The breath hydrogen test is the diagnostic test of choice for lactose intolerance. It measures lactose malabsorption and is simple to perform, is noninvasive, and has a sensitivity and specificity that are superior to the absorption test. Individuals 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.
The lactose tolerance test is rarely performed in clinical practice. Serial blood glucose levels are measured after an oral lactose load. The diagnosis is confirmed if the serum glucose level fails to increase by 20 g/dL above the baseline. 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.
Small bowel biopsy is invasive and rarely performed. 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 study is not readily available and is not usually necessary.
For more on the workup of lactose intolerance, read here.
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