The reading and subsequent interpretation of the TST reaction (zone of induration) depends on the risk group. Per current CDC guidelines, the TST is considered "positive" when:
≥15mm zone size andthe individual:
- Has no known risk factors for TB
≥10mm zone size and the individual:
- Was born in a TB-endemic country or
- Abuses drugs, or
- Works in a mycobacteriology laboratory, or
- Lives in high-risk congregate settings (e.g., homeless shelters, correctional facilities, nursing homes), or
- Has specific medical condition(s) that places them at risk for TB (e.g., silicosis, cancer, severe kidney disease, etc.), or
- Has a low body weight (<90% of ideal body weight), or
- Is a child under the age of 5 years, or
- Is an infant, child, or adolescent exposed to an adult in any high-risk categories
≥5mm zone size and the individual:
- Is living with HIV infection or is immunosuppressed
TST Interpretation of Results
- Positive TST = An immune response is present. However, a positive may occur due to cross-reactivity, as the TST is not highly specific. Additional testing must be performed.
- Negative TST = Does not always mean the individual is free from tuberculosis infection. (See limitations.) A negative result means that there was inadequate or no immune response.
