A positive skin test, specifically referring to a tuberculin skin test (TST) for tuberculosis, isn't defined by a single size. The size considered "positive" depends on several factors, including the individual's risk factors.
Interpreting Tuberculin Skin Test (TST) Results
The TST measures the induration (hardened area) at the injection site, not the redness (erythema). The size of this induration determines the interpretation:
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15 mm or more: This size is considered positive in anyone, regardless of risk factors or suspected exposure to TB. This is a consistent finding across multiple sources. (CDC, Healthline)
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10 mm or more: This size is considered positive for specific groups:
- Individuals who have recently immigrated from countries where TB is prevalent.
- People who work or live in close contact with those who have TB.
- People with compromised immune systems (e.g., HIV-positive individuals). (NCBI Bookshelf, Wikipedia)
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10 mm induration in children under 4 years or people who use injected drugs: This is also considered a positive result ([Reference 1, not provided in original prompt]).
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5 mm or more: This size is considered positive in individuals with HIV infection or those with recent contact with a TB patient. (Wikipedia)
It's crucial to remember that only a healthcare professional can interpret the results of a skin test accurately. They will consider the individual's risk factors and medical history along with the measurement of the induration to determine if further evaluation is needed. A positive skin test doesn't automatically mean active TB; it suggests latent TB infection requiring further investigation. (CDC)
