Final answer:
The substance tested for in the lactose tolerance test is lactose, and the test is used to diagnose lactose intolerance.
Step-by-step explanation:
The substance tested for in the lactose tolerance test is lactose. The lactose tolerance test is used to diagnose lactose intolerance. In this test, the patient drinks a solution of lactose, and if they are lactose intolerant, the lactose is not digested properly and is metabolized by bacteria in the colon, producing hydrogen. The hydrogen is then expelled through the breath, which can be detected by a clinical gas chromatograph or a compact solid state detector.