Diagnosing Small Intestinal Bacterial Overgrowth (SIBO)

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Introduction

In the first article of this series about Small Intestinal Bacterial Overgrowth (SIBO), I covered what SIBO is, how common it is, and its primary symptoms. If you haven’t read that introduction yet, it provides essential context for our discussion. In this second article, I cover the different tests used in diagnosing SIBO, exploring the advantages and drawbacks of each method. In the next installment, we will cover various treatment options, including dietary protocols combined with antibiotic or herbal therapies.

The Challenges of Diagnosing SIBO

One of the first challenges in diagnosing SIBO is finding a physician knowledgeable about the condition and its current treatments. Historically, SIBO was diagnosed primarily by gastroenterologists using invasive and expensive surgical tests. Before the widespread use of endoscopy, diagnosis required a surgical procedure to collect liquid from the jejunum of the small intestine for culturing. A positive diagnosis was recorded when bacterial growth exceeded 104 colony-forming units per milliliter.

Endoscopy and the Gold Standard

The invention of the endoscope in the mid-1980s allowed for less invasive fluid collection from the duodenum. While the patient is sedated, a flexible tube is passed through the esophagus and stomach into the small intestine. Despite being the current “gold standard,” this procedure remains expensive and invasive. Furthermore, a significant drawback is that only about 30% of gut bacteria can actually be successfully cultured from these samples, and contamination during withdrawal is common.

The Discovery of Breath Testing

A brilliantly simple solution emerged with the discovery that gases like hydrogen and methane are produced in the small intestine only as a by-product of unabsorbed or incompletely absorbed carbohydrates. Breath tests detect these gases to provide evidence of carbohydrate malabsorption and identify the specific types of bacteria causing the fermentation. The two most common variants are the glucose breath test and the lactulose breath test.

Glucose vs. Lactulose Breath Tests

Clinicians often debate which substrate is more accurate. Glucose is absorbed completely in the upper small intestine, making it very accurate for that region, but it may miss SIBO located in the ileum (the far part of the small intestine). Conversely, lactulose is not absorbed by humans, allowing it to travel the full length of the small intestine and potentially detect overgrowth in the ileum. Preference for these tests varies by practitioner.

How the Breath Test Works

After a patient consumes the substrate, exhaled hydrogen or methane is measured using a gas chromatograph. These gases are produced by bacteria, absorbed into the bloodstream, and then exhaled via the lungs. Notably, 15%-30% of people host Methanobrevibacter smithii, which converts hydrogen into methane. These individuals may not exhale much hydrogen even if they have SIBO, making it vital to measure both gases.

Performing the Test and Interpreting Results

Testing requires an overnight fast and specific oral hygiene to ensure mouth bacteria do not skew results. Breath is analyzed every 15 minutes for 2 to 4 hours. According to the 2017 North American Consensus, a rise in hydrogen of ≥20 ppm within 90 minutes is considered positive for SIBO. Methane levels ≥10 ppm are considered methane-positive. However, some researchers, like Dr. Mark Pimentel, suggest even lower cutoff points for methane (reading >3 ppm) to avoid missing clinical cases.

Distinguishing SIBO from IBS

The symptoms of Irritable Bowel Syndrome (IBS) and SIBO overlap significantly, including bloating, pain, and altered bowel habits. Research has found that nearly 80% of subjects with an abnormal breath test also met the criteria for IBS. This raises the question of how many IBS patients might actually have SIBO. If you have been unsuccessful in resolving IBS symptoms through diet alone, breath testing may be a valuable next step.

Final Thoughts

While some recent studies call into question the validity of breath tests for detecting specific jejunal overgrowth, they remain a vital tool for identifying microbial dysbiosis. Identifying methane-predominant SIBO is particularly important because methane-producing bacteria are often resistant to standard antibiotics and are strongly associated with chronic constipation.

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To your good health!

Joy

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References

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