Lactose tolerance tests can be used to diagnose a hereditary problem digesting lactose, the primary sugar found in milk or other dairy products. Many people, especially those of Native American, African, Asian and Mediterranean descent, cannot produce enough lactase enzyme in their small intestines to break down this sugar. The result is abdominal bloating and cramps, nausea, flatulence and other intestinal problems within a few hours of ingesting milk products. 

Some people also develop a lactose intolerance later in their lives, due to surgery on their intestines or bacteria and viruses that damage the cells lining the small intestine and prevent lactase production. Sufferers of celiac sprue and related digestive disease can also develop difficulties processing milk and milk products. Many people self-diagnose or are diagnosed with this problem by their doctors without testing, but a lactose tolerance test is needed to be sure.

This test detects the presence of glucose in the blood after the consumption of dairy products. Several blood samples are required, so that laboratory technicians can identify blood glucose levels before and after the patient consumes the lactose. To produce accurate results, the patients should not eat or engage in strenuous exercise for at least 8 hours before testing. If the blood test shows an increase of more than 30 milligrams per deciliter within 2 hours of consuming lactose it is considered normal. Increases of less than 20 milligrams indicate a problem. Patients who show an abnormal response should have a glucose tolerance test after the lactose tolerance test.