What is the recommended approach to diagnosing and managing small intestinal bacterial overgrowth (SIBO) in adults?

Approach to Small Intestinal Bacterial Overgrowth (SIBO)

Diagnostic Strategy

In patients with high pretest probability of SIBO (anatomical abnormalities, prior small bowel surgery, dysmotility, diverticulosis, pseudo-obstruction), proceed directly to empirical antibiotic therapy without testing. 1

When to Test vs. Treat Empirically

High pretest probability scenarios—skip testing and treat empirically: 1

• Prior small bowel surgery or resection
• Intestinal diverticulosis
• Pseudo-obstruction or documented dysmotility
• Anatomical abnormalities (strictures, blind loops, dilated bowel)
• Loss of ileocecal valve

Low to moderate pretest probability—consider testing: 1, 2

• Unexplained bloating, abdominal pain, diarrhea, flatulence without clear anatomical risk factors
• When antibiotic stewardship is a priority
• When establishing a definitive diagnosis would change management

Diagnostic Testing Options (When Indicated)

Small bowel aspirate and culture is the most sensitive test but has significant limitations: 1

• Clinically significant overgrowth defined as >10⁵ CFU/mL (normal <10⁴ CFU/mL) 1
• Common species include Bacteroides, Enterococcus, and Lactobacillus 1
• Performed via endoscopy: flush 100 mL sterile saline into duodenum, flush channel with 10 mL air, allow settling, then aspirate ≥10 mL into sterile trap 2
• Critical limitation: Most clinically relevant bacteria cannot be cultured 1
• Potential contamination from oropharyngeal flora 1
• Requires coordination with microbiology lab before performing 2

Breath testing has poor diagnostic performance and should be interpreted cautiously: 1, 3

• Glucose breath testing: sensitivity <50%, positive predictive value <70%, negative predictive value <70% 1
• Lactulose breath testing: sensitivity 68%, specificity 70% when using 10g dose 1
• Major flaw: Assumes rather than measures transit time, leading to false positives from rapid orocaecal transit 1
• False negatives occur in 3-25% of individuals whose bacterial flora don't produce hydrogen 1
• The British Society of Gastroenterology states breath tests are not recommended for SIBO diagnosis due to poor sensitivity and specificity 3
• If used despite limitations, combine hydrogen and methane measurement (75g glucose or 10g lactulose) 1
• Breath testing is particularly unreliable after intestinal resection, with enteric fistulas, or in dysmotility 1

Treatment Approach

First-Line Antibiotic Therapy

Rifaximin 550 mg twice daily for 1-2 weeks is the most effective treatment with 60-80% efficacy: 2, 4

• Preferred due to lack of systemic absorption and minimal resistance risk 4
• Most effective for hydrogen-producing SIBO 4

Alternative effective antibiotics: 1, 2, 4

• Amoxicillin-clavulanic acid
• Ciprofloxacin
• Doxycycline
• Cephalosporins
• Tetracycline

Avoid metronidazole—lower efficacy for SIBO 1, 4

Managing Recurrent SIBO

For recurrent cases, address underlying predisposing factors rather than simply repeating antibiotics: 4

• Discontinue proton pump inhibitors if possible
• Evaluate for and treat dysmotility disorders
• Consider prokinetic agents (prucalopride, bethanechol, neostigmine, pyridostigmine) 1
• Assess for anatomical abnormalities requiring surgical correction

Antibiotic strategies for recurrence: 2, 4

• Rotate antibiotics with 1-2 week antibiotic-free intervals before retreating
• Consider cyclical antibiotic courses
• Low-dose long-term antibiotics may be necessary in persistent cases

Adjunctive Measures

Nutritional monitoring and supplementation: 2, 4

• Monitor fat-soluble vitamins (A, D, E, K) and vitamin B12 2, 4
• Vitamin D deficiency occurs in 20% of patients on bile acid sequestrants 2
• SIBO causes malabsorption through bacterial deconjugation of bile salts 2

Dietary modifications: 4

• Reduce fermentable carbohydrates that feed bacterial overgrowth
• Complex carbohydrates and fiber from non-cereal plant sources support gut motility

Probiotics have limited role: 4

• Probiotics alone show only 33% efficacy vs. 25% for antibiotics alone
• Combination therapy (antibiotics + probiotics) shows 55% efficacy
• Discontinue probiotics during antimicrobial treatment as they may counteract therapeutic effects 4

Critical Pitfalls to Avoid

Do not use antimotility agents (loperamide, codeine) when bowel dilation is present—this worsens bacterial overgrowth 1, 4

Intolerance to pancreatic enzyme replacement therapy often indicates underlying SIBO: 2, 4

• Treat SIBO first, then retry pancreatic enzymes

If symptoms persist after successful bacterial eradication, evaluate for: 4

• Pancreatic exocrine insufficiency (fecal elastase testing) 1
• Bile acid diarrhea
• Alternative diagnoses (inflammatory bowel disease, celiac disease, microscopic colitis)

A negative breath test does not exclude SIBO—clinical judgment supersedes test results 1