Diagnosis of lactase deficiency manifesting as lactose intolerance is based on a characteristic clinical history of precipitation of gastrointestinal and/or systemic symptoms within minutes to a few hours after consumption of products containing lactose (dairy and non-dairy).
A 2-week trial of dietary lactose elimination, with resolution of symptoms, and subsequent challenge, with recurrence of symptoms, is generally diagnostic. Biochemical diagnostic tests are rarely required in practice.
For more subtle cases, the lactose hydrogen breath test can be used to confirm the diagnosis. The milk-tolerance test, where blood glucose is measured after administration of 500 mL of milk, or a fixed mass of lactose (12.5 to 50.0 grams), has relatively low sensitivity and specificity and is no longer performed.
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