There are different types of tests to diagnose lactose intolerance, the most used is the Breath Test.
It is a simple and non-invasive test that measures the hydrogen in the breath before and after drinking a lactose drink.
The test is based on the fact that, if the enzyme lactase it works normally, lactose is broken down into glucose and galactose and is rapidly absorbed in the gastrointestinal tract without a significant production of gas, in particular hydrogen; on the contrary, if the lactose is not metabolized because the lactase is absent, or not functioning, the production of hydrogen will be high because the bacteria present in the colon (large intestine) will produce more than normal, having more lactose available (fermentation reaction).
The hydrogen produced will subsequently be absorbed into the blood and part of it will be eliminated from the lungs through breathing.
To ensure that the test results are not falsified, it is necessary to respect some precautions. For example, in the month preceding the examination, the use of laxatives and antibiotics must be avoided and in the 15 days before the use of lactic ferments (probiotics).
The day before it is necessary to drink still water, have lunch (on a free diet) by 2.30 pm, refrain from intermediate meals (do not have a snack, not even tea, gum and candies); have dinner, before 20:00, with grilled meat or fish seasoned only with raw oil and salt, avoiding carbohydrates and fibers (bread, pasta, rice, vegetables, legumes, fruit, cereals, desserts).
In the twelve hours prior to the exam, it is also necessary not to smoke and to avoid taking non-essential drugs. On the day of the test you can take "life-saving" drugs such as, for example, those for pressure and for the heart, avoiding, however, thyroid hormones (which can be taken immediately after).
The exam lasts 4 hours; during this time it is possible to drink water but neither eating nor smoking is allowed.
Usually, the test result is delivered the same day as the exam. If it highlights that after consuming the lactose drink, the breath contains a large amount of hydrogen, more than 20 parts per million (ppm) above the norm, it is likely that lactose intolerance is present.Other tests
The lactose tolerance test performed on blood is a secondary test, sometimes used as a support for assessing intolerance: in this case, the samples taken are blood instead of air.
In this type of analysis, the amount of sugar (glucose) present in the blood (glycaemia) is measured before drinking a lactose-based drink, or a half-liter glass of milk, and two hours after ingesting it: if the quantity sugar does not increase after drinking lactose or milk, intolerance is likely present.
Under normal conditions, in fact, the enzyme lactase it breaks down lactose into glucose and galactose and, as a result, the blood glucose value increases. When this does not happen, the enzyme may be little or no active at all.
In infants who cannot undergo other tests, a stool acidity test may be used. Fermentation of undigested lactose results in the formation of lactic acid and other acids that can be detected in a stool sample.
Very rarely, if celiac disease is suspected, an analysis of a fragment of tissue (biopsy) of the small intestine can be performed.
In any case, the results must always be viewed by the attending physician who will be able to recommend the best path to take with the help, if necessary, of a nutritionist who can indicate the foods to be preferred to still maintain a balanced supply of nutrients.