The stool acidity test is a test which is used to diagnose an inability to digest lactose – the sugar in milk.
Normally, lactose is digested in the intestine by an enzyme into two smaller sugar – galactose and glucose which are the absorbed into the body.
People with lactose intolerance have a lack of the intestinal enzyme so the lactose is not absorbed in the small intestine and it is used by the bacteria in the colon to produce chemicals that cause diarrhea, gas and abdominal pain and the stool is turned acidic.
During the test, an infant or very young child must drink a liquid that contains lactose. Then a sample of stool is taken to measure the acidity of stool and if it is high, the person is intolerant of lactose.
In adults, lactose intolerance is measured by lactose tolerance test (by multiple blood samples) or the lactose hydrogen breath test.