Interferon-gamma release assays (IGRAs) have been recently developed in order to overcome some limitations of the tuberculin skin test (TST).1 However, the superiority of IGRAs on TST in children is still controversial.1–3 The reported IGRAs sensitivity in paediatric studies ranges between 62% and 89% for T-SPOT.TB and between 66% and 83% for QuantiFERON-TB Gold In-Tube (QFT-G-IT), possibly due to different characteristics of the study populations (ie, proportion of immunocompromised/HIV infected and/or young children included).4 Few previous studies performed a simultaneous 3 way comparison with TST and both the commercially available IGRAs, using all the tests in the same child, allowing to reduce potential bias due to individual differences.5–8 However, the observed results still largely vary: QFT-G-IT sensitivity ranged from 47% to 93% and specificity from 90% to 100% while T-SPOT.TB sensitivity ranged from 39% to 93% sensitivity and specificity from 78% to 98%.5–8 The aim of the present study was to evaluate the performances of QFT-G-IT, T-SPOT.TB and TST executed simultaneously in a large population of children referring to our paediatric infectious unit for suspected tuberculosis (TB) infection.
Definitions of Latent Tuberculosis, Active Tuberculosis Cases and Uninfected Case
Latent tuberculosis infection (LTBI) diagnosis was assigned to any child with a positive TST and no clinical nor radiographic evidence of active TB, irrespectively from QFT-G-IT and T-SPOT.TB results. Cases of active TB were defined according to 2 categories: (1) definite active TB (ie, microbiologically confirmed TB); (2) probable active TB: absence of microbiologic confirmation but presence of all of the following criteria: clinical symptoms and signs of active TB, abnormal radiography and/or computed tomography scan consistent with lung TB, response to TB therapy, and either a history of TB contact or travel to a TB-endemic country within the last 24 months. Uninfected case was defined as an asymptomatic child with a negative TST or a child with symptoms potentially suggestive of TB but in whom results of all investigations for TB were negative or a child with an alternative diagnosis and complete recovery in the absence of specific TB treatment.
Consecutive children (<18 years) at risk for TB infection, referred to our Infectious Diseases Unit, between January 1, 2010, and June 1, 2013, were enrolled. Children with congenital or acquired immunodeficiency disorders were excluded from the study. The study was approved by local Ethics Committee and all parents/tutors of the enrolled patients gave written informed consent. Information regarding sociodemographic data, prior TB exposure and medical history were obtained from each child’s parents or tutors and/or from medical documentation and they were recorded into the study database. At first examination, all children underwent clinical evaluation, TST and venipuncture for IGRAs (QFT-G-IT and T.SPOT-TB). Blood was taken during the first examination and before starting any antitubercular treatment. Chest radiography was performed in all symptomatic children, in those with a positive TST and/or a positive IGRA and in all contacts aged less than 5 years. Children with suspected pulmonary TB had 3 sputum or early morning gastric aspirates samples collected for Mycobacterium tuberculosis detection (by means of microscopy, polymerase chain reaction and culture). Suspected or ascertained extrapulmonary TB cases were included in the study. The management of the study children was performed according to the American Academy of Pediatrics recommendations.9 TST, QuantiFERON-TB Gold In-Tube and T-SPOT.TB assays were performed as previously described.3 Statistical analyses were performed using SPSS 11.5 (SPSS, Chicago, IL). Significance was defined by P < 0.05.
Overall 338 children were included (males 170; 50.3%; median age 66 months; IQR: 32–115). Reasons for screening were international adoption in 128 (37.9%) children, TB contact in 145 (42.9%), and symptoms/other reasons in 65 (19.2%) children. 238/338 (70.4%) children were immigrants from high endemic countries. Bacille Calmette-Guérin (BCG) vaccination status was known in 252 (74.5%) children; 112/252 were BCG vaccinated (44.4%). Two hundred and ten (62.1%) children were uninfected, 58 (17.1%) were LTBI cases, 42 (12.4%) probable TB cases and 28 (8.3%) definite active TB cases, including 9 extra-pulmonary TB cases. QFT-G-IT results were available for 332/338 (98.2%) study children. No indeterminate result was observed. T-SPOT.TB results were available for 332/338 (98.2%) study children; 9 of whom (2.7%) were indeterminate (median age 40 months; IQR: 24–90), 4 of these being observed in children with latent TB. 19/332 (5.7%) T-SPOT.TB results were borderline (median age 61 months; IQR 43–114), in most of the cases (11/19) being observed in uninfected children.
When results for definite and probable active TB were combined (probable + definite: 70 cases), TST showed a sensitivity of 94.3% [95% confidence interval (CI): 89-99.7%: 66/70], QFT-G-IT of 84.3% (95% CI: 75.9-92.7%; 59/70) and T-SPOT.TB of 74.3% (95% CI: 64.0–84.5;52/70) including borderline results (considered as positive results), and 71.8% (95% CI: 60.8–82.78; 46/64) excluding borderline results. Considering only definite TB (28 cases), sensitivity was 96.4% (95% CI: 92.9–100; 27/28); 89.3% (95% CI: 83.3–95.3; 25/28); 75.0% (95% CI: 66.8–82.3; 21/28) for TST, QFT-G-IT and T-SPOT.TB respectively (Table 1). In children below 5 years with active TB TST sensitivity was 90.0% (95% CI: 79.3–100; 27/30); QFT-G-IT sensitivity was 73.3% (95% CI: 57.5–89.1; 22/30); and T-SPOT.TB sensitivity was 63.3% (95% CI: 46.1–80.6; 19/30), including borderline results. Excluding borderline results T-SPOT.TB sensitivity was 59.3% (95% CI: 40.1–77.8; 16/27). In the 6 children under 1 year of age with active TB disease TST was positive in all cases; while QFT-G-IT and T-SPOT.TB were positive in 5/6. In children over 5 years or age TST sensitivity was 97.5% (95% CI: 92.6-100 39/40); QFT-G-IT sensitivity was 92.5% (95% CI: 84.4–100; 37/40); T-SPOT.TB sensitivity was 82.5% (95% CI: 74.5-90.25 33/40) including borderline results and 73.0% (95% CI: 58.6–87.3; 27/37) excluding borderline results. In the 9 cases of extra-pulmonary disease, TST was positive in all cases; QFT-G-IT in 6/9 (66.7%) and T-SPOT.TB in 4/9 (44.4%).
QFT-G-IT specificity was 95.6% (95% CI: 93–98; 195/204) and T-SPOT.TB specificity was 93.5% (95% CI: 90.1–96.9; 187/200), including borderline results, and 98.9% (95% CI: 97.5–100; 187/189) excluding borderline results. In children below 5 years QFT-G-IT and T-SPOT.TB specificities were, respectively, 96.6% (95% CI: 93.3–99.9; 113/117), and 94.6% (95% CI: 90.4–98.8; 105/111) including borderline results and 99.1% (95% CI: 97.2–100; 105/106), excluding borderline results (Table 1). In children over 5 years or age QFT-G-IT specificity was 93.1% (95% CI: 88.1–98.1; 82/88); T-SPOT.TB specificity was 92.2% (95% CI: 86.7–97.7; 83/90) including borderline results and 98.8% (95% CI: 96.8–100; 83/84), excluding borderline results. In the 18 healthy controls <1 years of age QFT-G-IT was negative in all the children and T-SPOT.TB was indeterminate in 1 child and negative in the remaining ones.
Concordance between QFT-G-IT and T-SPOT.TB was good (k = 0.747); between TST and QFT-G-IT was good (k = 0.663) but between TST and T-SPOT.TB was moderate (k = 0.593) (Table 2).
In the present study a simultaneous 3-way comparison between TST and both IGRAs performance was evaluated in a large pediatric cohort. In children older than 5 years we found a similar sensitivity of TST and QFT-G-IT [97% (95% CI: 92–100) vs. 92% (95% CI: 84–100)] but T-SPOT sensitivity was significantly lower [82% (95% CI: 74–90)] than TST sensitivity. In young children (<5 years of age) TST sensitivity was 90% [95% CI: 79–100], QFT-G-IT sensitivity was lower but not significantly [73% (95% CI: 57–89)] while T-SPOT sensitivity was only 59% [95% CI: 40–77] excluding borderline results, as recommended by the Food and Drug Administration, or 63% [95% CI: 46–80], considering borderline results as positive ones. These differences were also demonstrated in the subgroup of children with definitive active TB. Both IGRAs specificity was high (92–98% in children below 5 years of age and 94–99% in older children), with no differences among tests. TST specificity was not calculated since TST results were used to define children with LTBI. The results of concordance tests suggest that IGRAs may be less influenced by BCG vaccine that TST, as previously reported10 (see Table, Supplemental Digital Content 1, http://links.lww.com/INF/B936).
Previous studies reported similar IGRAs and TST sensitivities in children, but other authors found contrasting results.5–8 In our study, in the 9 cases of extra-pulmonary disease, a high sensitivity of TST (100%) but lower for QFT-G-IT (66.7%) and T-SPOT.TB (44.4%) was observed. Kampmann et al.6 instead demonstrated that QFT-G-IT was unaffected by the type of TB disease (80% in both pulmonary or extra-pulmonary TB), but sensitivities of TST ant T.SPOT were lower in extra-pulmonary manifestation (TST sensitivity 93% in pulmonary TB and 60% in extrapumonary TB, T.SPOT-TB 67% in pulmonary TB and 40% in extra-pulmonary TB). The low number of extra-pulmonary TB in our study should be considered when interpreting our results in this subgroup of children.
Our study has several limitations. We decided to define LTBI status according to TST results and thus we may not exclude that some uninfected children with positive TST due to BGC vaccine or infectious with not tuberculous mycobacteria could have been erroneously identified as LTBI. However this finding may have affected the estimated IGRAs specificity but not sensitivity.
In conclusion, according to our results, IGRAs sensitivity in children below 5 years of age is particularly low and inferior to TST sensitivity. Considering that this group of children is at high risk for severe disease, the replacement of TST with IGRAs in young children appears to be unsafe. Our data suggest a high IGRAs specificity in young children. Simultaneous use of TST and IGRA in BCG vaccinated young children may be useful to avoid unnecessary treatment for LTBI. Larger studies may further elucidate this issue.
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