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Medical Author:

Jay W. Marks, MD

Jay W. Marks, MD

Jay W. Marks, MD, is a board-certified internist and gastroenterologist. He graduated from Yale University School of Medicine and trained in internal medicine and gastroenterology at UCLA/Cedars-Sinai Medical Center in Los Angeles.

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Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

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.

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In this Article

• What is a stool acidity test?
• What is lactose intolerance?
• How does the stool acidity test work?
• How does testing for lactose intolerance differ in adults?
• Stool Acidity Test Index

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 acid. The acidity of stools that are passed after ingestion of the lactose then is measured. If the stool becomes acid, the individual is intolerant of lactose.

How does testing for lactose intolerance differ in adults?

Lactose intolerance in older children and adults can be detected by using the lactose tolerance test or the lactose hydrogen breath test.

• 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.