Which of the following statements regardingGamma-Release-Assays for diagnosis of Tuberculosis is true –
**Core Concept**
Gamma-Release-Assays (GRAs) are a type of immunodiagnostic test used to diagnose latent tuberculosis infection (LTBI) by measuring the release of gamma interferon (IFN-γ) in response to specific antigens of Mycobacterium tuberculosis. This test is primarily used for individuals who have been exposed to TB but do not exhibit symptoms.
**Why the Correct Answer is Right**
The correct answer is based on the principle that GRAs detect cell-mediated immunity to M. tuberculosis antigens, which is a hallmark of LTBI. The IFN-γ release is a result of the activation of T cells that recognize M. tuberculosis antigens presented by antigen-presenting cells. This immune response is specific to M. tuberculosis and is not elicited by BCG vaccination or other non-tuberculous mycobacteria. The use of specific M. tuberculosis antigens, such as ESAT-6 and CFP-10, further enhances the specificity of the test.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct antigens used in GRAs. While M. tuberculosis antigens are used, the specific antigens mentioned are not accurate.
**Option B:** This option is incorrect because it implies that GRAs are used to diagnose active TB, which is not the case. GRAs are primarily used for diagnosing LTBI.
**Option C:** This option is incorrect because it suggests that GRAs are a type of serological test, which is not accurate. GRAs measure cell-mediated immunity rather than serum antibodies.
**Clinical Pearl / High-Yield Fact**
It is essential to note that GRAs are not a substitute for the Mantoux test (tuberculin skin test) but can be used as an alternative in certain situations, such as in individuals with a history of BCG vaccination or those with a high risk of false-positive results.
**Correct Answer:** C.