Home3.7.1. Contact investigations
In the context of a contact investigation, to identify a true conversion (ie, new infection), a single TST should be performed as soon as possible after an exposure to TB is recognized and the contact is identified. If the first TST is negative and performed less than 8 weeks after contact with the index patient, then a second TST should be scheduled no sooner than 8 weeks after the contact was broken. This also means for contacts that are identified more than 8 weeks after contact with an index patient is broken (eg, casual contacts), a single TST will identify all those with new infection.7,89 For a complete discussion on TB infection testing in contact investigations see Chapter 11: Tuberculosis Contact Investigation and Outbreak Management.
3.7.2. Health care workers and other populations
Certain populations may be repeatedly tested for TB infection because they are at elevated risk of exposure. These persons should undergo two-step TSTs prior to any exposure to account for the booster effect.89 This is because the initial TST may elicit an anamnestic immune response in persons with remote TB infection or prior BCG vaccination. This “boosted” immune response results in a greater response if a second TST is administered anytime from a week to more than a year later. In the absence of exposure, the likelihood that this greater reaction to a second test is indicative of true infection is low. This phenomenon is important to detect as it can be confused with new infection if the second test is only performed following exposure to a person with infectious pulmonary TB (see Appendix 1, Section A.3 for conduct and interpretation of two-step TST and Chapter 14: Prevention and Control of Tuberculosis Transmission in Healthcare Settings).
3.7.3. Nursing-home and long-term care residents
Performing universal TB infection testing among nursing-home and long-term care residents, prior to or shortly after entry, is discouraged. This is for three reasons. The most important is the high risk of age-related toxicity and the potential for drug-drug interactions in this population,98 which results in low rates of being offered TPT and, if offered, low rates of treatment completion.99–102 Second, the sensitivity of both the TST and IGRA are reduced in older age.103–106 Third, available evidence suggests that TB disease risk is low101 and transmission is rare in residents of these facilities who develop TB disease.101 In situations of exposure to a potentially infectious person with TB disease, testing for TB infection should be performed on a case-by-case basis, considering the balance of individual risks and benefits associated with treatment and the epidemiological context (see also Chapter 14: Prevention and Control of Tuberculosis Transmission in Healthcare Settings).
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