Two tests are commonly used to measure the digestion of lactose.
Hydrogen Breath Test: The person drinks a lactose-loaded beverage and then the breath is analyzed at regular intervals to measure the amount of hydrogen. Normally, very little hydrogen is detectable in the breath, but undigested lactose produces high levels of hydrogen. Smoking and some foods and medications may affect the accuracy of the results. People should check with their doctor about foods and medications that may interfere with test results.
Lactose tolerance Blood Test: Oral lactose is given to the subject who is on overnight fast. 0.75g to 1.5 g per kg body weight is administered. Blood samples at different periods of time are collected and analyzed for glucose which is the breakdown product of lactose metabolism.
If the glucose levels are more than 25 mg/dl, the result is considered as positive and the subject can tolerate lactose. If the subject cannot tolerate lactose and he experiences cramping and bloating it may be inferred that he lacks the enzymes necessary for breaking down lactose
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