Lactose Intolerance Workup

Updated: Dec 04, 2019
  • Author: Praveen K Roy, MD, MSc; Chief Editor: Burt Cagir, MD, FACS  more...
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Laboratory Studies

Lactose tolerance test

This test is rarely done in clinical practice.

Measure serial blood glucose levels after an oral lactose load. A fasting serum glucose level is obtained, after which 50 g of lactose is administered. [22] Measure the serum glucose level at 0, 60, and 120 minutes.

The lactose tolerance test has a sensitivity of 75% and a specificity of 96%.

False-negative results occur in the presence of diabetes and small bowel bacterial overgrowth. Abnormal gastrointestinal emptying can also affect the results of the lactose tolerance test. [23]

The diagnosis is confirmed if the serum glucose level fails to increase by 20 g/dL above the baseline.

Milk tolerance test

Administer 500 mL of milk and measure the blood glucose level.

An increase of less than 9 mg/dL indicates lactose malabsorption. [24]


Other Tests

Imaging studies

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

Breath hydrogen test

This is the diagnostic test of choice.

Subjects are administered lactose after an overnight fast, after which expired air samples are collected before and at 30-minute intervals for 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. [22]

Dietary elimination

Resolution of symptoms with the elimination of lactose-containing food products and resumption of symptoms with their reintroduction are findings suggestive of lactose intolerance. [25, 26]



Small bowel biopsy

This 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 is not readily available and is not usually necessary.