Usefulness of a novel diagnostic method of tuberculosis infection, QuantiFERON TB-2G, in an outbreak of tuberculosis

OBJECTIVE: The purpose of this study was to evaluate QuantiFERON TB-2G (QFT), a novel method of detecting tuberculosis infection among contacts of a tuberculosis patient by determining the whole-blood interferongamma response to the specific antigens. SUBJECTS AND METHODS: A teacher of a college who had been coughing for the preceding two months was diagnosed with smear-positive tuberculosis. About 270 students of the college were considered to have been exposed to tuberculosis infection, of whom 73 were in closer contact with the index case because they participated in a one-week group excursion attended by the teacher. Two of the contact students developed active tuberculosis shortly thereafter. Tuberculin tests were conducted to almost all students, and QFT was performed for only those with tuberculin reactions having erythema diameters of 30 mm or larger. RESULTS: Tuberculin tests of students, all of whom had been vaccinated with BCG at least once, revealed that the distribution of the close contact group was slightly shifted to right (larger side) than those with less close contacts. The QFT positive rate for close contacts was 45.5%, while that for less close contacts was only 7.1%, which obviously indicates that QFT is hardly affected by the tuberculin allergy due to past BCG vaccination. The distribution of interferon-gamma measurements (log-transformed) of the close contacts showed typical bimodality, one mode representing the infected, another the non-infected. This was not clear for the less close contacts. The correlation of interferon-gamma measurements (log-transformed) with tuberculin reaction erythema size was weak, if not non-significant. CONCLUSION: It was concluded that QFT was a useful method for diagnosing tuberculosis infection and was unaffected by the BCG-caused tuberculin allergy. In the case of the outbreak mentioned above, QFT greatly reduced the indication of chemoprophylaxis, from 28% of all the contacts solely based on tuberculin test to only 7%. Although there remains some problems to be overcome for QFT to be widely used with high confidence, this technology will provide a high possibility for wider and more accurate indication of chemoprophylaxis and will be one of the essential tools of tuberculosis control of the 21st century in Japan.