How to Diagnose Small Intestinal Bacterial Overgrowth

This problem in the small intestine can lead to a variety of symptoms and disrupt digestion
By Ashley Turner
Ashley Turner
Ashley Turner
June 16, 2021 Updated: June 24, 2021

In part one of this series, we gave an overview of small intestinal bacterial overgrowth—otherwise known as SIBO. It can be an insidious disease because so many of its symptoms overlap with other gut ailments. Misdiagnosis, or missing a diagnosis altogether, is very common.

Yet the longer SIBO goes untreated, the more difficult it can be to resolve.

Furthermore, its treatment is specific and must be handled in the right way in order to achieve full recovery. That makes an accurate diagnosis all the more imperative. This article will cover the various kinds of SIBO, as well as the tests used to reveal them.

A Quick Refresher

Your digestive system has several sections with specific roles. After food is chewed and broken down by digestive juices, it lands in your small intestine. This is where the work of drawing nutrients from your food really takes place. Then, what’s left passes into your large intestine, where a host of microbes help finish the job, and waste products make their way out of your body.

While those bacteria are essential in the large intestine, they can wreak havoc in the small intestine. When bacteria proliferate in the small intestine, that is the condition known as SIBO.

SIBO Variations

It’s important to note that there are three types of SIBO. This is because multiple bacteria—as well as microorganisms—contribute to overgrowth in the small intestine. The way they combine can produce a buildup of three different gases in your gut: methane, hydrogen, or sulfide. While there are foundational aspects to treating SIBO as a whole, it’s essential to determine the type of SIBO you have, as it could determine the treatment you receive.

Hydrogen-dominant SIBO occurs when food lingers in your small intestine for too long and begins to ferment before it can be broken down. This creates excess hydrogen gas in your gut and can cause symptoms unique to this kind of SIBO: diarrhea and flatulence that smells like rotten eggs.

Methane-dominant SIBO occurs when the normal digestion process is slowed and excess methane accumulates, causing abdominal pain, bloating, and constipation. While essential at normal levels for regulating hydrogen and carbon dioxide in the gut, an excess of methane produced by archaea in the small intestine slows down intestinal motility and increases the transit time of food going through the gastrointestinal tract. Sometimes people with this form of SIBO feel inadequate evacuation of the bowels, straining, or will go extended periods of time without a bowel movement.

Hydrogen sulfide-dominant SIBO is when there’s an overproduction of hydrogen sulfide in the small intestine. In a balanced body, hydrogen sulfide is a beneficial gasotransmitter, which is a gaseous molecule that acts as a neurotransmitter. At balanced levels, hydrogen sulfide is a protective anti-inflammatory agent with antioxidant and immune-supporting properties. If hydrogen sulfide levels proliferate, this can cause damage to the gut, immune system, and cells along with initiating systemic inflammation.

There’s also a condition called small intestinal fungal overgrowth (SIFO). This is when high levels of fungi are found in the small intestine, which contributes to various symptoms similar to SIBO. Oftentimes, those with unexplained gastrointestinal symptoms who test negative for SIBO actually have SIFO.

Diagnosing SIBO

Currently, the most commonly used and least invasive way to test for SIBO is a breath test.

Preparation for the test can begin 2 to 4 weeks beforehand if you’re on certain medications or probiotics. Usually, about 48 hours before the test, a strict diet and the elimination of nonessential medications are required to reduce baseline gases in your small intestine and produce a more accurate result. Restrictions on smoking, alcohol, and exercise are also recommended just prior to the test.

During the testing process, patients will consume a sugar solution—such as glucose or lactulose—and then, at various intervals over a several-hour span, blow into a special vial. If bacteria are fermenting in your small intestine—that is to say, the sugar isn’t being properly metabolized by your system—elevated levels of hydrogen, methane, or hydrogen sulfide will be detected. This indicates the likelihood of SIBO. In the past, hydrogen sulfide gas wasn’t able to be found with breath testing. Thankfully, reliable breath testing for hydrogen sulfide-dominant SIBO is now available.

Although this method of breath testing is widely available and relatively easy, false negative and positive results are common. If results are unexpected or symptoms persist, it may be recommended to test further or repeat testing. Small bowel aspiration procedures can be done, but these are incredibly invasive and usually utilized for research purposes only.

At-home breath test kits are available through many functional and integrative medicine doctors. It’s highly recommended to not only test for SIBO, but also work to resolve SIBO with the supervision of your physician.

Next Up

In the third and final part of our SIBO series, we’ll discuss treatment options, as well as dietary and lifestyle changes you can make today to avoid developing SIBO in the future.

Dr. Ashley Turner is a traditionally trained naturopath and a board-certified doctor of holistic health for Restorative Wellness Center. An expert in functional medicine, Turner is the author of the gut-healing guideRestorative Kitchen” andRestorative Traditions,” cookbooks featuring non-inflammatory holiday recipes.


Ashley Turner
Ashley Turner