Introduction: Two cases of tuberculosis were diagnosed at a high school. A contact investigation using Mantoux test (TST) and QuantiFERON TB in Tube test (QFT) was conducted in order to compare QFT with TST.
Methods: A total of 787 individuals were invited to participate, of whom 734 had a TST applied and 490 a QFT test done.
Results: Of the skin test applied 689 (94%) were read, and 41 (5.9%) were positive. TSTs were positive in 17/603 (2.8%) students and 24/87 (28%) staff members. A total of 490 (62%) had a QFT performed; 11 (2.2%) were positive and four (0,8%) inconclusive. QFT positive results were found in 8/419 (1.9%) students and 3/71 (4.2%) staff members. Agreement between the TST and the QFT was moderate 97% (Kappa 0.407; confidence interval (CI) 0.124-0.689) among students and poor among staff 68% (Kappa 0.036; CI -0.102-0.175). The majority of the TST-positive and all the QFT positive students were found in relation to the index case. Positive TST results and BCG vaccination (p < 0,05) were associated. Only 5/10 (50%) of the QFT-positive individuals were TST positive, which indicates a suboptimal TST sensitivity.
Conclusion: The two tests identified an equal amount of Mycobacterium tuberculosis infected students, whereas a high proportion of the staff were TST positive. The discordant results underline that the two tests do not measure the same and that their sensitivity is not identical. Choice of test should rest on a case-by-case analysis and should be tailored to the desired level of safety and the cost for society and for the patients.