Recommended Test for Lactose Intolerance
The lactose hydrogen breath test is the recommended first-line diagnostic test for lactose intolerance, as it is non-invasive, widely available, and superior to lactose tolerance tests while having similar sensitivity and specificity to mucosal lactase assay. 1
Test Procedure
The lactose hydrogen breath test should be performed as follows:
- Patient preparation: Overnight fast required 1
- Lactose dose: Administer 25-50g lactose dissolved in 200-500ml water 1
- Sampling intervals: Collect end-expiratory breath samples at 15-30 minute intervals for three hours 1
- Positive result: Rise in hydrogen concentration >20 parts per million (ppm) above baseline 1
Critical Limitations and How to Address Them
False Negative Results
The hydrogen breath test has a false negative rate up to 25%, primarily due to hydrogen non-excretion which occurs in approximately 18% of tested subjects 2, 3. This is a critical pitfall that must be anticipated.
When to suspect false negative results:
- If breath test is negative but clinical suspicion remains high, proceed with a 2-week trial of lactose-free diet 2
- Consider adding methane measurements, as 6% of patients are hydrogen-negative but methane-positive 4
Baseline Hydrogen Considerations
- High baseline hydrogen (>20 ppm) occurs in 15.8% of patients and makes breath testing unreliable 5
- In patients with elevated baseline hydrogen, the lactose tolerance test becomes the preferred alternative 5
Alternative: Lactose Tolerance Test
When breath testing is contraindicated or unreliable (high baseline hydrogen, hydrogen non-excretors, or infection control concerns), use the lactose tolerance test:
- Procedure: Measure serum glucose at baseline and 30 minutes after 50g lactose ingestion 3
- Positive result: Failure of blood glucose to rise by >15-20 mg/dL above baseline 5, 6
- Advantage: Not influenced by hydrogen non-excretion 5
- Correlation: Positive lactose tolerance test reflects more severe lactose malabsorption with higher breath hydrogen scores 6
Practical Clinical Algorithm
Screen appropriateness: Only test patients consuming >280ml (0.5 pints) milk or dairy equivalent daily 2
First-line approach: Lactose hydrogen breath test with methane measurement 1, 4
If baseline hydrogen >20 ppm: Use lactose tolerance test instead 5
If negative breath test but symptoms persist: Trial of lactose-free diet for 2 weeks 1, 2
Consider secondary causes: Screen for celiac disease, gastroenteritis, NSAID use, or small bowel disease, as lactose intolerance may be reversible when underlying condition is treated 2
Important Caveats
- Self-reported intolerance is unreliable: Patient reports correlate poorly with objective test results 2
- Ethnic considerations: Lactase deficiency is normal in most non-Caucasian populations after age 2 years (68% of world population), approaching 100% in Han Chinese 1
- Symptom overlap: Bloating, pain, and diarrhea are non-specific and overlap with IBS 2
- No gold standard exists: All available tests have limitations 1