Diagnosis of Latent Tuberculosis Infection: Comparison between the Tuberculin Test and Quantiferon-TB Gold Plus in Household Contacts from the City of Buenos Aires / Diagnóstico de infección tuberculosa latente: comparación de la prueba de tuberculina y el quantiferon-tb gold plus en contactos domiciliarios de la Ciudad de Buenos Aires

ABSTRACT

The epidemiological control strategy of tuberculosis (TB) includes both the diagnosis and early treatment of baciliferous patients and the identification of individuals with latent tuberculosis infection (LTBI), who represent the pathogen reservoir within the population. Objective: To compare the results obtained using the tuberculin test (PPD, Purified Protein Derivative) and the QuantiFERON-TB Gold Plus (QTF) in a group of household contacts of patients with bacillary pulmonary TB in the city of Buenos Aires. We used two cutoff points to consider PPD as positive: ≥ 5 mm (PPD-5) and ≥ 10 mm (PPD-10). Materials and Methods: Blood samples for QTF were taken from household contacts of patients with bacillary TB, followed immediately by the application of PPD. The QTF was considered as the reference test from which to compare the PPD by calculating sensitivity (S), specificity (E), positive predictive value (PPV), negative predictive value (NPV) and Kappa correlation coefficient. Results: 48 contacts were included (33 women, 69%), age 38.8 ± 19 years, 27 (56%) Argentinean, 18 (38%) Bolivians and 3 (6%) Peruvians, 37 were cases of TB. Only one contact informed She/she hadn't received the BCG; and 44 subjects showed the BCG scar. The QTF was positive in 23 individuals (47.9%) and undetermined in 2 cases (4.2%). Excluding the indeterminate cases from the analysis, there were no significant differences between positive and negative QTF contacts regarding age (33.8 ± 16 vs. 42.1 ± 20 years), nationality: Argentinean (12 of 26.46%) vs. foreigners (11 of 20.55%) and sex: women (18 of 32, 56%) vs. men (5 of 14, 36%). There were 28 (60.9%) positive cases with PPD-5 and 13 (28.3%) with PPD-10. Comparison between PPD-5 vs. PPD-10: S = 73.9 vs. 34.8%, E = 52.2 vs. 78.3%, PPV = 60.1 vs. 61.5% and NPV = 66.7 vs. 54.5%. The coincident results (positive and negative) between QTF and PPD were 29 (63%) for PPD-5 and 26 (56.5%) for PPD-10. Expressed with the Kappa coefficient, they show weak (0.261) and insignificant (0.130) concordance, respectively. Conclusions: Considering the QTF as the reference method for its higher specificity, PPD-5, although less specific, was more sensitive than PPD-10 for the diagnosis of latent tuberculosis infection in the group of household contacts under study.