The goals with testing are to diagnose lactose intolerance and distinguish it from other conditions with similar symptoms. Testing may include:
- Lactose tolerance tests, including:
- Hydrogen breath test – currently, this is the most reliable test used to diagnose lactose intolerance. Breath samples are taken before and at timed intervals after a patient drinks a fluid containing a standard amount of lactose. With lactose intolerance, undigested lactose is broken down by bacteria in the large intestine, producing excess hydrogen gas. The hydrogen moves to the circulation and is eventually exhaled by the lungs. Increasing levels of hydrogen in the breath samples over time strongly indicates lactose intolerance.
- Lactose tolerance blood test – sometimes performed to help diagnose lactose intolerance; blood samples are collected and tested for glucose at timed intervals after a standard oral dose of lactose is given. If glucose levels do not rise, then lactose intolerance may be present. This test is rarely done in clinical practice.
- Stool acidity (pH) test – occasionally performed, usually on infants or children who cannot undergo other types of testing; stool can be acidic with lactose intolerance.
- Potential testing that is rarely ordered clinically includes:
A healthcare practitioner may have a patient abstain from all dairy products for a couple of weeks to see if symptoms resolve (dietary elimination), then re-introduce dairy to see if symptoms re-emerge. This is not diagnostic of lactose intolerance but is consistent with it.