Positive tuberculin test signifies –
**Core Concept**
The tuberculin skin test (TST) is a diagnostic tool used to assess the immune response to Mycobacterium tuberculosis. It measures the body's cellular immune reaction to the tuberculin antigen, which is a protein derived from M. tuberculosis.
**Why the Correct Answer is Right**
A positive TST result indicates that the individual has been previously exposed to M. tuberculosis, either through infection or vaccination with the Bacillus Calmette-Guérin (BCG) vaccine. This exposure triggers a cell-mediated immune response, leading to the production of cytokines and the activation of T lymphocytes. The TST is considered positive if the induration (not redness) at the injection site is ≥5 mm in diameter. This reaction is a result of the delayed-type hypersensitivity response, which is a hallmark of cell-mediated immunity.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because a negative TST result does not rule out latent TB infection, as it may be due to a variety of factors, including immunosuppression or a false-negative test result.
**Option B:** Incorrect because a positive TST result does not necessarily indicate active TB disease; it may indicate latent TB infection or previous vaccination with BCG.
**Option C:** Incorrect because the QuantiFERON-TB Gold test is a different diagnostic tool that measures interferon-gamma release in response to M. tuberculosis antigens, rather than the tuberculin skin test.
**Clinical Pearl / High-Yield Fact**
It's essential to note that a positive TST result in individuals who have received the BCG vaccine may not necessarily indicate latent TB infection. Instead, it may reflect a reaction to the vaccine antigens.
**Correct Answer: B. Previous exposure to Mycobacterium tuberculosis.**