Evaluation of the filter paper IP-10 tests in school children after exposure to tuberculosis: a prospective cohort study with a 4-year follow-up

OBJECTIVES: The prevalence of active tuberculosis (TB) is low in Finland, but outbreaks do occur. Following exposure national guidelines recommend either tuberculosis skin test or interferon-γ-release assay-testing of asymptomatic children. The aim of this study was to compare QuantiFERON-TB Gold In-Tube test (QFT) and interferon-γ-inducible protein (IP-10) release assay for detection of Mycobacterium tuberculosis infection following exposure to TB in a primary school. DESIGN: A prospective cohort study. SETTING: School children in Helsinki, Finland. PARTICIPANTS: Two siblings of the index case and 58 classmates exposed to M tuberculosis. INTERVENTION: All the children were screened using the QFT, which was used to guide preventive treatment. All those exposed were followed up through the national TB registry. OUTCOME MEASURES: IP-10 was measured in plasma supernatants from the QFT test supernatants and in plasma dried and stored for 1 year on filter paper. IP-10 test results were calculated using preset algorithms for positive and indeterminate tests. The negative predictive values of the tests were assessed. RESULTS: At an initial screening 2 months after the debut of symptoms in the index case, QFT was positive in two children; 56 tests were negative; one was indeterminate and one was borderline. IP-10 showed a perfect concordance between the dried plasma spot and plasma method; two children were IP-10 positive and two were IP-10 indeterminate. There were two (3%) discordant results between the QFT and IP-10 tests. Four children converted to positive QFT at a 1–3 month follow-up. None of the QFT negative/borderline children developed TB in the 4-year period since exposure. CONCLUSIONS: We demonstrated that IP-10 and QFT perform comparably as screening tools for infection with M tuberculosis in a contact investigation. IP-10 determined in dried plasma spots was at par with IP-10 determined in plasma, which further supports the usefulness of this alternative approach.