American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the most common diagnostic test for suspected small intestinal bacterial overgrowth (SIBO)?

1. Colonoscopy

2. Stool culture

3. Hydrogen breath testing

4. CT scan of the abdomen

The correct answer is: Hydrogen breath testing

Hydrogen breath testing is the most common diagnostic test for suspected small intestinal bacterial overgrowth (SIBO) because it is non-invasive, relatively easy to perform, and specifically designed to measure the amount of hydrogen or methane produced in the breath after ingestion of a carbohydrate substrate, typically lactulose or glucose. In SIBO, there is an abnormal growth of bacteria in the small intestine, and these bacteria ferment carbohydrates, producing hydrogen or methane. By measuring the levels of these gases in the breath over time following ingestion of the substrate, clinicians can determine whether bacterial overgrowth is present based on abnormal rises in hydrogen or methane levels. Other diagnostic tests such as stool culture, colonoscopy, and CT scans do not directly assess for bacterial overgrowth in the small intestine. Stool culture evaluates the composition of bacteria in the stool, which is not indicative of SIBO. Colonoscopy is primarily used for visualizing and diagnosing issues in the colon, not the small intestine, and typically requires a more invasive procedure. A CT scan may help visualize structural abnormalities but does not directly confirm SIBO. The non-invasive nature and targeted approach of hydrogen breath testing make it the preferred diagnostic tool in cases of suspected SIBO.