| Objectives:The newly developed Interferon-gamma Release Assay (IGRA) is a new Enzyme-linked Immunospot(ELISpot) technology based on two tuberculosis-specific antigens, providing a fresh direction for the diagnosis of tuberculosis. Owning fantastic sensitivity and specificity, this technology has been employed as a main method for the diagnosis of tuberculosis infection. However, regarding the diagnosis of active tuberculosis, it is quite disputed in different researches in different countries. Furthermore, less attention has been paid to the diagnosis of extra-pulmonary active tuberculosis, particularly in China. In this study, we assessed the diagnostic value of one of the IGRA assays—T SPOT. TB in those clinically suspected active tuberculosis cases prospectively.Methods:In 182 recruited HIV-negative patients who are clinically suspected of active tuberculosis, we employed IGRA(T SPOT. TB), Tuberculin Skin Test with 1 or 5 unit Purified Protein Derivatives(PPD), medical history collection. After following up at least 3 months, getting the bacteriological, pathological or other diagnostic evidences to reach the final diagnosis for each case, we analyzed the following data such as the diagnostic value of IGRA, and negative cases, the comparison of TST with IGRA. Furthermore, in those cases which were initially diagnosed as Fever of Unknown Origin(FUO), we evaluated the diagnostic value of IGRA in identifying the clinically obscure active tuberculosis.Results:IGRA(T SPOT. TB)has a high sensitivity of 94.7%(95%CI 86.9%-98.5%) in active tuberlosis, and a high Negative Predictive Value of 95.7%(95%CI 89.5%-98.8%) while its specificity is 84.1%(95%CI 78.5%-90.5%) and Positive Predictive Value is 80.7%(95%CI 70.9%-88.3%) IGRA (T SPOT. TB) has a high sensitivity in both pulmonary and extra-pulmonary tuberculosis,95.6%(95%CI 84.9%-99.5%) and 93.3%(95%CI 77.9%-99.2%), respectively. IGRA has a higher specificity in extra-pulmonary tuberculosis than in pulmonary tuberculosis, with 88.9%(95%CI 80.0%-94.8%) versus 69.2%(95%CI 48.2%-85.7%) (P<0.05). In the risk factors analysis for IGRA false positive cases, the OR value of multi-factor logistic regression analysis is 0.1965(95%CI 0.0494-0.7819) for age≥46years old, and 0.0994 (95%CI 0.0159-0.6194)for past tuberculosis history. The sensitivity, specificity and kappa value of TST with 5 unit PPD versus IGRA is 56.4%(95%CI39.6%-72.2%),61.5%(95%CI 31.6%-86.1%)and 0.2841, respectively. This study does not find the relationship between T SPOT. TB false negative cases and immunosuppressive factors, anti-tuberculosis treatment. The sensitivity, specificity and kappa value of TST with 5 unit PPD versus IGRA is 27.8%(95%CI 9.7%-53.3%), 82.8%(95%CI 70.6%-91.4%) and 0.1866, respectively. In the group of FUO, IGRA(T SPOT. TB) has a high sensitivity of 92.5%(95%CI 79.6%-98.4%), and a even higher Negative Predictive Value of 97.4%(95%CI 86.2%-99.9%).Conclusions:IGRA (T SPOT.TB) has high sensitivity and NPV, thus the negative result may be used to rule out active tuberculosis. It also has a relatively high specificity and PPV, thus the positive result may be used to help diagnose active tuberculosis in the cases that are highly suspected of tuberculosis. The specificity of IGRA has a higher specificity in extra-pulmonary tuberculosis than in pulmonary tuberculosis. Age≥46 years old and past tuberculosis history are independently correlated to IGRA false positive cases. Patients older than 46 years old or patients with past tuberculosis history may be easier to have a false positive result of T SPOT. TB. Our study does not identify any risk factor for T SPOT.TB false negative cases. T SPOT.TB has better specificity and sensitivity than TST with 5 unit PPD while better sensitivity than TST with 1 unit PPD. They both have very poor concordance with IGRA. In the FUO group, IGRA shows excellent ability to rule out active tuberculosis.
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