The oral lactose tolerance test can be used to support the diagnosis of lactase deficiency and lactose intolerance. The patient is monitored after administration of an oral lactose load.


NOTE: The oral test is rarely used today. Empiric diagnosis (occurrence of symptoms on lactose high diet, remission on lactose free diet) is often sufficient. Breath tests are usually preferred by gastroenterologists. However, the test may have a role when resources are limited.


Patient preparation: The patient should be fasting overnight or at least 8 hours. The patient should remain seated or in bed after the lactose dose is administered.


The lactose dose to administer is variable:

(1) adults: 50 grams in 400 mL water, 50 grams per square meter BSA, 1 g per kg

(2) children: 1-2 grams per kg

(3) for severe disease, a lower dose is administered (for children: 15 grams in 250 mL water)

(4) for mild disease, a higher dose may be administered (up to 100 g in adults)


Specimen collection: before and then 30, 60 and 120 minutes after ingestion. Additional specimens can be collected at 180 and 240 minutes after ingestion.


Criteria for a positive test (evidence of lactose intolerance):

(1) The maximal change in blood glucose from baseline is < 20 mg/dL for any of the post-ingestion specimens (normal >= 30 mg/dL).

(2) The presence of symptoms (gas, diarrhea, cramps, bloating, nausea)


Control test to demonstrate absorption: 25 grams glucose and 25 grams of galactose



• False positive tests are relatively common, most often due to delayed gastric emptying. A positive result may be seen in malabsorption disorders other than that due to lactase deficiency.

• False negative tests (a normal rise in glucose during the test) may occur with some frequency.


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