[Diagnosis of lactase deficiency with the expired hydrogen (H2) test]

To determine the sensitivity and specificity of breath hydrogen (H2) in detecting lactase deficiency, breath H2 collected by end-expiratory sampling and capillary blood glucose were measured after ingestion of 50 g of lactose in 36 patients with biopsy-proved isolated lactase deficiency, 42 with normal lactase activity and 6 with lactase deficiency secondary to mucosal lesions. All patients had digestive symptoms clinically compatible with lactose malabsorption. The maximum increase in breath H2 concentration was more than 1.1 mumol/l (25 ppm) in all patients with isolated lactase deficiency, and less than 0.88 mumol/l (20 ppm) in 88% of patients with normal lactase activity; there were 5 false-positive results, attributed in one case to small bowel colonization and in another case to rapid transit after gastric surgery. Secondary lactase deficiency was accurately detected by neither breath H2 nor blood glucose.