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Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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The most common cause of lactose intolerance is secondary lactose intolerance. The use of an Exclusion diet is the simplest diagnostic method:

  • exclusion diet (low lactose) (symptom improvement) and then reintroduction (symptom recurrence). Usually improve within 48 hours of exclusion

More detailed investigation may be undertaken in some cases:

Diagnosis is often strongly suggested by the presence of reducing sugars in the stool.

Breath tests may also be used

  • when colonic bacteria act upon undigested lactose, hydrogen is formed. Breath hydrogen analysis may be useful in the diagnosis of lactose intolerance


  • testing for reducing substances in the stools is not always helpful. Infants with positive reducing substances may have sufficient lactase activity
    • because lactase breaks down lactose into glucose and galactose, the presence of 20-25 milligrams per deciliter of glucose above fasting levels indicates sufficient levels of lactase

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