The diagnosis of TB depends on the observation of clinical symptoms, smears and cultures of clinical samples, and radiographic examination [4]. However, the symptoms of TB have become increasingly insidious and the signs more atypical. Clinical diagnosis remains a major challenge due to the limitations of existing diagnostic methods and the high negative rate of cultures and smears. TB-IGRA is widely used for the diagnosing PTB. Compared to the Xpert MTB/RIF assay, TB-IGRA is highly specific for M. tuberculosis infection, reducing the risk of false positives that can occur with the GeneXpert assay because of non-tuberculous mycobacteria (NTM) or Bacillus Calmette-Guérin (BCG) vaccination, which is also particularly useful for diagnosing latent TB infection [7]. However, the body’s inflammatory status can influence TB IGRA results. mNGS is becoming a promising option for detecting M. tuberculosis, results within a short timeframe, similar to the GeneXpert assay, but with the added benefit of pathogen identification from a single test, allowing the detection of unexpected or novel pathogens [13]. This could also improve our understanding of tuberculosis transmission. However, M. tuberculosis migh not be detected by mNGS in some patients due to its thick capsule [24]. In this study, the TB-IGRA was combined with mNGS to determine whether this combination could reduce the rate of misdiagnoses and missed diagnoses.
We evaluated the clinical characteristics and demographics of 29 patients with PTB and 32 patients without TB. We found that the levels of total albumin, protein, and IFN-γ in the sera of PTB patients were significantly higher compared with those in non-TB patients. Our results are consistent with previous studies [25–27], indicating that IFN-γ release could be influence by the immune function of the body. We also found that the incidence of cough and thoracalgia was significantly lower in PTB patients than in non-TB patients, indicating that pulmonary symptoms were relatively mild in these patients. It is worth noting that TB patients experienced fewer coughs, and thoracalgia might have led to delayed diagnosis because the onset of TB might have gone unnoticed. Therefore, early diagnosis is crucial for the effective treatment and prevention of disease progression. The levels of total protein and albumin in the sera of PTB patients were significantly higher than those in non-TB patients (p < 0.05 ). This might be due to the small sample size; however, it did not affect the overall analysis results as the inclusion criteria for PTB were positive for TB-IGRA and/or mNGS, with positive mycobacterial tuberculosis cultures, while non-TB was defined as a pulmonary infection caused by pathogens other than tuberculosis. There were no significant differences in age, sex, WBCs count, lymphocyte count, PCT level, neutrophil count, or chest CT findings between PTB and non-TB patients. Additionally, IFN-γ release in PTB patients with positive sputum smears were higher compared to PTB patients with negative smears. However, Ji et al. found there was no difference in the IFN-γ levels of smear-negative and smear-positive PTB patients (89.6% vs. 90.8%, p > 0.05) [28]. The different results might be due to the different sources of specimens and number of cases.
Our results showed that the AUC of TB-IGRA, mNGS, and TB-IGRA combined with mNGS for the diagnosis was 0.939, 0.879, and 1, respectively. This indicates that the sensitivity of TB-IGRA combined with mNGS was significantly higher than that of TB-IGRA and mNGS alone (p < 0.05). Clinical studies have shown that T lymphocyte reduction can lead to a lower positivity rate of TB-IGRA in immunocompromised patients [29, 30]. However, Our study did not address this issue. Therefore, additional data are required for further analysis. We found that the sensitivity was 86.2% and the specificity was 96.9%, when the optimal cut-off value for IFN-γ (14.3 pg/mL, Youden’s index 0.831) was applied. This suggests that the cut-off value for TB-IGRA is consistent with the diagnostic criteria, aligning with of previous studies [25, 28], indicating that TB-IGRA is a reliable diagnostic tool for PTB infection. Additionally, mNGS has been gradually applied to the diagnosis of TB infection due to its rapid and highly sensitive characteristics [31]. However, the specificity of mNGS is not high compared to the final clinical diagnosis [32]. The principle of TB-IGRA is that IFN-γ is secreted by T lymphocytes in PTB patients under specific antigen stimulation, making its sensitivity higher than that of mNGS. The main advantage of mNGS is its ability to eliminate interference from non-TB patients. Our results revealed that the combination of TB-IGRA and mNGS was superior to mNGS alone for PTB diagnosis. Therefore, it is necessary to perform a TB-IGRA combined with mNGS for PTBinfections. These data may be helpful in evaluating and diagnosing PTB in elderly and immunocompromised patients in future studies, as well as in the patients at stages of infection that are currently difficult to diagnose. We hope that the combination of TB-IGRA and mNGS can be implemented in clinical settings.
A limitation of our study was the small sample size owing to the design of this study, which obtained examination results from medical records. Meanwhile, TB-IGRA and mNGS examinations were not routinely performed at the First Affiliated Hospital of Anhui Medical University. This resulted in a substantial exclusion of subjects. Further prospective studies with larger sample sizes are required to evaluate the combination of TB-IGRA and mNGS for rapid diagnosis of TB.