Which of the following is true about tuberculin test?
**Core Concept**
The tuberculin test, also known as the Mantoux test, is a diagnostic tool used to assess an individual's immune response to Mycobacterium tuberculosis (M. tuberculosis). It involves the intradermal injection of tuberculin, a protein derived from M. tuberculosis, to measure the skin's hypersensitivity reaction.
**Why the Correct Answer is Right**
The tuberculin test is based on the concept of cell-mediated immunity, where T cells recognize and respond to the presence of antigens from M. tuberculosis. The reaction is typically measured in millimeters of induration (not redness) 48-72 hours after the injection. A positive result indicates that the individual has been exposed to M. tuberculosis and has developed a cellular immune response. The test is commonly used to diagnose latent tuberculosis infection (LTBI) and to assess the effectiveness of treatment.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the tuberculin test is not a blood test, but rather a skin test that measures the body's immune response to M. tuberculosis.
**Option B:** This option is incorrect because the tuberculin test is not used to diagnose active tuberculosis, but rather to diagnose latent infection.
**Option C:** This option is incorrect because the tuberculin test does not measure IgG antibodies, but rather the cellular immune response to M. tuberculosis.
**Clinical Pearl / High-Yield Fact**
The tuberculin test is a crucial tool in diagnosing latent tuberculosis infection, which can progress to active disease if left untreated. It is essential to remember that a positive result does not necessarily indicate active disease, but rather exposure to M. tuberculosis.
**Correct Answer:** B. The tuberculin test is used to diagnose latent tuberculosis infection (LTBI).