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.