Diagnosis of lactose intolerance

Lactose intolerance can be informally identified with a lactose challenge test. Gastrointestinal upset after drinking 125 mL of full-fat milk suggests lactose intolerance. To diagnose lactose intolerance, low-fat milk should be used because fat can induce gastrointestinal symptoms independently of lactose. It is important to note that not all symptoms that occur after consuming milk indicate lactose intolerance. If lactose intolerance is suspected after a lactose challenge test, advise the patient to avoid lactose-containing foods for a few weeks and record their symptoms to see if they improve. If a low-lactose diet improves symptoms, suggest that the patient consume some lactose-containing foods again (rechallenge)—this helps to confirm the diagnosis and avoid unnecessary long-term food restriction.

Other diagnostic tests include formal lactose tolerance testing, hydrogen breath testing and measurement of lactase enzyme concentration in a small bowel biopsy taken at endoscopy. However, correlation between these tests and symptoms is poor, so they are rarely useful.

Cow’s milk protein allergy is distinct from lactose intolerance. Symptoms of cow’s milk protein allergy include hives, blotchy skin rashes, facial swelling, diarrhoea and wheeze; anaphylaxis may occur. For more information about cow’s milk protein allergy, see the Australian society of clinical immunology and allergy website.

Lactose intolerance may occur after gastroenteritis but is usually transient in infants and young children (see Transient lactose intolerance after gastroenteritis).