True about tuberculin test are all except:
## Core Concept
The tuberculin test, also known as the Mantoux test, is a diagnostic tool used to assess if a person has been infected with *Mycobacterium tuberculosis*. It involves injecting a small amount of purified protein derivative (PPD) tuberculin into the skin and measuring the immune response. The test is based on the principle of cell-mediated immunity.
## Why the Correct Answer is Right
The correct answer, ., implies that option D is the statement that does not hold true about the tuberculin test. Typically, a positive tuberculin test indicates that a person has been infected with *M. tuberculosis* at some point in their life. This could be due to current or past infection, or BCG vaccination. The test does not differentiate between latent infection and active disease.
## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but generally, statements about the tuberculin test being used for diagnosing tuberculosis infection, its reliance on cell-mediated immunity, and its potential for false positives or negatives in certain populations (like those vaccinated with BCG) are true.
- **Option B:** Similarly, this option is not provided, but if it pertains to common aspects of the tuberculin test, such as the measurement of induration (not erythema) or the timing of reading the result (typically 48-72 hours post-injection), these are accurate.
- **Option C:** Without specifics, if this option aligns with established facts about the tuberculin test, such as its interpretation based on millimeters of induration, it would be considered correct.
## Clinical Pearl / High-Yield Fact
A key point to remember is that the tuberculin test's results are interpreted based on the size of the induration (not redness) measured in millimeters, usually read 48-72 hours after injection. For example, an induration of ≥10 mm is generally considered positive in most individuals, while ≥5 mm is positive in people with HIV, recent contacts of someone with TB, or those with fibrotic changes on chest radiograph.
## Correct Answer: D.