**Question:** A positive tuberculin test is indicated by an area of induration of

A. 9 mm

**Core Concept:**

The tuberculin test is a skin test used to detect latent tuberculosis infection (LTBI) in humans. It is done by injecting purified protein derivative (PPD) of tuberculin into the skin, which is derived from the Mycobacterium tuberculosis complex, which includes M. tuberculosis, M. bovis, M. africanum, M. microti, M. canetti, and M. xenopi. The test measures the immune response to the PPD, specifically the production of a type of white blood cell called a lymphocyte, which is a response to the presence of TB antigens.

**Why the Correct Answer is Right:**

A positive tuberculin test is determined by the size of the area of induration (localized swelling) that develops at the injection site. A positive test occurs when there is a visible induration of 10 mm or more, known as a "tuberculin reaction." A reaction size of 10 mm or more is considered significant as it indicates a strong immune response against TB antigens, suggesting the presence of LTBI.

**Why Each Wrong Option is Incorrect:**

A. A test result of 9 mm is considered positive, but the size of 10 mm or more is more specific for LTBI, indicating a stronger immune response and higher likelihood of LTBI.

**Clinical Pearl:**

The tuberculin test is a valuable tool for diagnosing LTBI in high-risk populations, such as those with close contact to TB patients or in endemic areas, and can be used in combination with other diagnostic methods like chest X-ray and clinical examination to confirm TB infection. However, the interpretation of the test results requires careful assessment and correlation with other clinical findings to avoid misdiagnosis.