Fast Five Quiz: Lactose Intolerance

B.S. Anand, MD


March 21, 2022

The breath hydrogen test is generally considered the diagnostic test of choice. Patients are administered lactose after an overnight fast, after which expired air samples are collected before and at 30-minute intervals over the course of 3 hours to assess hydrogen gas concentrations. A rise in breath hydrogen concentration greater than 20 parts per million over the baseline after lactose ingestion suggests lactase deficiency.

Small bowel biopsy is the criterion standard; however, it is invasive and rarely performed. A major advantage is that it provides definitive information. Biopsy samples from the small bowel are assayed for lactase activity. The biopsy results may be normal if the deficiency is focal or patchy. This test is not readily available and is not usually necessary.

In the lactose tolerance test, serial blood glucose levels are measured after an oral lactose load. A fasting serum glucose level is obtained, after which 50 g of lactose is administered. The serum glucose level is measured at 0, 60, and 120 minutes. Failure of blood glucose levels to increase by 20 mg/dL suggests lactose intolerance. False-negative results occur in the presence of diabetes and false-positive results are seen in small bowel bacterial overgrowth. Abnormal gastrointestinal emptying can also affect the results of the lactose tolerance test.

Imaging tests are not helpful in the diagnosis of primary lactose intolerance, but they may be helpful for excluding secondary causes.

This Fast Five Quiz was excerpted and adapted from the Medscape article Lactose Intolerance.

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