When to use Quantiferon?

when they are foreign and have had the BCG vaccine

see uptodate one diagnosis of TB or the WHO- executive summery on the quant gold for more info.

Guidelines for the diagnosis of active and latent tuberculosis (TB) were published in 2017 by the American Thoracic Society (ATS), United States Centers for Disease Control and Prevention (CDC), and Infectious Diseases Society of America (IDSA). The guidelines state the following:

●For individuals with low-to-intermediate risk of progression to active disease (table 3), the IGRA is preferred over TST for diagnosis of LTBI. IGRA is especially useful for patients who are unlikely to return to have the TST read and for patients with a history of Bacille Calmette-Guérin (BCG) vaccination (administered at birth in most TB-endemic countries). The TST is an acceptable alternative to IGRA, especially in situations where IGRA is not available or is too costly (even though it is less specific than the IGRA).

●For individuals with high risk of progression to active disease (table 3), either the IGRA or TST may be used; in such cases, the higher false-positive rate of the TST is acceptable. A dual testing strategy (perform one test and, if negative, perform the other) may be used, in which a positive result from either test would be considered positive.

UptoDate (2018)