A referral to a Paediatric Gastroenterologist may be required for cases of persistent symptoms of lactose intolerance in infants and young children. The following tests may be used to diagnose of lactose intolerance:

A trial of a lactose-free diet
When lactose intolerance is suspected, a lactose-free diet can be tried. During a diagnostic lactose-free diet, it is important that all sources of lactose be eliminated, requiring the reading of food labels to identify “hidden” sources of lactose. Generally, a 2-week trial of a strict lactose-free diet with resolution of symptoms and subsequent reintroduction of dairy foods with recurrence of symptoms can be diagnostic.1

Many lactose containing foods are rich in calcium and Vitamin D, important nutrients for infants, thus an elimination diet should always be performed for a specified period, under strict medical supervision and preferably in consultation with a Paediatric Dietitian.

Stool acidity test
This test measures the amount of acid in the stool.  Undigested lactose fermented by colon bacteria creates lactic acid and other short-chain fatty acids that can be detected in a stool sample.1 Fecal pH will normally be lower (5.0–5.5) in infants compared with older children and adolescents due to relative enzyme inadequacy in relation to the high amount of lactose in breastmilk.1

Hydrogen Breath Test
Measures the presence of hydrogen in the expired air. Hydrogen is produced when lactose is fermented in the colon and diffuses into the blood stream and is expired through the lungs. This test is simple, inexpensive and non-invasive but is not specific to lactose since any unabsorbed carbohydrates can cause hydrogen production.1 Antibiotic use within one month of the test may modify the gut flora and produce false negative results.1