Tuberculin test is –
**Core Concept:**
The tuberculin test, also known as the Mantoux test or skin test, is a diagnostic procedure used to detect latent tuberculosis infection (LTBI) and to assess the response to tuberculosis (TB) vaccination (BCG). It involves injecting purified protein derivative (PPD) into the skin to stimulate a cellular immune response.
**Why the Correct Answer is Right:**
The correct answer is D: **True-False**. A true-false response is expected after injecting PPD because it triggers a delayed-type hypersensitivity reaction. This reaction is a result of T-cell activation and subsequent production of cytokines, such as interferon-gamma (IFN-Ξ³). IFN-Ξ³ binds to tuberculosis-specific antigens, leading to the formation of granulomas and induration (skin swelling) at the injection site. The size of the induration is a measure of the immune response and helps to determine the likelihood of LTBI or past BCG vaccination.
**Why Each Wrong Option is Incorrect:**
A) False: This option is incorrect because the tuberculin test does not assess the presence of active TB disease but rather detects LTBI or past BCG vaccination.
B) False: Although the test is used to diagnose LTBI, it does not specifically detect Mycobacterium tuberculosis infection, but rather indicates a T-cell response to mycobacterial antigens.
C) False: The tuberculin test is not solely for assessing the immune status in HIV-positive patients. It is useful in general for diagnosing LTBI and evaluating the immune response to BCG vaccination. However, the test interpretation requires caution in HIV-infected individuals.
**Clinical Pearl:**
Understanding the true-false nature of the tuberculin test allows for accurate interpretation of the test results. The test should be performed in a controlled environment under the guidance of a trained medical professional. The reading of the test is done 48-72 hours post-injection, and positive results are defined as a diameter of induration β₯5 mm in BCG-vaccinated individuals or β₯10 mm in unvaccinated individuals.
**Correct Answer:** **True**
The tuberculin test is a useful diagnostic tool for assessing the immune response to mycobacterial antigens, such as those present in the PPD. It helps in diagnosing latent tuberculosis infection (LTBI) and evaluating the immune response to BCG vaccination.