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).

com.bmj.content.model.Caption@61123aa4[Figure caption and citation for the preceding image starts]: Lactose-containing productsCreated by BMJ using content from Dr Mohammad Azam [Citation ends].

A 2-week trial of dietary lactose elimination, with resolution of symptoms, and subsequent challenge, with recurrence of symptoms, is generally diagnostic.[2] Biochemical diagnostic tests are rarely required in practice.

For more subtle cases, the lactose hydrogen breath test can be used to confirm the diagnosis.[4][24][25] 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),[26] has relatively low sensitivity and specificity and is no longer performed.

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