Objectives: To evaluate the performances of the QuantiFERON TB-Gold assay (QFT) and tuberculin skin test (TST) and to examine how a latent tuberculosis infection (LTBI) should be diagnosed in contact investigations of children aged 6 to 17 years.
Methods: A total of 232 boys and girls aged 6 to 17 years who were in contact with 134 culture-confirmed pulmonary tuberculosis patients (index cases) were examined both with QFT and TST. Factors influencing the results of the tests and their interactions were evaluated with multivariate analyses.
Results: Two variables (whether household contact and with/without contact with a cavitary disease patient) were found to significantly predict a positive QFT result. Positive TST defined with erythema being either greater than 20 mm or 30mm correlated significantly with two variables (whether household contact and with/without contact with a smear positive patient). There was moderate agreement between QFT and TST (positivity defined as with erythema greater than 30mm), with kappa=0.49, for contacts aged 6 to 11 years. Among contacts aged 6 to 11 years, 14 had a negative QFT result and TST with erythema greater than 30 mm. Of these 14, 7 contacts (50%) of smear positive index case were not indicated for LTBI treatment.
Conclusions: When diagnosing LTBI among contacts aged 6 to 11 years who show negative QFT and strong TST reactions, we should take into consideration the factors related with a higher probability to the risk of infection. Because a history of past BCG vaccination is more likely to affect TST results in those aged 12 to 17 years than in younger subjects, greater care must be taken when evaluating the TST of these contacts.