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Lactose Intolerance

How Is Lactose Intolerance Diagnosed?

Usually lactose intolerance is diagnosed based on symptoms and relief of those symptoms when avoiding dairy products. However, certain tests for lactose intolerance may be used to help confirm the diagnosis. Many doctors will ask patients who suspect they have lactose intolerance to avoid milk and dairy products for one or two weeks to see if their symptoms subside. One of the following tests may be given to confirm the diagnosis.

  • Milk challenge test: A milk challenge is a simple way of diagnosing lactose intolerance. A person fasts overnight and then drinks a glass of milk in the morning. Nothing further is eaten or drunk for three to five hours. If a person is lactose intolerant, the milk should produce symptoms within several hours of ingestion.
  • Hydrogen breath test: The hydrogen breath test measures the amount of hydrogen in the breath after drinking a lactose-loaded beverage. Raised levels of hydrogen in the breath three to five hours after ingestion of lactose indicate improper digestion of lactose.
  • Lactose tolerance test: During the lactose tolerance test your blood sugar is measured over a two-hour period after drinking a lactose-loaded beverage. You are required to fast before the test. By measuring the level of sugar in the blood, the test indicates how well the body is digesting lactose.
  • Stool acidity test: The stool acidity test is a test for lactose intolerance in infants and young children. The child is given a small amount of lactose to drink. Lactic acid turns the stool acidic. Therefore, children with lactose intolerance will develop acidic stool after consuming lactose.
  • Intestinal biopsy: The most direct test for lactose intolerance is biopsy of the intestinal lining to measure lactase levels in the lining. However, these biopsies are invasive and require specialized analysis that is not available at most doctor's offices. Thus, lactase levels are not usually measured through biopsy except for research purposes.

The blood glucose test and hydrogen breath test are not given to infants and very young children, because they may cause severe diarrhea. If an infant or young child is having symptoms of lactose intolerance, your child's health care provider will likely recommend changing from a cow's milk formula to a soy milk formula until the symptoms disappear. Milk and dairy products may be slowly reintroduced at a later time. If needed to confirm the diagnosis, a stool acidity test may be given to infants and young children.

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