To evaluate the role of the lactose breath hydrogen test for the detection of lactose malabsorption in children with chronic nonspecific abdominal complaints, breath hydrogen excretion was measured in 131 children with recurrent abdominal pain (n = 75) or chronic nonspecific diarrhea (n = 56) following a lactose load (2 gm/kg; maximum 50 gm). The data were compared to those obtained from lactose tolerance tests (n = 113) and symptom response following a lactose load (n = 109) performed simultaneously with the lactose breath hydrogen test, and with results from small bowel biopsies obtained in 31 children to determine dissacharidase activity and mucosal histology. The results indicate that an increase in breath hydrogen of greater than 10 ppm above base line values (Δppm) by 120 minutes ("early increase" response) completely discriminates between biopsyproven isolated lactase-insufficient and lactase-sufficient children. A similar increase after 120 minutes ("late increase" response) is consistent both with normal mucosal function and partial lactase insufficiency due to mucosal injury. Breath hydrogen responses predicted assayed lactase activity in all patients with isolated lactase insufficiency, but were "falsely negative" in four of ten children whose lactase insufficiency was secondary to mucosal injury. In both clinical groups, lactose malabsorbers report significantly more symptoms than absorbers ( P .001), but neither symptom reports nor tolerance tests are accurate methods for distinguishing lactose malabsorbers from absorbers. Although the lactose breath hydrogen test provides objective documentation of lactose malabsorption, it is not equally predictive of assayed lactase activity in all clinical groups.
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