What blood tests should I ask my healthcare provider to order for a patient with Small Intestine Bacterial Overgrowth (SIBO)?

Blood Tests to Order for SIBO

When evaluating a patient with SIBO, you should request vitamin B12, fat-soluble vitamins (A, D, E, K), and consider screening for anemia, as bacterial overgrowth causes malabsorption through bacterial consumption of B12 and bile salt deconjugation. 1, 2, 3

Essential Blood Tests

Vitamin B12 (Cobalamin)

• Vitamin B12 deficiency is significantly associated with hydrogen-producing SIBO because bacterial overgrowth causes B12 malabsorption through direct bacterial consumption and bile salt deconjugation 1
• B12 malabsorption is a key laboratory finding that may indicate SIBO presence 3
• The presence of B12 deficiency can guide treatment selection 1

Fat-Soluble Vitamins (A, D, E, K)

• Screen for fat-soluble vitamin deficiencies as bacterial overgrowth causes bile salt deconjugation and subsequent malabsorption 1, 2
• Fat-soluble vitamin deficiencies, particularly vitamins A and E, are indicative of SIBO according to the American Gastroenterological Association 3
• These deficiencies occur due to bacterial deconjugation of bile salts, which impairs fat absorption 2

Complete Blood Count (CBC)

• Check for anemia, which is significantly associated with SIBO 4
• Anemia may result from B12 deficiency or other malabsorption issues 4

Additional Considerations

Pancreatic Function Assessment

• If steatorrhea persists after antibiotic treatment, evaluate for pancreatic exocrine insufficiency, which may coexist with SIBO 2
• Important caveat: Falsely low fecal elastase may be secondary to diarrhea when screening for pancreatic exocrine insufficiency 2
• If there's intolerance to pancreatic enzyme replacement therapy, this often indicates underlying SIBO; once SIBO is eradicated, enzyme therapy is usually better tolerated 1, 2

Metabolic Markers

• Consider testing for high anion gap acidosis from D-lactic acid production, which may be indicative of SIBO according to the European Society for Clinical Nutrition and Metabolism 3

Clinical Context

The blood tests should be interpreted alongside your clinical presentation and risk factors:

• Proton pump inhibitor use is a well-established risk factor for SIBO development 1, 4
• Patients with structural GI changes, hypomotility, or loss of ileocecal valve are at higher risk 2, 3
• The presence of B12 deficiency can guide treatment selection and monitoring 1

Do not rely solely on blood tests for SIBO diagnosis—hydrogen and methane breath testing with glucose or lactulose remains the recommended first-line diagnostic approach 2, 3. Blood tests primarily identify complications and malabsorption consequences rather than diagnosing SIBO itself.