Gaxilose, also known as 4-Galactosylxylose, a synthetic disaccharide and a substrate of intestinal lactase. Gaxilose is used in a new noninvasive diagnostic test based on urine or serum measurement of D-xylose after lactase cleavage of orally administered 4-galactosylxylose (gaxilose). Gaxilose is hydrolyzed into galactose and xylose in vivo. Galactose is converted into glucose in the liver and xylose is absorbed from a passive way and partially metabolized endogenously (50%). The rest appears in blood and it is excreted finally through urine. The amount of xylose in urine is proportional to the activity of intestinal lactase. This represents a direct measure of hypo or normo lactasia.