Immunodiagnostics of tuberculosis: goals and agents
The main objective of immunodiagnostics is to detect the body's sensitization to mycobacteria using specific antigens. This is the priority screening method in children.
The main agents for intradermal administration are:
- Purified tuberculosis allergen in standard dilution (Mantoux test with 2 TU), the biological activity of which is measured in tuberculin units.
- Recombinant tuberculosis allergen (RTA), containing specific CFP-10-ESAT-6 proteins.
Age indications are strictly divided: children up to 7 years of age inclusive are given the Mantoux test for selection for BCG revaccination, while individuals aged 8 to 14 years undergo the RTA test for screening purposes.
Pathogenesis and mechanism of skin reactions
The action of tuberculin and recombinant allergens is based on the development of a cellular immune response. The intradermal injection triggers a specific skin reaction, which is a classic manifestation of delayed-type hypersensitivity (DTH).
Criteria for evaluating Mantoux test results:
- Negative — complete absence of infiltrate or a puncture reaction (0–1 mm).
- Doubtful — papule 2–4 mm or hyperemia only.
- Positive — papule with a diameter of 5 mm or more.
- Hyperergic — in children, a papule from 17 mm; in adults, from 21 mm, as well as the presence of vesicles, necrosis, or lymphangitis.
Infectious and post-vaccination allergy
A positive result of the intradermal test can have two fundamentally different causes requiring differential diagnosis:
| Sign | Infectious allergy | Post-vaccination allergy |
|---|---|---|
| Cause | Infection with a wild-type M. tuberculosis strain | Response to previously administered BCG vaccination |
| Mantoux test | Positive, tuberculin conversion and hyperergic reaction possible | Positive due to shared antigens |
| Diaskintest (RTA) | Positive | Negative (no wild-type proteins) |
This is precisely why the RTA test is indispensable for ruling out false-positive reactions caused by the BCG vaccine.
Criteria for tuberculin conversion and indications for a phthisiatrician consultation
Tuberculin conversion is the initial appearance of a positive reaction following previously negative results. This is an important marker of primary infection and tuberculosis intoxication.
Key immunological criteria for infection include:
- Transition of a negative test to positive ("conversion").
- Increase in reaction intensity over time.
- Appearance of a hyperergic reaction.
Referral to a phthisiatrician is required upon detection of conversion, an increase in the Mantoux test induration, positive or doubtful RTA test results, as well as in the presence of clinical symptoms suggestive of tuberculosis.
Mantoux for Tots (under 7 years for BCG selection), RTA for Active disease recognition (specific for tuberculosis patients with no BCG reaction).
Class and exam questions
What is the main difference between the Mantoux test and the RTA test (Diaskintest)?
The Mantoux test reacts to both infection and post-vaccination immunity after BCG. The RTA test contains recombinant proteins specific only to actively multiplying Mycobacterium tuberculosis, which is why it yields a negative result in the presence of post-vaccination allergy alone.
What is tuberculin conversion?
This is the initial appearance of a positive reaction to the Mantoux test following preceding negative results. This sign indicates primary infection of the body with mycobacteria.
Is Diaskintest used to select children for BCG revaccination?
No. Diaskintest is not used for selection for BCG vaccination or revaccination, as it does not detect post-vaccination immunity, but is intended for the diagnosis of tuberculosis infection and differentiation of allergies.
What else to study on this topic
- Clinical presentation of tuberculosis intoxication in children
- Specifics of performing tests before prescribing immunosuppressive therapy
- Causes of tuberculin anergy in sarcoidosis
- Methodology for administering and reading Mantoux test results at 72 hours
- Indications and contraindications for BCG and BCG-M vaccination and revaccination
Master the entire topic in the "Immunology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Intradermal vaccine administration" and 723 more subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 23 video lessons on the subject
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.