PPD is the standard screening test to identify the presence of Mycobacterium tuberculosis infection and, thus, assist in the diagnosis of tuberculosis. Usually, this test is performed on people who have been in direct contact with patients infected by the bacteria, even if they do not show symptoms of the disease, due to the suspicion of a latent infection with tuberculosis, when the bacteria is installed but has not yet caused the disease. Find out what the symptoms of tuberculosis are.
The PPD test, also known as the tuberculin skin test or Mantoux reaction, is done in clinical analysis laboratories through a small injection containing proteins derived from the bacteria under the skin, and must be evaluated and interpreted preferably by a pulmonologist so that it can be done. correct diagnosis.
When the PPD is positive there is a high chance of being contaminated by the bacteria. However, only the PPD test is not enough to confirm or exclude the disease, so in case of suspected tuberculosis, the doctor should order other tests, such as chest X-rays or sputum bacteria, for example.
How the PPD exam is done
The PPD exam is done in a clinical analysis laboratory by injecting a purified protein derivative (PPD), that is, purified proteins that are present on the surface of the tuberculosis bacteria. The proteins are purified so that there is no development of the disease in people who do not have the bacteria, however the proteins react in people who are infected or have been vaccinated.
The substance is applied to the left forearm and the result must be interpreted 72 hours after application, which is the time that the reaction normally takes to happen. Thus, 3 days after the application of the tuberculosis protein, it is recommended to go back to the doctor to know the result of the test, which must also take into account the symptoms presented by the person.
To take the PPD exam it is not necessary to fast or take any other special care, it is only recommended to inform the doctor if you are using any type of medication.
This test can be performed on children, pregnant women or people with compromised immune systems, however, it should not be done on people who have the possibility of severe allergic reactions, such as necrosis, ulceration or severe anaphylactic shock.
PPD exam results
The results of the PPD test depend on the size of the reaction on the skin, as shown in the image and, therefore, can be:
- Up to 5mm: in general, it is considered a negative result and, therefore, does not indicate infection with the tuberculosis bacteria, except in specific situations; 5 mm to 9 mm: it is a positive result, indicating infection by tuberculosis bacteria, especially in children under 10 years old who have not been vaccinated or vaccinated with BCG for more than 2 years, people with HIV / AIDS, with weakened immunity or who have tuberculosis scars on the chest X-ray; 10 mm or more: positive result, indicating infection by tuberculosis bacteria.
In some situations, the presence of a skin reaction greater than 5 mm does not mean that the person is infected with the mycobacterium that causes tuberculosis. For example, people who have already been vaccinated against tuberculosis (BCG vaccine) or who have infection with other types of mycobacteria, may experience a skin reaction when the test is performed, being called a false positive result.
A false-negative result, in which the person has the infection by the bacteria, but does not form a reaction in the PPD, can arise in cases of people with weakened immunity, such as people with AIDS, cancer or using immunosuppressive drugs, in addition to malnutrition, age over 65, dehydration or with some serious infection.
Because of the chance of false results, tuberculosis should not be diagnosed by analyzing this test alone. The pulmonologist should request additional tests to confirm the diagnosis, such as chest X-rays, immunological tests and sputum smear microscopy, which is a laboratory test in which the patient's sample, usually sputum, is the bacillus that causes the disease. These tests should also be ordered even if the PPD is negative, as this test alone cannot be used to exclude the diagnosis.
