IFN-γ release assay: a diagnostic assistance tool of tuberculin skin test in pediatric tuberculosis in China.

BACKGROUND

Prompt diagnosis of Mycobacterium tuberculosis (MTB) infection is an essential step in tuberculosis control and elimination. However, it is often difficult to accurately diagnose pediatric tuberculosis (TB). The tuberculin test (TST) may have a low specificity because of cross-reactivity with antigens present in Mycobacterium bovis bacillus Calmette-Guerin (BCG) and other mycobacteria, especially in China with a predominantly BCG-vaccinated population. Early-secreted antigenic target 6-kDa protein (ESAT-6) and culture filtrate protein 10 (CFP-10), stand out as suitable antigens that induce an interferon-gamma (IFN-γ) secreting, T-cell-mediated immune response to infection. While, considered the higher costs and complexity of the IFN-γ release assay (TSPOT), we aimed to evaluate the TSPOT and TST test in the clinical diagnosis of pediatric tuberculosis and to establish a diagnostic process suitable for China.

METHODS

The sensitivity and specificity of the assay were evaluated in total seventy four children with active tuberculosis and fifty one nontuberculous children with other disease, and then the results were compared with TST. Logistic regression models were used to identify variables that were associated with positive results for each assay. The independent variables included sex, age, birth place, vaccination history, close contract with an active TB patient.

RESULTS

The sensitivity of TSPOT was higher than TST in active TB children with or without BCG vaccination, as well as in children with culture-confirmed TB. But the difference was not significant statistically. Combining results of the TSPOT and TST improved the sensitivity to 94.6%. Agreement of the TST and TSPOT was low (77.0%, κ = 0.203) in active TB patients. The difference in specificity between TSPOT and TST test was statistically significant (94.1% vs. 70.6%, P = 0.006). Specificity of the two tests in patients without prior BCG vaccination history was similar (80.0% vs. 60.0%). The concordance between the two tests results in BCG vaccinated subjects was low (71.7%, κ = 0.063). For TSPOT, none of the included risk factors was significantly associated with positive results. For TST, BCG vaccination (OR: 1.78; 95%CI: 1.30 - 2.00) was significantly associated with positive results.

CONCLUSIONS

Although IFN-γ release assay had relatively high sensitivity and specificity, we also should consider the higher costs and complexity of this test. Therefore, TSPOT could be used as the complementary tool of TST in circumstances when a suspected patient with negative TST results, or to exclude a positive TST result caused by BCG vaccination.