Abstract: Objective To explore the value of PPD positive rate in school tuberculosis outbreaks risk assessment by using retrospective analysis of close contacts of sporadic cases of tuberculosis registered in the Dalian city. Methods We collected and analyzed the sporadic student TB cases (228 cases in total) registered in the National TB management information system between December 2010 and December 2013, and followed up their 9863 close contacts for one-year. We observed the TB incidence and PPD strong positive (PPD average diameter ≥15 mm)distribution of these contacts, and used ROC curves to get cut-off value of PPD strong positive rate of TB outbreaks and occurrence of new cases among contacts. Results In 228 sporadic cases, 39 cases were smear-positive pulmonary tuberculosis, and the incidence of accidental occurrence aggregation and tuberculosis outbreaks were 7/39(17.9%) and 4/39(10.3%) respectively among close contacts; in 189 smear-negative TB cases, both incidence of accidental occurrence aggregation and tuberculosis outbreaks were 15/189(7.9%), with no statistical significance, all P＞0.05 (χ²=0.60, 0.23). PPD positive rate of close contacts of smear-positive TB cases is (16.0±0.8)%; comparing with close contacts of smear-negative TB cases of (15.3±0.9)%, with no statistical significance (χ²=2.41, P>0.05). PPD strong positive average rate of freshmen in Dalian was (6.5±0.5)%, the average PPD strong positive rates varied with TB epidemic levels, χ²=26.14, P＜0.01. Using the continuous cases occurred in one year as a criterion to draw PPD strong positive rate ROC curve, specificity is 85.0%,sensitivity is 70.0%,AUC=0.78, the predictive cut-off value is 25.5%. When use occurrence of TB outbreaks as criterion to draw ROC, AUC=0.81, both specificity and sensitivity were 0.78,cut-off value is 21.5%. Conclusion Both smear-positive and smear-negative patients have the same risk of causing tuberculosis outbreaks. Over 21.5% of PPD strong positive rate is the warning indicator of continuous cases occurrence, it also suggests that close contacts have the possibility of TB incident and intervention should be given in a timely manner.