The Mantoux test

The Mantoux or tuberculin test or PPD consists of demonstrating the organism's hypersensitivity to the proteins of the tubercle bacillus, a reaction capacity of the immune system that is acquired once the organism has been infected by M. tuberculosis. It is essential to note that a positive test implies that there has been infection but not that the disease has developed.

Who should be tested?

This test for detection of infection by M. tuberculosis should not be carried out in the general population, but only in people who have a high probability of having contracted tuberculosis and who can benefit from treatment even though they do not show signs of tuberculosis. the illness. These people are:

• People in contact with patients with overt tuberculosis.
• HIV-infected patients.
• People with lung radiographic images of old healed tuberculosis lesions that were not adequately treated.
• Patients with risk factors such as diabetes, low weight, silicosis, gastrectomy, chronic renal failure, chronic malabsorption, head and neck neoplasms, hematological neoplasms, treatment with corticosteroids or treatment with immunosuppressants.
• Workers or residents of hospitals, prisons, nursing homes or drug addiction centers.
• People who come from countries with a high incidence of the disease.

What is the Matoux test?

The Mantoux test consists of introducing a small amount of liquid (0.1 ml) intradermally through a syringe containing a purified derivative of the tuberculosis bacillus antigen. When the liquid is introduced there is an elevation of the skin, a wheal, less than one centimeter in diameter. It is important that the person does not scratch, rub or manipulate this area, nor should it be covered.

After 48 or 72 hours, what is done is to measure the diameter of the inflamed skin area at the injection site. Only the diameter of the indurated area should be measured, not the redness (erythema) that occurs around it. Depending on the millimeters of said induration, the test will be considered positive or negative. The cut-off point for considering the Mantoux test positive or negative will vary depending on the situation of each patient. Thus, a reading of diameter 5 mm or more will be positive in:

• HIV patients
• People in close contact with patients with active tuberculosis
• People with radiological images of cured old tuberculosis

A reading of 10mm diameter or more will be positive in:

• People with risk factors other than HIV
• History of drug use or active drug use by vein
• People who live in nursing homes, hospitals, prisons, or detoxification centers
• Health personnel
• Under 5 years old

A reading of 15 mm in diameter or more will be positive in other people (that do not meet any of the above criteria).

If the induration that is measured is less than these diameters for these groups of people, it will be considered negative in people under 65 years of age. In those people over 65 years it would be advisable to repeat the test at 7-10 daysIt is the first, since the capacity to respond to tuberculin decreases over the years. If the result is negative again then it can be confirmed that there is no infection.

Some people may be vaccinated against tuberculosis by a vaccine called BCG (Bacillus Calmette-Guérin), a fact that can alter the results of the Mantoux test if the vaccination has been carried out in the last year. In vaccinated patients, it will be considered negative if the induration measures less than 15 mm, except if it measures 5 mm or more or 10 mm or more in the same cases described above for these diameters respectively.

Likewise, if a negative tuberculin test is performed and after two years it is repeated and an increase in the diameter of the induration of 10 mm or more is observed with respect to the first test, it will always be considered positive. This is called tuberculin conversion and implies that the infection has recently been acquired.

What if the result is positive?

Chest x-ray and sputum cultures should be tested to rule out an unknown active infection. In the case of active disease, a treatment with several antibiotics should be carried out for a long time.

If, on the contrary, there is no active infection, it should be carried out prophylaxis with an antibiotic, isoniazid, for 6 months As long as the risk factors mentioned above exist (HIV carriers - in this case the treatment will be 12 months - living with people with active tuberculosis, health personnel, etc.) Isoniazid can have side effects such as skin rashes, gastrointestinal discomfort or liver involvement, so it is very important not to drink alcohol during antibiotic treatment.

Having a positive Mantoux test does not imply that it can necessarily be spread to other people, unless it has been proven that there is an active infection. The vast majority of people with a positive Mantoux do not have the ability to infect others.

Even after correct treatment after having a positive tuberculin test there is a minimal possibility of develops active tuberculosis, since there are tuberculosis bacilli that have developed resistance to isoniazid and other antibiotics. It is advisable to take a relative rest, not become excessively exhausted, and maintain healthy lifestyle habits (diet, hours of sleep, moderate physical activity) during treatment with isoniazid.

• A positive test implies that there has been infection but not that tuberculosis has developed.
• The vast majority of people with a positive Mantoux do not have the ability to infect others.
• Even after correct treatment after having a positive tuberculin test there is a minimal chance of developing active tuberculosis.

General Medicine Specialist