Discussion on evidence for principle of tuberculosis contact investigation--from the experience in Osaka City

Risk factors for tuberculosis transmission from patients to contacts were found to be younger age, presence of cavitary lesion in chest X-ray findings, shedding higher number of organisms, longer duration of respiratory symptoms, longer hours of contact, and smaller space of contact environment. It should be noted that even casual contacts identified by contact investigation developed tuberculosis later on. In the past contact investigation and subsequent latent TB infection (LTBI) treatment, preventive effects of tuberculosis was obvious, although there might be some over-diagnosis of LTBI with tuberculin skin testing (TST). Introduction of interferon-gamma release assay (IGRA) added to TST seems to increase specificity and reduce over-diagnosis of TB infection. However in case of outbreak investigation in schools, screening by TST is still effective and efficient. Only those case with 30mm or over of redness (equivalent of 15 mm of induration) of TST were tested with QFT, IGRA. If anyone shows positive QFT, test of QFT would be expanded to the contact with TST of less than 30 mm.