[ESTIMATION OF POSITIVE RATES OF INTERFERON-GAMMA RELEASE ASSAY BY AGE GROUP IN JAPAN]

[Background] Interferon-gamma release assay (IGRA) is necessary for evaluating Mycobacteriun tuber- culosis infection in Japan. Application of IGRA for contact surveys has been extended for the aged population; how- ever, there is little information on positive rates with Quanti- FERON® in Tube (QFT-3G) and T-SPOT.®TB (T-SPOT), which sometimes makes it difficult to interpret the results of IGRA performed in contact investigation including the aged population. [Objective] To estimate the positive rate of IGRAs by age group in the general population as well as among healthcare workers. [Methods] We requested all public health centers in Japan to provide contact investigation data for which the risk of infection is limited. Collected data included results of IGRAs in the target group, sputum bacteriological examinations and chest-image findings, and symptoms of the index cases as well as closeness and duration of contact between the index case and the target group. We scrutinized all the cases and exclude data that were not eligible for this study. Positive rates by age group were calculated by summing the number of contacts who were "positive" and dividing by the number of examinees. [Results] In spite of our effort to exclude newly infected persons from the index case, a small portion (probably 3%) may be due to those newly infected by a source case, as it is difficult to exclude those who get infected by casual contact. It is sometimes difficult to collect information on the close- ness and overall duration of contact with the index case, which is a limitation in the questionnaire. Positive rates of IGRA by age group in the general popula- tion were one third to one fifth of the predicted prevalence of infection, which is consistent with findings in the study using QFT Gold (QFT-2G) that IGRA wanes after infection. There were no differences of IGRA positive rate between the general population and health care workers. It may be because the risk of infection for health care workers is similar, as the number of infectious TB patients has been decreasing and infection control in hospitals has generally improved. It may be also because targets for IGRA in contact examina- tion among health care workers tend to be broad including a certain number of low risk staff. [Conclusion] With reference to past studies, we estimated that IGRA positive rates were 5% in the 60's and 15% in the 70's. It will be useful in assessing the possibility or spread of infection for aged groups in contact investigation.