The lactose tolerance test is a test for diagnosing an intolerance of ingested lactose. Lactose intolerance is a genetic condition in which individuals are unable to digest and absorb the lactose sugar in cows milk. With normal lactose tolerance, all of the lactose is broken down (digested) in the intestine by lactase into two smaller sugars (galactose and glucose) which then are absorbed into the body. As a result, normally no lactose reaches the colon. On the other hand, in individuals with lactose intolerance who lack lactase that breaks down the lactose, the ingested lactose is neither digested nor absorbed in the small intestine and reaches the colon where it is used by the bacteria in the colon. The bacteria produce chemicals that cause diarrhea, abdominal pain, nausea, vomiting, constipation, joint pain, and other symptoms. This test is performed by giving the patient lactose orally and then several blood samples are taken over a two hour period following the ingestion of the lactose to measure the blood glucose level. If lactose tolerance is normal, the glucose level in the blood rises due to the absorption of glucose from the intestine. If there is lactose intolerance, the glucose level does not rise.