Teste diagnóstico para intolerância à lactose.

DIAGNOSIS — The term lactose malabsorption is generally reserved for those patients with typical symptoms in whom the intestinal malabsorption of lactose has been confirmed by a test of absorption (eg, lactose absorption test) or malabsorption (lactose breath hydrogen test). Less direct tests, such as low fecal pH or reducing substances in the stool, are only valid when lactose has been ingested, intestinal transit time is rapid, stools are collected fresh, assays are performed immediately, and bacterial metabolism of colonic carbohydrate is incomplete. The importance of confirming the diagnosis was illustrated in the preceding study of African Americans [44].

Lactose tolerance test — The capacity for lactose absorption can be measured using a lactose absorption test. Following oral administration of a 50 g test dose in adults (or 2 g/kg in children), blood glucose levels are monitored at 0, 60, and 120 minutes. An increase in blood glucose by less than 20 mg/dL (1.1 mmol/L) plus the development of symptoms is diagnostic. False negative results may occur in patients with diabetes or bacterial overgrowth. Abnormal gastric emptying also can lead to spurious results; the blood glucose may be relatively higher with rapid emptying and depressed with delayed gastric emptying.
In adults, the lactose tolerance test has a sensitivity of 75 percent and a specificity of 96 percent [48]. However, it is cumbersome (particularly in children) and time consuming, and has largely been replaced by the lactose breath hydrogen test.

Lactose breath hydrogen test — The breath hydrogen test measures lactose nonabsorption. It is simple to perform, noninvasive, and has a sensitivity and specificity that are superior to the absorption test [48,49].
The test is begun by giving oral lactose in the fasting state, at a usual dose of 2 g/kg (maximum dose, 25 g). Breath hydrogen is sampled at baseline and at 30-minute intervals after the ingestion of lactose for three hours. The post-lactose and baseline values are compared. We generally consider a breath hydrogen value of 10 ppm (parts per million) as normal. Values between 10 and 20 ppm may be indeterminate unless accompanied by symptoms, while values over 20 ppm are considered diagnostic of lactose malabsorption.
Both false-positive and false-negative results can occur. False-positive results are seen with inadequate pretest fasting or recent smoking; false-negative results can be seen after the recent use of antibiotics, in patients with lung disorders, or in the approximately 1 percent of subjects who are nonhydrogen producers. As noted above, intestinal lactase levels do not begin to fall until after age five. Thus, an abnormal lactose breath hydrogen test in children less than five years reflects either abnormal intestinal mucosa or bacterial overgrowth, both of which require further evaluation by appropriate diagnostic tests. A normal breath hydrogen test does not rule out an intestinal mucosal lesion and should not be used to avoid an intestinal biopsy.