Diagnosing lactose intolerance can be done using several procedures. Your doctor, depending on your body condition and its reaction to lactose-rich foods, may suggest any of the below tests. Before undergoing the tests, you will be required to make observations about your self and your body’s reaction to milk or milk products. Better to make notes about the observations and intimate your doctor on it.
How to diagnose lactose intolerance? Along with diagnosing lactose intolerance, you need to find out how much of lactase your body is producing. This can be done with advanced tests like Hydrogen Breath test, Stool sample test etc. 
1. Hydrogen Breath Test: This is one of the most common and accurate of testing methods. You will be made to fast overnight and given a drink of lactose solution in the morning. Your breath will be checked at regular intervals for its hydrogen levels. The concentration of hydrogen measured in the air that your breathe out will tell how much of the lactose (you took) is still undigested in your body. If you breathe out a large amount of hydrogen (say, 20 ppm than the baseline), you are definitely lactose intolerant. The baseline in this case will the amount of hydrogen you breathed before drinking the lactose solution.
2. Lactose Tolerance test: Your blood sugar levels will be checked and noted. Then, you will be given a drink of lactose solution. After sometime, your blood sample will be tested for blood glucose levels. If the sample shows only a slight increase or no increase, it is confirmed that you are lactose intolerant. This is because, if you are not lactose intolerant, your body will have broken down the lactose into glucose and galactose. Since there is no sign of increase in glucose, it is a clear-cut case of lactose intolerance.
3. Stool sample test: This is mostly done for babies or children who cannot be given huge amounts of lactose for tests. The stool samples are taken and tested for acids. High presence of acetate may indicate lactose intolerance. This is because, undigested lactose in the intestine reacts with bacteria in the colon to produce a highly fatty acid like acetate. This test will also reveal if there are any parasites in the child’s body – like giardia lambia or crystosporidia.
4. Small bowel biopsy: An invasive surgical procedure, small bowel biopsy is used only in rare cases, especially to diagnose the presence of any other condition that lactose intolerance, like coeliac disease. In the test, a sample of the small intestinal lining is taken using an endoscope under the guidance of a local anesthetic. Then, the sample is tested for presence of lactase. If the sample contains only a small amount of lactase, there is a heavy chance of lactose intolerance. If, on the other hand, there is normal amount of lactase, there is chance of coeliac disease.
