Tuberculin test is positive if induration is –
**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. It involves injecting a small amount of purified protein derivative (PPD) into the skin, and measuring the resulting induration (not redness) at 48-72 hours.
**Why the Correct Answer is Right**
A positive tuberculin test indicates that the individual has been exposed to M. tuberculosis and has mounted an immune response. The induration is a result of cell-mediated immunity, primarily involving T lymphocytes and macrophages. The size of the induration is directly proportional to the intensity of the immune response, with larger indurations indicating a more robust response. The correct threshold for a positive test is a minimum of 10 mm of induration in individuals with no known risk factors for tuberculosis.
**Why Each Wrong Option is Incorrect**
**Option A:** 5 mm of induration is too small to be considered a positive test, even in individuals with known risk factors. This option is incorrect because it does not meet the minimum threshold for a positive test.
**Option B:** Redness is not a reliable indicator of a positive tuberculin test, as it can be caused by various factors such as irritation or allergic reactions. This option is incorrect because it focuses on the wrong aspect of the test.
**Option C:** 20 mm of induration is excessive and would be considered a false positive in individuals with no known risk factors. This option is incorrect because it exceeds the expected range for a positive test.
**Clinical Pearl / High-Yield Fact**
Remember that the tuberculin test is not a definitive diagnostic tool for tuberculosis, but rather a screening test to identify individuals who may require further evaluation.
**Correct Answer: D. 10 mm of induration.**