria, yeasts and parasites, often with “dysbiosisDysbiosis An imbalance in the makeup or activity of the microbes living in the gut.” scores and healthy ranges. In 2025, an international panel of experts reviewed microbiome testing and concluded³:

• There is insufficient evidence to recommend the routine use of microbiome testing in clinical practice.
• The Firmicutes-to-Bacteroidetes ratio should not be reported, and there is not enough evidence for any dysbiosis index.
• There is generally not enough information to set strict healthy reference ranges for individual species.
• Direct-to-consumer tests have no proven clinical value and risk wasting money and misdirecting care.

Zonulin

Many panels include zonulinZonulin A protein that loosens the tight junctions between intestinal cells. High levels are associated with increased intestinal permeability. as a marker of leaky gut. A 2018 study found that a widely used commercial zonulin test does not actually detect zonulin (pre-haptoglobin 2), but other, related proteins¹⁰. Results from these tests are hard to interpret.

Summary

Common questions

Can a GI-MAP diagnose SIBO?

No. It analyzes stool, which reflects the colon, not the small intestine². A breath test or small-intestinal fluid culture is needed to diagnose SIBO. Our SIBO diagnosis guide explains which test to use.

Should I get a stool test if I think I have SIBO?

Talk to your doctor about targeted tests that rule out other causes, such as calprotectin, Giardia and, if you have fatty or floating stools, elastase⁷. Broad microbiome panels are unlikely to change treatment³.