Diagnostic Testing for Lactose Intolerance
The hydrogen breath test is the most widely used and recommended diagnostic method for lactose intolerance, involving ingestion of 25-50g lactose after an overnight fast with breath samples collected every 15-30 minutes for three hours. 1
Primary Diagnostic Test: Hydrogen Breath Test
The hydrogen breath test (HBT) is superior to other available methods because it is non-invasive, relatively inexpensive, and has good sensitivity and specificity comparable to mucosal lactase assay. 1, 2
Test Protocol
- Patient preparation: Overnight fast required 1
- Lactose dose: 25-50g dissolved in 200-500ml water 1
- Sampling frequency: End expiratory breath samples every 15-30 minutes 1
- Test duration: Minimum 3 hours for adequate sensitivity 1, 3
- Positive result: Rise in hydrogen concentration >20 parts per million above baseline 1
Critical Limitations to Recognize
- False negative rate of up to 25% due to hydrogen non-excretion in approximately 18% of tested subjects 1, 4
- A negative test does NOT exclude lactose intolerance - if clinical suspicion remains high, proceed with a trial of lactose-free diet 1, 4
- Shortened tests (1-2 hours) have unacceptably low sensitivity (37% for 1-hour test vs 95% for 3-hour test) and should be avoided 3
Alternative Diagnostic Approach: Genetic Testing
In populations with high prevalence of lactase-persistent alleles (like Northern Europeans), DNA genotyping for LCT-13,910 C/T may replace breath testing entirely. 5
- Genotyping has 97% negative predictive value and 94% specificity versus HBT 5
- The 13,910 C/C genotype indicates primary lactase deficiency with 2-15% prevalence in Northern European Caucasians 1
- Genotyping is particularly useful in populations where 68% globally (approaching 100% in Han Chinese) are lactase-deficient 1
Practical Clinical Algorithm
Step 1: Assess Dairy Consumption
- Only test patients consuming >280ml (0.5 pint) milk or dairy equivalent daily 4
- Those consuming less rarely experience symptoms even if lactose intolerant 4
Step 2: Consider Secondary Causes First
Screen for reversible causes before testing: 1, 4
- Celiac disease
- Gastroenteritis
- NSAID use
- Small bowel disease
- Chemotherapy (causes reversible lactose intolerance in 35% of patients) 4
Step 3: Choose Testing Method
- In Northern European populations: Consider genetic testing first 5
- In other populations or when genetic testing unavailable: Perform 3-hour hydrogen breath test 1, 3
- If HBT negative but symptoms persist: Trial 2-week lactose-free diet 4
Step 4: Interpret Symptoms Cautiously
- Self-reported lactose intolerance correlates poorly with objective testing 1, 4
- Symptom specificity ranges from 69% (bloating) to 90% (diarrhea) 6
- At least 50% of patients with presumed lactose intolerance actually have broader FODMAP intolerance 4
- Consider IBS with visceral hypersensitivity as alternative diagnosis 4
Common Pitfalls to Avoid
- Don't rely on patient symptom reports alone - objective testing is essential 1, 4
- Don't use shortened breath tests (<3 hours) - sensitivity drops dramatically 3, 7
- Don't stop at negative breath test - false negatives are common; trial dietary restriction if suspicion remains 1, 4
- Don't eliminate all dairy unnecessarily - yogurt and hard cheeses are generally well-tolerated even in lactose-intolerant individuals 4
- Don't ignore broader FODMAP intolerance - consider low FODMAP diet trial if lactose restriction alone fails 4