51. Diagnostic skin tests

Diagnostic skin tests. Skin reactions to the administration of tuberculin are based on the detection of delayed-type hypersensitivity to antigens of tuberculous mycobacteria and are of great importance in the diagnosis of tuberculosis infection. Positive reactions appear 6-10 weeks after pathogens enter the body. The test involves intradermal injection of an antigen drug into the patient. A positive reaction is expressed by the appearance of indurate at the injection site. It is caused by migration of activated lymphocytes and macrophages to the area of ​​antigen injection. Two different tuberculin preparations are used: old Koch tuberculin (alt-tuberculin, ATK) and purified protein-free tuberculin PPD.

Mass screening of tuberculosis infection in pediatric practice is carried out using multiple puncture methods. The disadvantage of this relatively sensitive method is the weak specificity, therefore, in cases of a positive or doubtful reaction, it is usually necessary to additionally examine using the Mantoux test. The most common is the Tine-test, which involves the use of a plate with four steel spikes soaked in ATK. The test results are taken into account after 48-72 hours. A positive reaction is expressed by the appearance of vesicles or more often papules with a size of at least 2 mm at the site of one or more punctures.

The Heaf test involves the use of a special device that simultaneously produces 6 skin punctures to a depth of 1 mm through a layer of concentrated PPD tuberculin. The sample can be taken into account within the next 3-7 days. A positive reaction is expressed by the appearance of 4 or more papules at the puncture site. False positive reactions are not uncommon with all multiple puncture techniques. In addition, all positive and doubtful reactions require confirmation of the Mantoux test.

The Mantoux test is more complex than multiple puncture methods, but more accurate, since it introduces a strictly defined amount of antigen. The results of the reaction are taken into account after 48-72 hours. The appearance of an indurat with a diameter of 10 mm at the injection site indicates an infection with tuberculosis and is regarded as a positive reaction.

With an indurate of 5 to 10 mm, the reaction is regarded as doubtful, and with an indurate diameter of up to 5 mm, as negative.

Under certain circumstances, an indurat of 5 to 10 mm can be interpreted as a dubious reaction and treatment can be prescribed. False-negative Mantoux test results can occur for many reasons: they are negative in the early stages of the disease, even with the introduction of 250 IU; as a result of technical errors in the storage of tuberculin and during the test; as a result of the suppression of tuberculin reactions by preventing activation by lymphocytes and the development of delayed-type hypersensitivity.

Any reaction to intradermal administration of tuberculin larger than 10 mm, occurring 3 years or more after BCG vaccination, should be considered as an indicator of tuberculosis infection.

Author: Pavlova N.V.

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