Examination of lactose intolerance: how to read the result?

Present in yogurts, cheeses, chocolates and dairy products, lactose is part of several products consumed daily. However, a study indicates that up to 75% of the world population may have some level of lactose intolerance.

Although there are currently several products for lactose intolerant or allergic people, in addition to vegans (which are free of animal ingredients), lactose is easily found on labels and also in home recipes, and can cause a lot of discomfort to patients. intolerant.

The correct diagnosis helps to plan life changes and prevent health risks. For this, it is necessary to carry out tests that confirm intolerance.

What is the lactose intolerance test?

The lactose intolerance test is indicated to assess the intestine’s ability to absorb and process lactose from food. The performance can be indicated when there are suspicions due, mainly, to gastrointestinal symptoms and, quickly, the diagnosis can be confirmed.

Lactose is responsible for the sweet taste of dairy products, so it is also called sugar or milk glucose.

Thus, the traditional test consists of collecting blood from the fasting patient, checking blood glucose levels.

After the puncture (blood withdrawal) in the fasted state, a determined amount of liquid lactose is administered and the collection is done again every 30 minutes, in order to measure the blood glucose rate.

When lactase enzymes are not able to break down lactose, generating energy for the body, it is called lactose intolerance, which usually results in varied symptoms, but which prevail in the digestive system.

If the lactase enzymes are working correctly, the glycemic rate will change by at least 20mg / dL in relation to the fasting value. If the measurements show a smaller variation, the suspicion of intolerance is confirmed.

Despite checking for the presence of lactase deficiency, it is not possible, through laboratory tests, to measure the degree of intolerance. That is, how much lactose is possible to ingest without presenting symptoms.

There are also 2 alternatives to drawing blood. The hydrogen test is done with the ingestion of liquid lactose and, after a few minutes, the concentration of hydrogen expelled or exhaled is measured on the breath. And also the acid meter exam, which investigates the acidity present in the stool.

These tests are, in general, done by observing the patient’s people, who can opt for the total exclusion of milk and its derivatives or identify the degree of ability to digest the substance.

What is lactose?

Lactose is, chemically, a carbohydrate – popularly called carbohydrate. Its chemical structure has a disaccharide, composed of 2 monosaccharides, which are glucose and galactose.

The substance is exclusive to animal milk, as it is produced only by the mammary glands, that is, by mammals. For this reason, vegetable-based milks, such as almond or soy milk, are naturally lactose-free.

Cow’s milk has about 4.7% lactose, but human milk has the highest concentrations, with about 7.2%.

When the human organism receives milk sugar (lactose), it starts the process of breaking down to simpler substances, glucose and galactose, through the enzymatic action of lactase. The process is known as hydrolysis or hydrolysis.

From the degradation, glucose is used as an energy source by the body, while galactose makes up glycolipids and glycoproteins and which have different actions in the body.

The enzyme is produced in the small intestine, therefore, in patients with intolerance there is a difficulty or deficiency in enzyme production and performance. As there is no breakdown of the molecules, the substance reaches the large intestine unchanged, where it begins to accumulate and ferment due to the presence of intestinal bacteria.

In general, the most obvious symptoms are water retention, gastrointestinal changes, such as constipation or diarrhea, and cramps.

Types of intolerance

There are surveys that indicate the presence of some degree of lactose intolerance in up to 75% of the population.

Although quite common, the condition can be well tolerated, as there are people with mild enzyme deficiency and, in general, asymptomatic. That is, it is possible that some patients are able to consume quantities of milk and dairy products without experiencing discomfort.

However, intolerance can be classified into 3 types:

Congenital deficiency

It is characterized by a chronic malformation of the enzymatic production of lactase. Generally, the condition is diagnosed in the first years of life, as the child’s body is unable to properly break down lactose. The problem is rare and hereditary.

Primary disability

It is situated as a gradual decrease in the production of lactase. In general, the condition appears between 4 and 6 years or after adolescence, and continues to increase, decreasing the lactose digestion action more and more.

In the first months of life, lactase production is higher because the baby’s food source is essentially breast milk. With dietary changes, the body tends to change and, therefore, decrease the amount of enzymes produced. However, the reduction does not always remain adequate.

Usually the condition is permanent, that is, there is no treatment capable of curing primary intolerance.

Secondary disability

There are other factors that can cause intolerance, for example, when the small intestine decreases or alters lactase production because of disease.

In general, this condition is temporary and tends to be mitigated with the control or treatment of the causative problem.

Several organic changes can cause the condition, such as irritable bowel syndrome, celiac disease, gastroenteritis and milk protein allergy .

Allergy vs. intolerance

The allergy comprises an organism’s response to the substance, in which the immune system reacts to some protein present in milk and initiates a defense process of the organism.

In allergies, the consequences can be quite varied, involving different organs and parts of the body. In general, there is itching, skin blemishes, difficulty breathing and gastrointestinal dysfunctions.

The condition is common in the first years of life, when the body can still react allergic to various substances and nutrients. But the tendency is for the allergy to disappear even in the childhood stage.

Intolerance, on the other hand, is a difficulty or decrease in enzymatic action. The condition does not present itself as a disease, but as a deficiency of the organism.

If caused by other illnesses or factors, intolerance tends to cease as soon as the primary cause is controlled. For example, in celiac disease, after the injection of gluten , the intestine tends to become irritated and develop a degree of intolerance to milk sugar. With intestinal stabilization, lactose is normally digested again.

However, if intolerance is primary, the indication is that foods with milk are avoided or switched to versions without lactose.

In intolerance, it is still possible to combine drugs that supplement the enzymes, making the body capable of properly degrading lactose, without harming the intestine or causing symptoms.

How is the exam done?

In general, different types of exams can be performed in clinics and laboratories, and are not considered complex procedures.

Blood test

In general, laboratories require that the patient be fasted for 8 hours. Blood samples are collected before and after ingesting a lactose-rich liquid.

Usually the compound is prepared with the dissolution and powdered lactose in the water.

Fasting blood is withdrawn and others are taken after lactose intake.

The puncture (blood collection) is performed in a vein and, in cases of difficulty in accessing the blood flow, access can be maintained at the site of the first withdrawal, preventing the need for more than one incision. The amount of blood collected can vary between 2 ml and 5 ml.

In people with the correct lactase function, blood glucose levels should rise at least 20mg / dL from the first value (before lactose intake).

Hydrogen Measurement

Under normal conditions, the amount of hydrogen expelled from the lungs is quite small. However, in patients with lactose intolerance, the intestine ferments the milk carbohydrate, as it has not been properly degraded.

Fermentation produces gases , including hydrogen, which are absorbed and transported to the lung, where they are eliminated by breathing. High levels of hydrogen indicate lactose intolerance.

In general, it is necessary to fast for 12 hours and a restricted diet on the day before the exam. The menu consists of foods that do not favor fermentation in the intestine and therefore cause the formation of gases, as it can result in a false positive.

Initially, the patient slowly breathes in a measuring device (similar to a breathalyzer), in order to check the basal concentration of hydrogen.

The lactose-rich liquid is ingested and, after periods of 15 or 30 minutes, the measurement procedure is performed again. The total exam time is usually 2 to 4 hours.

Pre-examination care

The hydrogen test requires that the diet be followed correctly the day before and that the patient has not used antibiotics in the last 7 days.

Small children, 1 to 4 years old, should only fast for 6 hours. Above 5 years of age, in general, a period of 8 hours is indicated, depending on the criteria adopted by the laboratory.

In this examination method, food on the previous day should follow the laboratory’s recommendations, generally avoiding cabbage, broccoli, alcoholic beverages, ice cream, industrialized sweets, canned products, processed meats (bologna, ham, hamburger), among others.

The patient must not smoke before the test and, also, he must not have had a colonoscopy exam less than 4 weeks ago.

Acid deposition

Performed through stool analysis, the test measures the presence of lactic acid and fatty acids in the samples.

Due to poor digestion and the consequent lactose fermentation in the intestine, acids accumulate and are concentrated in the faeces.

The test, therefore, consists of collecting a stool sample and there are, in general, no specifications for its performance.

Stool acidity, or pH, varies according to age and eating habits. However, it is estimated that the values ​​should be between 6.0 and 7.5. In intolerant patients, the pH is below 6.0, characterizing the sample as acidic.

Intestinal biopsy

The test is more invasive and requires anesthesia, so when compared to other laboratory methods, biopsy is more difficult to perform.

Through an examination of intestinal endoscopy, a small sample of the tissue is taken and, subsequently, the presence of the enzyme lactase is verified.

Although less used for the diagnosis of lactose intolerance, the procedure still presents itself as an option. The differential of the method is that the patient does not need to ingest lactose, however it is much more invasive compared to the others.

Genetic diagnosis

The method is used in diagnoses of primary intolerance. The technique uses a blood or saliva sample and analyzes the structures of DNA seeking to identify genetic variants common to lactose intolerance.

When should the exam be done?

Lactose intolerance is, in general, requested when there is suspicion of poor digestion of the substance in the presence of symptoms.

When there is a family history or diseases of the intestinal and autoimmune tract, such as celiac disease, the test may also be requested, as there is a high incidence of intolerance in patients with other autoimmune diagnoses.

Usually, suspicion starts when there is presence of gas, vomiting, reflux, diarrhea or constipation, bloating and bloating.

However, the patient may not show symptoms constantly, even if some milk derivative is ingested. This is due to the low degree of intolerance.

That is, there are cases in which the patient may be able to ingest daily doses of lactose and not present any discomfort, but if the dose is exceeded, symptomatic manifestations occur.

In this case, it is important to consider the examination even when the malaise is infrequent, daily or, necessarily, after each lactose ingestion.

As the organism’s signs are, in general, quite comprehensive, some diseases can be confused with the diagnosis. It is therefore necessary to correctly eliminate the possibility of other causes. Among the most recurring ones then:

  • Allergy to milk protein (casein);
  • Irritable bowel syndrome;
  • Celiac disease;
  • Crohn’s disease;
  • Ulcerative colitis;
  • Food allergies;
  • Endometriosis.


In general, there are no contraindications for the exam. Even in cases of high sensitivity, the amount of lactose ingested for the exam does not represent health risks.

Allergic to milk protein with high sensitivity should not be tested , at risk of anaphylactic shock.

There are laboratories that restrict the exam to patients with diabetes , due to the alterations caused by the lack of insulin.

Symptoms after examination (side effects)

After ingesting the lactose necessary for the exam, the patient may present symptoms common to intolerance, including:

  • Diarrhea or constipation;
  • Flatulence;
  • Stuffing;
  • Reflux;
  • Vomiting;
  • Feeling of poor digestion;
  • Headache;
  • Weakness;
  • Abdominal swelling.

However, the tendency is for the signs to quickly disappear and the picture to stabilize.

Post-examination care

The care for blood collection follows the standard procedure, in which it is indicated to press the region, in order to avoid bleeding or edema.

After performing any method of examination, it may be suggested that the patient eat lactose-free meals, light and that help the body in recovery, especially if there is the presence of symptoms of intolerance.

How to interpret the results

The interpretation of the examination must be done by a specialist doctor. It will compare the values ​​and consider the patient’s personal and clinical history.

However, in general, changes in lactose tolerance can be seen if the values ​​do not follow the following references:

Blood test

Fasting blood glucose, for non-diabetic patients, is usually between 70mg / dL and 90mg / dL. Thus, lactose intolerance is evaluated when, after ingesting the liquid substance, the measured values ​​do not indicate an increase of at least 20mg / dL.

For example:

  • Fasting lactose (basal) : 91.00 mg / dL;
  • Lactose after 15 minutes : 101.00 mg / dL (10mg / dL increase compared to baseline);
  • Lactose after 30 minutes : 108.00 mg / dL (17mg / dL increase compared to baseline);
  • Lactose after 60 minutes : 109.00 mg / dL (18 mg / dL increase compared to baseline).

In the case above, after 1 hour, the result indicated an increase of only 18mg / dL, suggesting that there is poor digestion of milk carbohydrate.

It is noteworthy that the results must be interpreted with the doctor, who will assess and consider the patient’s individual conditions.

Hydrogen measurement

After measuring the amount of basal hydrogen (before ingesting lactose), changing the amount of hydrogen in the breath may indicate intolerance.

Most laboratories and doctors adopt values ​​above 20ppm (parts per million) as sufficient to indicate changes in absorption. This is because, in healthy intestines, there should be no gas formation during digestion.

Acid deposition

The acid-base balance of the stool can be altered according to the patient’s eating habits. However, in general, below the value 6, the sample is considered acidic, representing the low pH concentration, being an indicator of poor digestion or absorption of lactose.

What can change the results?

The test can be changed if the patient has taken any medication that supplements the lactase enzyme shortly before the test. But, in general, the fasting period ensures that no medication or food interferes with the result.

In the hydrogen measurement method, results can be affected if the patient smokes before or during the test, does not perform the correct fast, uses laxatives or diuretics, contrast tests or antibiotics recently.


In patients with intolerance, the test may cause discomfort and reactions to the ingested lactose. The malaise is, in general, mild to moderate, but it can present to a high degree if there is high sensitivity.

Still, for most people, there are no risks in taking the exam.

In the traditional method (blood collection), the risks involve edema and bleeding at the puncture site.

In general, dizziness, weakness and fainting may occur due to the fasting period, but they tend to pass quickly after eating food.

In associated allergic cases, the patient may trigger an immunological process to lactose and present swelling, redness of the skin, itching and, in severe cases, anaphylactic shock.


The traditional lactose intolerance test (blood collection) is offered by SUS and can be performed free of charge by anyone through medical advice. Private laboratories also offer the exam, on average, starting at R $ 20 reais.

The tests for measuring hydrogen and deposition and acids can be found, on average, for R $ 60 and R $ 15, consecutively.

Common questions

At what age can I take the lactose intolerance test?

There is no specific age to take the exam. As there is a difficulty in keeping young children still (and also because it is painful), the blood test may not be suitable for babies.

It is up to the doctor to check the need to order the exams.

Does it hurt to take the exams?

Blood tests are simple procedures. The collection is fast and, when there is pain, it is usually caused by the needle stick and tends to be well tolerated.

Measurements of hydrogen and stool samples are not considered invasive tests.

Biopsies and other imaging tests may require anesthesia, so there is no perception of pain.

The biggest complaint of the intolerance test tends to be about lactose intake, as it can cause adverse reactions if the patient is intolerant.

The exams make up the most accurate part for the diagnosis of any disease, confirming or ruling out suspicions.

It is important that the care and recommendations of each laboratory are followed so that there is no change in the values, indicating false results.

In addition, it is always necessary for a doctor to interpret the values ​​and refer the patient how to proceed correctly.

In Hickey solution you can find more information about lactose intolerance, milk-free diets, as well as information on how to maintain a more balanced life!