Is Lactose Intolerance a Clinical Diagnosis?
Yes, lactose intolerance is primarily a clinical diagnosis that can be made based on a careful history and dietary manipulation, though objective testing with hydrogen breath testing is available when confirmation is needed. 1, 2
Diagnostic Framework
Clinical Diagnosis Approach
A 2-week dietary restriction trial with symptom resolution serves as a simple and economical diagnostic approach that can confirm lactose intolerance without requiring formal testing 1
The diagnosis can usually be established through careful history taking supported by dietary manipulation, particularly in patients who develop abdominal pain, bloating, flatulence, and watery diarrhea after consuming lactose-containing foods 2
Patient self-reports of lactose intolerance correlate poorly with objective evidence and cannot be relied upon alone, as a sizable number of adults believe they are lactose intolerant but do not actually have impaired lactose digestion 1, 2
When to Consider Objective Testing
Testing should be reserved for patients who are regular consumers of more than 280 ml (0.5 pint) of milk or equivalent dairy products daily, as those consuming less can generally tolerate moderate quantities even if lactose intolerant 1
The hydrogen breath test is the most widely used and recommended confirmatory method when objective diagnosis is needed, as it is non-invasive, relatively inexpensive, and has good sensitivity and specificity 1, 3, 4
The test involves ingestion of 25-50g lactose after an overnight fast, with breath samples taken at 15-30 minute intervals for three hours; a rise in hydrogen concentration by more than 20 parts per million indicates a positive diagnosis 1
Important Diagnostic Caveats
A negative breath test does not exclude the diagnosis due to false-negative rates as high as 25%, often from hydrogen non-excretion occurring in approximately 18% of tested subjects 1
If symptoms are suggestive, a trial of lactose-free diet should still be considered even with negative testing 1
Symptoms alone are not specific to lactose intolerance and may overlap with irritable bowel syndrome or other conditions 1, 5
Key Distinction from Food Allergy
Lactose intolerance is explicitly not an allergic condition because the response is not immune-based; it results from inability to digest the sugar lactose, leading to excess fluid production in the gastrointestinal tract 6
This distinguishes it from milk protein allergy, which triggers an adverse immunologic reaction and is considered a true food allergy 6
Secondary Causes to Evaluate
Before confirming primary lactose intolerance, consider reversible secondary causes including:
- Celiac disease 1
- Gastroenteritis 1
- Non-steroidal anti-inflammatory drug use 1
- Small bowel disease 1
- Chemotherapy (as noted in cancer patients where up to 35% may have abnormal lactose breath tests during treatment, though only up to 11% become symptomatic) 6
Lactose intolerance is generally reversible once these underlying conditions are treated 1
Epidemiologic Context
Lactase deficiency should be regarded as normal in most non-Caucasian populations after age two, with the exception being northern European populations where lactase activity may persist as an autosomal dominant trait 1
Prevalence ranges from 15% in persons of northern European descent to 80% in blacks and Latinos, and up to 100% in American Indians and Asians 2