What is the procedure for a lactose tolerance test?

Lactose Tolerance Test Procedure

The lactose tolerance test involves ingestion of 25-50g lactose dissolved in 200-500ml water after an overnight fast, followed by collection of end-expiratory breath samples at 15-30 minute intervals for three hours, with a rise in hydrogen concentration from basal levels by more than 20 parts per million indicating a positive diagnosis. 1

Test Preparation

• Patient must fast overnight before the test
• Avoid antibiotics for at least 2-4 weeks before testing (may affect colonic bacteria)
• Avoid high-fiber foods the day before the test
• Stop smoking and physical exercise on the day of testing

Test Procedure Steps

1. Baseline sample collection: Collect an initial breath sample before lactose administration
2. Lactose administration: Patient ingests 25-50g lactose (equivalent to 1-2 cups of milk) dissolved in 200-500ml water
3. Timed breath sample collection: End-expiratory breath samples are taken at 15-30 minute intervals for three hours
4. Monitoring for symptoms: During the test, monitor for symptoms such as bloating, flatulence, abdominal distention, and diarrhea

Interpretation of Results

• Positive test: Rise in hydrogen concentration from baseline by more than 20 parts per million
• False negatives: Up to 25% of tests may be falsely negative 1
• Symptoms correlation: Higher breath hydrogen levels correlate with increased symptom severity 2

Alternative Testing Methods

1. Lactose tolerance test measuring serum glucose:

   • Measure blood glucose at intervals after lactose ingestion
   • A rise in blood glucose less than 20mg/dL suggests lactase deficiency
   • Less reliable than hydrogen breath testing 1

2. Mucosal lactase assay:

   • Direct measurement of lactase enzyme in jejunal biopsy
   • Invasive procedure
   • Similar sensitivity and specificity to hydrogen breath test 1

3. Genetic testing:

   • Testing for lactase-persistent alleles (LCT-13,910 C/T)
   • High sensitivity (89%) and specificity (94%) in populations with high prevalence of lactase-persistent alleles 3

Important Considerations

• A negative test does not exclude lactose intolerance due to the 25% false negative rate
• If clinical suspicion remains high despite a negative test, a trial of lactose-free diet should be considered 1
• The standard test dose (25-50g) is significantly higher than typical dietary lactose intake, which may lead to overdiagnosis of clinical lactose intolerance
• Most lactose maldigesters can tolerate up to 12 oz of milk daily without symptoms 4

Pitfalls to Avoid

• Confusing lactose intolerance (enzyme deficiency) with milk protein allergy (immune-mediated) 3
• Relying solely on patient-reported symptoms without objective testing 3
• Using IgG antibody testing for diagnosis, which is not recommended 3
• Failing to consider secondary causes of lactose intolerance such as celiac disease, SIBO, or intestinal infections 3

The hydrogen breath test is considered superior to blood glucose measurement for diagnosing lactose malabsorption due to its stronger correlation with symptoms after lactose challenge 2.