[Evaluation of tuberculosis contact investigation & examination in congregate settings in Osaka City]

To evaluate tuberculosis index case, group contact investigation and usefulness of tuberculin skin test (TST) results.

We analyzed data for 833 groups examined by contact investigation registered in 2003-2005 in 24 wards of Osaka City characteristics of their index cases, TST results, and follow-up outcome of radiograph screening up to 2 years.

71.3% (7406/10378) of contacts completed 2 years follow-up radiograph screening. There were 54 secondary cases. Characteristics of 38 index cases of contact groups were compared by the rate of causing secondary cases. Results showed that the following characteristics were statistically significant in causing secondary cases: sex (male: 5.6%, female: 1.8%, chi2 test; p<0.05), age (49 or less: 6.8%, 50-64: 5.7%, 65 or over: 0, p<0.01), chest radiography (cavitary: 6.0%, non-cavitary: 2.0%, p<0.05), initial AFB smear grade (G5 or over: 6.5%, G4 or less: 2.2%, p<0.01), cough duration (2 months or more: 7.0%, less than 2 months: 2.8%, p<0.01), transmission risk index (10 or over: 8.4%, 0.1-9.9: 3.4%, 0: 0, p<0.001). As initial part of contact investigation, decision was made for each group whether TB secondary transmission was "likely" or "unlikely". If it was considered as "likely", TST was conducted for contacts. Incidence rate of secondary case was 0.44% in groups TST was conducted, and 0.09% in groups TST was not conducted. The difference was statistically significant (p<0.05). According to tuberculin results, 760 persons in 215 groups were given LTBI treatment. Comparing the number of secondary cases in the age group of 10-39 eligible for LTBI treatment, 19 secondary cases were detected immediately or 2 months after the initial contact examination, and it was reduced to 7 cases after 6 months to 2 years. However in the age group of 40 and over not eligible of LTBI treatment, the number of secondary cases increased from 10 immediately or 2 months after the initial contact examination to 18 after 6 months to 2 years, and the difference was statistically significant (p< 0.01).

Results of contact investigation and successive LTBI treatment showed that the prevention of secondary cases was effective, despite of some over-diagnosis of LTBI by TST.