Lactose tolerance testing is ordered to help diagnose the cause of lactose intolerance. It may be ordered by itself or as part of a larger panel of tests when a secondary cause, such as malabsorption conditions, is suspected. The hydrogen breath test is the most commonly ordered test. The blood test for lactose tolerance is not ordered as frequently but is used for the same reason.
Hydrogen breath test A baseline breath sample is taken before giving a lactose-loaded drink. If the hydrogen gas in a person's breath significantly increases from the baseline, then it is likely that the person is lactose intolerant.
If the breath samples are negative or low for hydrogen, then it is less likely that the person is lactose intolerant. The signs and symptoms may be due to another cause. However, some people may have lactose intolerance even with a negative result. In these cases, the bacteria in the intestine may not produce hydrogen. This can be confirmed after ingestion of lactulose, a synthetic, non-digestible disaccharide sugar that is similarly broken down to hydrogen gas by intestinal bacteria. Since lactulose is not absorbed by the intestine, the ongoing lack of hydrogen gas production suggests a false negative indicating the person may still be lactose intolerant.
Lactose tolerance blood test Timed samples of blood are taken and measured for glucose. If the glucose levels do not increase, yet the person still has symptoms consistent with lactose intolerance, then the condition is likely present. Increasing blood glucose levels over the course of the test indicates that signs and symptoms are unlikely due to lactose intolerance.
Care must be taken when interpreting results of the test. People who have diabetes may have an increase in blood glucose even when they do not produce enough lactase.
Antibiotics taken within the last month or two prior to testing may decrease the number of normal bacteria in the large intestine and give a false negative hydrogen breath test.
If food moves more quickly than usual through a person's intestinal tract, that person may experience symptoms associated with lactose intolerance because the lactose has a shorter amount of time to be exposed to and broken down by lactase.
Bacterial overgrowth in the intestines (more bacteria present than normal) can cause symptoms similar to lactose intolerance, as can a variety of other gastrointestinal disorders.
Although it is not commonly done, it is possible to test for mutations in the gene that regulates lactase production (the LCT gene). Potentially, this could identify likely lactase deficiency and take the place of hydrogen breath testing.
This article was last reviewed on September 7, 2012. | This article was last modified on September 7, 2012.
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