How is congenital lactose intolerance diagnosed?

Most of the time, doctors don't need to perform tests to diagnose congenital lactose intolerance. Usually, symptoms of congenital lactose intolerance occur pretty soon after birth. If a baby has symptoms like persistent diarrhea, a doctor will usually suspect congenital lactose intolerance and recommend feeding the baby with a lactose-free formula for several weeks. If symptoms go away during this period, then the doctor knows it's congenital lactose intolerance. However, if there are doubts, or if a person is not tested till later in life, there are a few tests that can be performed. In a lactose tolerance test, a doctor will measure glucose levels in the blood after a person drinks a liquid high in lactose to see if the sugar is being absorbed by the body. A similar test, the hydrogen breath test, will analyze the level of hydrogen in a person's breath after drinking a high-lactose liquid.