A lactose intolerance test usually costs about$300. To have a lactose intolerance test, you must go to a regular hospital for a laboratory test to determine if there is a lactase deficiency. This is usually required for babies with prolonged diarrhea, as well as for routine stool tests to determine the specific cause. The actual lactose requires the body to secrete lactase to break down, and lactose intolerance is mainly due to insufficient lactase secretion, and may also be related to enteritis and dietary habits.