What is the SIBO Test?
The SIBO test refers to diagnostic methods used to detect Small Intestinal Bacterial Overgrowth, with breath testing (measuring hydrogen and methane gases after ingesting glucose or lactulose) being the most commonly used non-invasive approach, while small bowel aspiration and culture remains the gold standard but is invasive. 1, 2
Primary Testing Methods
Breath Testing (Most Common)
- Hydrogen and methane combined breath tests are more accurate than hydrogen-only testing for identifying SIBO 3, 4
- The patient ingests either 75g glucose or 10g lactulose, then breath samples are collected serially over 2-3 hours to measure hydrogen and methane gas production 1
- Gas production occurs when intestinal bacteria ferment the ingested carbohydrate, with elevated levels indicating bacterial overgrowth 1
- Important caveat: In IBS patients, the lactulose hydrogen breath test often measures small intestinal transit rather than true SIBO, as gas production may come from colonic bacteria rather than small bowel overgrowth 1
- Breath tests are inexpensive, safe, and non-invasive but lack optimal sensitivity and specificity depending on the clinical context 2
Small Bowel Aspiration and Culture (Gold Standard)
- Clinically significant overgrowth is defined as bacterial counts exceeding 10⁵ colony-forming units per milliliter (normal is less than 10⁴/mL) 1
- The procedure involves endoscopic aspiration from the proximal jejunum or duodenum 1
- Common bacterial species identified include Bacteroides, Enterococcus, and Lactobacillus 1
- Critical limitation: Most bacteria relevant to causing symptoms cannot be cultured using standard methods 1
- The American Gastroenterological Association recommends a specific technique: avoid aspirating on intubation to prevent contamination, flush 100mL sterile saline into the duodenum, flush the channel with 10mL air, allow fluid to settle, then aspirate ≥10mL into a sterile trap 4
Alternative Testing Methods
- Urinary excretion tests using chemically synthesized bile acid conjugates (PABA-CA, PABA-UDCA, or 5-ASA-UDCA) measure bacterial bile acid hydrolase activity 2
- These tests track urinary excretion rates of PABA or 5-ASA metabolites as a measure of intestinal bacterial activity 2
- Clinical trials are limited but show promise as SIBO diagnostic substrates 2
Test Selection Strategy
When breath testing is available, use it as first-line rather than empirical treatment to support antibiotic stewardship and establish the cause of symptoms 3, 4
If breath testing is unavailable, qualitative small bowel aspiration during upper endoscopy is the recommended alternative 3
Key pitfall: Breath tests may produce false negatives compared to small bowel culture, particularly when there are variations in test performance and interpretation 1
Clinical Context
SIBO testing is indicated when patients present with:
- Bloating, abdominal pain, gas, and diarrhea 2, 5, 6
- Steatorrhea in more severe cases 7
- Nutritional deficiencies (elevated folate, vitamin B-12 deficiency, fat-soluble vitamin deficiencies) 4, 7
- Weight loss and malabsorption 2, 6
The relevance of SIBO in IBS remains unclear due to methodological problems and conflicting study results 1