LACTULOSE, a synthetic disaccharide not absorbed by the digestive system, produces hydrogen after coming into contact with bacteria in the gut. Typically, after drinking a 10-g lactulose solution, breath samples are collected every 15 min for 2-3 h.

This test provides good sensitivity and specificity.  The challenge dose is carried further toward the jejunum than glucose, and therefore continues testing for bacterial overgrowth more distal in the intestinal tract. Interpretation, however, is more difficult than that of glucose due to the possible late peak of colonic fermentation.  Lactulose can also cause mild discomfort or diarrhea in some patients, although this usually occurs at higher doses than used in the breath test.

The lactulose challenge typically causes a two-phase response.  Hydrogen may increase early, between 1 and 1-h after the challenge, as lactulose comes into contact with bacteria in the small intestine. This rapid response distinguishes bacterial overgrowth from normal colonic flora, which produce a later more prolonged increase in breath hydrogen.  It is therefore advisable to monitor breath gas during the first 2 h so that colonic fermentation is either not detected or seen as a rise in the last breath specimens. 

A breath peak hydrogen peak greater than 12ppm above the fasting level, observed within 30 mins of ingesting lactulose and preceeding the colonic resposnse by 15 min, is considered indicative of bacterial overgrowth of the small intestine.