The diagnosis can be done with different medical tests (tolerance test, genetics test etc). The test most often used is the hydrogen breath test. After consumption of a solution with lactose (approximately 50 g, which is much more than a person normally consumes), the study subject breathes out into a small machine. In case of lactose intolerance, the hydrogen that is produced in the colon by fermentation of the lactose is absorbed in the blood and excreted by the lungs. The quantity of hydrogen that is breathed out thus increases very rapidly over the hours that follow ingestion of lactose.