Diagnosis

There is no perfect test for SIBO. Diagnosis usually combines symptoms, risk factors, exclusion of alternative conditions, and testing when appropriate.

Breath Testing

Hydrogen and methane breath tests are commonly used. The patient drinks a sugar substrate, commonly glucose or lactulose. Breath samples are collected over time to measure gases produced by intestinal microbes.

Commonly measured gases include:

• hydrogen
• methane
• sometimes hydrogen sulfide, depending on test availability

Glucose vs Lactulose

Glucose and lactulose breath tests behave differently.

Glucose is absorbed in the proximal small intestine, which may make it less likely to detect distal overgrowth. Lactulose is not absorbed and travels through the intestine, but results may be affected by intestinal transit time.

Neither test is perfect.

Methane and Constipation

Methane positivity is often discussed in patients with constipation-predominant symptoms. Some clinicians use the term intestinal methanogen overgrowth because methane is produced by archaea rather than bacteria.

Test Preparation

Preparation instructions vary by laboratory. Patients are commonly asked to follow specific diet restrictions before the test and to avoid selected medications, laxatives, probiotics, or antibiotics for defined periods.

Follow the instructions from the ordering clinician or laboratory. Incorrect preparation can make results harder to interpret.

Limitations

Breath test results can be affected by:

• rapid transit
• slow transit
• recent antibiotics
• laxatives
• colonoscopy preparation
• diet before the test
• smoking or exercise during the test
• substrate selection
• interpretation criteria

Alternative Testing

Small bowel aspirate culture can be obtained during endoscopy in selected cases. It is invasive, less commonly used, and has technical limitations.

Broader Evaluation

Depending on the presentation, clinicians may also consider evaluation for:

• celiac disease
• inflammatory bowel disease
• anemia
• thyroid disease
• pancreatic insufficiency
• bile acid diarrhea
• infection
• medication effects
• nutritional deficiencies