A comparison between QuantiFERON and tuberculin skin test during screening for tuberculosis infection in a contact investigation among students

Abstract number: O310

Jepsen M., Krause T., Moryl D., Seersholm N., Ruhwald M., Søborg B., Brock I., Ravn P.

Introduction: Two cases of tuberculosis (TB) were discovered at a high school in Copenhagen. A contact investigation was performed with Mantoux skin test (TST) and QuantiFERON^® TB In Tube test (QFT) and all staff and students were asked to participate in the study.

Objective: To compare QFT with TST in a contact investigation.

Methods: Students and staff were screened with TST, QFT and risk factors were registered in a questionnaire.

Results: A total of 724 had a TST applied of which 689 (94%) were read. Twenty-seven (4%) were TST positive. A total of 490 (62%) had a QFT done and 11 (2.2%) were positive and 4 (0.8%) inconclusive. We found that 17/603 (2.8%) of the students and 24/87 (28%) of the staff were TST positive whereas, 8/419 (1.9%) of the students and 3/71 (4.2%) of the staff were QFT positive. Agreement between the TST and the QFT was moderate 97% (Kappa 0.407, CI (0.124–0.689)) among students and poor among staff, 68 (Kappa 0.036, CI (-0.102–0.175)). The majority of positive QFT and TST persons were found in the index class and the neighbouring classes. Logistic regression analysis showed a significant association between positive TST results and BCG vaccination and we concluded that prior BCG vaccination could explain the high prevalence of TST positive staff. Only 5/10 (50%) of the QFT positive individuals were also TST positive indicating a suboptimal sensitivity of TST among persons at risk.

Conclusion: The two tests found an equally amount of M tuberculosis infected amongst the young non BCG vaccinated group taking into account that not everyone volunteered for both tests. 50% (4/8) of the QFT positives students were not detected with the TST. Among the staff, the high frequency of TST positives was explained by former BCG vaccination.

The choice of test preferred for a TB contact investigation should be based on the profile of the screening population. BCG status, age, country of origin, the use of both tests at the same time or in two steps should be evaluated according to the risk in the population of developing TB.