Tuberculin Skin Test (TST) - Test Frequency
Two-step testing has been recommended, involving a baseline and a retest. This is to rule out "booster phenomenon" which may occur if many years have passed since an individual first became infected. (Immune reaction wanes over time, especially in older people.) The first TST result may be read as negative, while the second baseline TST [1-3 weeks later] is positive, leading to a correct interpretation of TB infection, but an incorrect interpretation as a recent conversion. More likely, the boosted reaction occurred because the first TST stimulated an old TB infection in the body.
For individuals who will be retested periodically, two-step testing should be used for the initial TST. In this scenario, a positive TST (after negative baseline and retest) would indicate the individual likely has TB infection. This result could be due to a conversion after an exposure to TB.
In either case (old infection or recent conversion), the individual will need to be further evaluated.
The reading and subsequent interpretation of the TST reaction (zone of induration) is dependent upon risk group. Per current CDC guidelines, the TST is considered "positive" when:
≥15mm zone size and:
- The 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 (homeless shelters, correctional facilities, nursing homes), or
- Has certain medical condition(s) that places him or her at risk for TB (eg., silicosis, cancer, severe kidney disease, etc.), or
- Has a low body weight (<90% of ideal body weight), or
- Is a child under age 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 as a result of 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.
False Positives
- Nonspecific reactions (hives, redness, inflammation)
- Inaccurate reading by the provider
- Interference from nontuberculosis infections, such as Mycobacterium avium complex disease (Ra, 2011)
- Reaction from prior BCG vaccination
- Boosted reactions (misread as a new infection, rather than old infection)
False Negatives
- Immunosuppressed patients (anyone who lacks lymphocytes and is unable to mount an immune response)
- Recent TB exposure (immune response has not yet occurred)
- Very young infants (<6 months)
- Live virus vaccination - recent vaccine for measles, smallpox (can suppress reactivity)
- Overwhelming reaction (TB disease can suppress reactivity)
- Poor TST administration technique (too small a dose)
- Inaccurate reading by the provider
9. CDC. Two-step TST Testing. Slide #37 from Targeted Tuberculosis (TB) Testing and Treatment of Latent TB Infection – Download complete slide set. Centers for Disease Control and Prevention site - the slide set is in the public domain. September 2016. Accessed March 17, 2022. https://www.cdc.gov/tb/publications/slidesets/ltbi/default.htm
