Tuberculosis infection by tuberculin skin test or QuantiFERON and related factors in contact investigations

OBJECTIVE: We investigated the factors related to secondary infections in contact group investigations. METHOD: From March, 2008 to February, 2009, a total of 457 tuberculosis (TB) notifications were reviewed by the Health Examination Committee of the Health Centers of Osaka City over indications of a contact investigation. A contact investigation was considered necessary for 92 cases with 620 contacts. For the contacts of these cases, the tuberculin skin test (TST) or/and QuantiFERON-TB 2nd Generation (QFT) was used. The contacts testing positive for QFT were classified as "infected". The contacts with "doubtful" QFT were classified as either "infected" or "non-infected" according to the QFT-positive rate of the group as a whole. Those with blisters in TST were classified as "infected". RESULTS: Among the total of 84 "infected" cases thus defined, 56 were QFT-positive, 17 were doubtful, and 11 had blisters with TST. On the other hand, among the total of 515 "uninfected" cases, 18 were doubtful on the QFT test, 323 were QFT-negative and 174 were cases with TST of erythema less than 30 mm without QFT test. The relationship between characteristics of index cases and whether a contact was infected or not was as follows: Severe chest X-ray findings, sputum smear positivity, and a cough persisting for more than two months were significantly associated with infection of the contacts. The closeness of contact was also significantly related with a higher risk of infection, so that contact with an index case for more than 100 hours, and contact in a space less than 100 square meters were more likely to cause transmission of infection. CONCLUSION: Severity of chest X-ray findings, degree of smear positivity and period of cough were confirmed to be important in judging whether investigations are necessary. The time and space of contact were also considered to be important factors. We should consider these factors comprehensively in order to decide on indications for a contact investigation.