Tuberculosis infection in young children: a screening based on skin and blood testing

Objectives: progression from latent tuberculosis infection (LTBI) to tuberculosis (TB) is higher in children younger than 5 years because of anergy of their immune system. Accurate diagnosis of LTBI is a desiderable goal in achieving effective TB control particularly in young children, because the preventive chemotherapy is effective and relatively safe. Aim of this study was to evaluate the "performance" of a commercially available Interferon-gamma (IFN-gamma) assay [QuantiFERON-TB Gold In-Tube (QFT-IT); Cellestis Ltd., Victoria, Australia] in this vulnerable population. Methods: in a retrospective study, QFT-IT test was performed in 145 young children ≤ 5 years of age, recent close contact of active pulmonary TB during the period April 2007-December 2010. Tuberculin skin test (TST; 5UI Biocine Test PPD; Chiron, Siena) was available only in 112 (77.2%) children. Results: in our pediatric population: mean age was 2.5 ± 1.5 years, 74 (51%) subjects were born in Italy from immigrant families, 9 (6.2%) were BCG-vaccinated. QFT-IT was positive in 24 (16.6%) children, negative in 112 (77.2%) and indeterminate in 9 (6.2%). TST was positive (cut-off 10 mm) in 28 (25%) of 112 evaluated patients. By statistical analysis, TST and QFT-IT showed a fair agreement: 82.9%, k=0.52. Among the discordant results (n=19): 12 (63.2%) children were TST-positive/QFT-IT-negative, of them 7 (58.3%) were BCG-vaccinated. Three (42.9%) of 7 patients TST-negative/QFT-IT-positive had TB and 4 (57.1%) had LTBI. Among 16 children TST-positive/QFT-IT-positive, 10 had clinical diagnosis of active TB and 6 of LTBI. In this group the analysis of IFN-gamma level as continuous variable revealed: 7/10 (70%) children with active pulmonary TB showed high level of IFN-gamma (10 UI/ml) and a TST size ranging from 10 mm to 15 mm; 3/6 (50%) children with a diagnosed LTBI had high IFN-gamma level 10 UI/ml and a TST size of 11-20 mm. Conclusion: these preliminary data suggest and support the potential advantages of using blood test, e.g. QFT-IT, in young children ≤ 5 years for diagnosing LTBI