Objective: To diagnose M. tuberculosis infection and select subjects for chemoprophylaxis in a contact investigation, we used the whole blood interferon-gamma response test, QuantiFERONTB-2G (QFT), and examined the usefulness of QFT.
Subjects and methods: The index case (heavily positive for sputum smear, at grade 6 by Gaffky system, the duration of coughing being 8 months; hence the infectious risk index is 48) was found at a periodic mass health examination before proceeding to the second grade in a university. Since TB outbreak was suspected based on the results of tuberculin skin test (TST) in the contact investigation, QFT test was carried out to determine the subjects for chemoprophylaxis and to define the target of further contact investigations.
Results: In the first TST, 57 contacts showed erythema of more than 30 mm in diameter, and these contacts would have been indicated for chemoprophylaxis based on TST results. Thus, this case would have been designated as a TB outbreak, and further investigation should be necessary for less close contacts. However, twice QFT tests revealed that only five contacts were positive for QFT (three showed erythema diameter of more than 30 mm, and two less than 29 mm). These five contacts were indicated chemoprophylaxis. Thus, the number of the secondary infections did not fulfill the criterion defined as a TB outbreak, and therefore an extended contact investigation was stopped. No contact has developed TB so far.
Conclusion: QFT test was shown to be useful for determining subjects for chemoprophylaxis and selecting the range of the contact investigation.