Another way is for her to keep a diary of foods and symptoms and evaluate it for patterns. If a change in diet brings no results, she should see a doctor to make sure she is not suffering from another digestive condition. Still, some people find it hard to reach a conclusion with self-monitoring because of the lag in the development of symptoms after eating, the subjectiveness of symptoms, and the number of other factors that confuse analysis. Also, when there is a suspicion of other gastrointestinal problems, more conclusive evidence is needed. A doctor can perform a breath hydrogen test, which involves checking the patient's breath hydrogen levels one hour after consuming a lactose meal.
Undigested lactose produces hydrogen gas as it passes through the gut.
Lactose intolerance can also be diagnosed through a biopsy of the small intestine. But, this is virtually never done for lactose intolerance itself. Rather, it is discovered incidentally when looking for other, more serious problems. But, IBS has no physical signs or diagnostic tests. The dietary and other provisions for IBS are much different than for lactose intolerance.