Abstract: Objective  To investigate TB infection risk and related factors on the closely contacts of smear positive TB patients. Methods  7884 persons closely contacted with smear positive pulmonary tuberculosis patients were checked-up by PPD tests within 1 month after the TB patients were diagnosed, and persons with negative result of  PPD tests were checked-up again by PPD tests 3 months later. Results  The infection rate of tubercle bacillus in contacts <15 years old was 39.03% (153/392), and in contacts ≥15 years old was 60.65% (4014/6618). The ratio of reaction size ≥15mm was 28.06% (110/392) in contacts <15 years old without BCG scar, it’s 1.67 times of contacts <15 years old who have BCG scar（16.82%,147/874）, and the ratio of reaction size ≥20mm in contacts ≥15 years old (13.55%,897/6618) was the lowest. The number of tubercle bacillus in sources patients sputum was high the infection rates of contacts was high, and they were different of having different number tubercle bacillus in sources patients sputum（χ²=38.42，P<0.01；χ²=235.11，P<0.01）. The infection rates in contacts who were spouse, parent, schoolmate, colleague of the sources patients were higher, and in contacts who were grandparent or grandchildren, brother or sister of the sources patients were lower. The infection rates and the ratio of reaction diameter ≥20mm in person<15 years and ≥20mm in person ≥15 years of  PPD test in different relationship contacts were different separately（χ²=127.14，P<0.05；χ²=18.97，P<0.05）.Annual risk of contacts of sputum smear positive pulmonary TB was 3.53%. Conclusion  Infection risk of tubercle bacillus in closely contacts of sputum smear positive pulmonary TB is higher than general population. It’s related with the number of tubercle bacillus in known patient’s sputum, the relationships between them and the age of contacts.

Key words: Tuberculosis,pulmonary/transmission, Age factors