[Interpretation of QFT results in several contact investigations]

QuantiFERON TB-2G (QFT-2G) is now widely used for contact investigations, as QFT-2G is not affected by BCG vaccination, whereas the conventional tuberculin skin test (TST) is confounded by BCG vaccination. We applied the QFT-2G tests to numerous contact investigations and found that the majority of contacts who had been supposed to be infected with M. tuberculosis (Mtb) based on TST were negative in the QFT-2G tests, strongly suggesting the possibility that the unnecessary preventive chemotheraphy was indicated for many TST positive contacts due to BCG vaccination in conventional contact investigations. Although QFT-2G positive results implicate that Mtb in certain active phases presents in the body, when Mtb had been infected cannot be decided by QFT-2G. Therefore, QFT-2G results should be carefully interpreted, especially in elder persons who had been exposed to Mtb in the past in Japan. Furthermore, since there were some active TB cases in QFT-2G negative contacts, attention must be paid for QFT-2G negative contacts in highly exposed groups. In this paper, I will discuss about current usages of QFT-2G based on our experiences of contact investigations.