Stool Acidity Test
William C. Shiel Jr., MD, FACP, FACR
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
What is a stool acidity test?
The stool acidity test is a test for diagnosing intolerance to lactose, the sugar contained in milk, in infants and very young children.
What is lactose intolerance?
Lactose intolerance is the inability to digest the lactose sugar in milk. Milk containing the sugar lactose is a main source of nutrition for infants and young children. With normal lactose tolerance, all of the lactose is digested (broken down into two smaller sugars) and absorbed in the small intestine so that none reaches the colon.
On the other hand, in individuals with lactose intolerance who lack the intestinal enzyme that breaks down the lactose, the ingested lactose is neither digested nor absorbed in the small intestine and reaches the colon where it is used by the bacteria in the colon. The bacteria produce chemicals that cause diarrhea and also produce gas. Some of the chemicals produced by bacteria include lactic and other acids that turn the stool acidic.
Individuals with intolerance to lactose cannot drink regular cow's milk without developing diarrhea, gas, and often abdominal pain. They instead must drink substitutes like rice or soy milk or treat cow's milk with an enzyme that digests the lactose (Lactaid) so that the smaller sugars can be absorbed in the small intestine.
How does the stool acidity test work?
During the stool acidity test, infants and very young children are given lactose to drink. With normal tolerance to lactose, all of the lactose is digested and absorbed in the small intestine. In individuals who are lactose intolerant, some or all of the lactose is not digested and absorbed in the small intestine and reaches the colon. Through the action of the colonic bacteria, the stool becomes acidic. The acidity of stools that are passed after ingestion of the lactose then is measured. If the stool becomes acidic, the individual is intolerant of lactose.
How does testing for lactose intolerance differ in adults?
- The lactose tolerance and lactose hydrogen breath tests are not given to infants and very young children who are suspected of having lactose intolerance because they require cooperation for either multiple samplings of blood or breath, which can be difficult if the infant or child is not cooperative.
- The dose of lactose that is needed for the lactose tolerance and hydrogen breath tests also may present problems for infants and very young children since if they are intolerant of lactose, they may develop diarrhea and become dehydrated.
- Nevertheless, special masks have been developed for infants and very young children that can make the sampling of breath for the lactose hydrogen breath test easier to perform.
Medically reviewed by Margaret Walsh, MD; American Board of Pediatrics
Montgomery, Robert K., PhD. "Lactose intolerance: Clinical manifestations, diagnosis, and management." UpToDate.com. Updated Jun. 29, 2015.
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