Tuberculin skin test in bacille Calmette-Guérin-vaccinated children: how should we interpret the results?

Background: The tuberculin skin test (TST) is the most useful method for the diagnosis of tuberculosis (TB). There is no evidence about the effect of bacillus Calmette-Guerin (BCG) vaccine on the interpretation of TST results.

Objective: The aim of this study was to evaluate TST results in a population of immigrants and adopted children, analyzing the effect of the vaccine on TST.

Methods: Cross-sectional observational study including immigrants or adopted children evaluated in our unit between January 2003 and December 2008 was made. Children diagnosed with TB, live attenuated virus vaccinated 2 months earlier, HIV-infected, chronically ill, or under treatment with immunosuppressive agents were excluded. TST was considered the dependent variable. Independent variables were gender, age, geographical origin, BCG scar, nutritional status, immune status, and intestinal parasites infestation.

Results: One thousand seventy-four children were included, 69.6 % are female; their origin includes China (34.7 %), Latin America (20.8 %), India/Nepal (19.4 %), Eastern Europe (15.7 %), and Africa (9.3 %). BCG scar was present in 79 % of children. Mantoux = 0 mm in 84.4 %, <10 mm in 4.1 %, and ≥10 mm in 11.4 %. Only two variables, age and BCG scar, influenced TST result. Risk of a TST false-positive due to BCG disappears 3 years after vaccine administration.

Conclusions: A history of BCG vaccination at birth does not interfere with TST results in children >3 years old. Under 3 years of age, BCG does interfere with and may cause a false-positive TST result. In these cases, the use of interferon-gamma release assays (IGRAs) is recommended. If IGRAs are not available or when results are indeterminate, ignoring the antecedent of the vaccine is recommended.