- THE DOC: Lactose Intolerance Test - This measures your glucose levels before and after drinking a lactose laden drink. If your glucose levels do NOT rise, this is an indication of lactose intolerance. Noteworthy: you will feel like you just consumed a bunch of lactose (and all the symptoms that implies) when you get done with this test because, well, you have! AT HOME: You can unofficially try a version of this test. Eat something high in lactose on an empty stomach and keep track of how you feel for the next several hours.
- THE DOC: Hydrogen Breath Test - This measures your hydrogen levels in your breath after drinking a lactose laden drink. Fermentation of lactose in the colon causes hydrogen to be released. AT HOME: you can unofficially do the same test. Eat some dairy and test the feeling/smell of your breath afterward. It is not uncommon that this will be up to 1-2 hours afterwards.
- THE DOC: Stool Acidity Test (mostly used in infants only) - This measures the acidity in stool (caused by undigested, fermenting lactose) for those too young to complete either of the above tests. AT HOME: don't do this at home - that would be gross; let the doc take care of stool samples!
The severity of your reaction of these tests will depend on how many enzymes you are still producing to break down the lactose. This often decreases over time, making your symptoms more severe as the years go by.