Risk factors for false-negative T-SPOT.TB assay results in patients with pulmonary and extra-pulmonary TB
Introduction
Tuberculosis (TB) is a major global infectious disease causing high mortality and morbidity, with 1.3 million deaths and 8.6 million new cases estimated by WHO each year worldwide.1 It shows a marked regional diversity and has highest prevalence in the South East Asian region (India), followed by the Western Pacific region (China and philippines).1 Early diagnosis and treatment are critical strategies for TB control in these high-burden developing countries. However, the definitive diagnostic methods, including the culture of Mycobacterium tuberculosis (M.TB) bacteria from sputum samples and smear of acid-fast bacilli, demonstrate some limitations in identifying all TB cases.2 Other adjunctive diagnostic methods, such as tuberculin skin tests, radiography, and nucleic acid amplification tests, exhibit poorer performances in diagnosis of active TB.3, 4 Moreover, the non-specific clinical presentations of extra-pulmonary TB (EPTB), and its paucibacillary load in poorly accessible sites of affected organs results in a lower chance of establishing a definitive diagnosis.5
Recently, interferon-gamma release assays (IGRAs) that detect IFN-γ responses to M. tuberculosis-specific antigens, including an enzyme-linked immunospot assay (T-SPOT.TB, Oxford Immunotec, UK) and the whole blood enzyme-linked immunosorbent assay (QuantiFERON-TB Gold and QuantiFERON-TB Gold In-Tube, QFT-TB, Cellestis Ltd, Australia) have been shown to be valid methods for detecting M.TB infection, due to their lack of cross-reactivity with bacillus Calmette–Guerin (BCG) and most non-tuberculous mycobacteria (NTM).6 However, the performances of these two commercial assays in the diagnosis of active TB have varied among populations and methods. Meta-analyses have shown that the overall sensitivity of T-SPOT.TB and QFT-TB for diagnosing active TB have ranged from 60% to 89%.7, 8, 9, 10, 11, 12 Meta-analysis focusing on pulmonary TB (PTB) diagnosis showed a sensitivity of 88% and 84% for T-SPOT.TB and QFT-TB,13 while 90% and 72% for EPTB in another meta-analysis.14
Some studies have recommended the use of IGRAs as an adjunct tool for diagnosis of active TB.7, 15, 16, 17, 18, 19, 20 However, its imperfect sensitivity may be a potential problem in interpreting negative IGRA results in suspected TB patients. Investigation of the factors that lower the sensitivity of IGRAs would facilitate better interpretation of IGRA results. Ethnicity has been reported as a risk factor for decreased sensitivity of the IGRAs test, the sensitivity of IGRAs in Malay and India patients were diminished compared to Chinese patients.21 It has previously been shown that HIV co-infection, immune suppression, increased age, body mass index (BMI), advanced disease, lymphocytopenia, and HLA-genotype are related to false-negative results in the QFT-TB assay, in different clinical settings.22, 23, 24, 25, 26 T-SPOT.TB assay is based on the ELISpot method, while QFT-TB assay is based on the ELISA method. This methodological difference may be the reason for the discordant sensitivities of these two assays, and may also suggest that factors which decrease sensitivity in these two assays are different. Up to date, only one study has found that increased age and steroid use were associated with false-negative T-SPOT.TB results in active TB,27 while another study found that TB meningitis is associated with false-negative T-SPOT.TB results in EPTB.28 Considering the limited information available on the risk factors associated with false-negative T-SPOT.TB results, we performed a prospective study to comprehensively explore the potential factors, including clinical characteristics and laboratory findings, which may influence T-SPOT.TB results in both microbiologically/histopathologically-confirmed pulmonary and extra-pulmonary TB.
Section snippets
Study population
This prospective study was conducted at the Beijing Chest Hospital (Beijing, China), a specialist tuberculosis and thoracic tumor disease clinic with a capacity of 533 beds. Between November 2012 and November 2013, a total of 1928 suspected TB patients who underwent valid T-SPOT.TB assays before the onset of anti-TB therapy were enrolled in the study. Patients gave written informed consent and were included if they had any clinical symptoms, signs or radiographic evidence of active TB. Patients
Clinical characteristics of participants
Among the 1928 patients suspected of TB prior to anti-TB therapy who underwent valid T-SPOT.TB tests, 774 (40.1%) patients were diagnosed as having microbiologically/histopathologically-confirmed TB, 209 (10.8%) patients were diagnosed as having no active TB, and 945 (49.1%) patients as having clinical TB. Only the 774 (40.1%) patients with microbiologically/histopathologically-confirmed TB were included for further analysis (Fig. 1). Among these patients, 554 patients were microbiologically
Discussion
Imperfect sensitivity gives rise to potential problems when using negative IGRA results as a ruling-out criteria in the diagnosis of active TB. Investigation of risk factors that decrease the sensitivity of IGRAs facilitates better interpretation of IGRA results. Previous studies investigating risk factors for false-negative IGRA results mainly focused on the QFT-TB test,22, 23, 24, 25, 26 with only two studies investigating the T-SPOT.TB test.27, 28 Information available on factors leading to
Conflict of interest statement
The authors declare that they have no conflicts of interest.
Acknowledgments
We thank all the subjects who participated in this study. This work was supported by grants from the National Science and Technology Major Project of China (No. 2013ZX10003003-004 and 2015ZX10004801-003), the Collaborative Innovation Center of Infectious Diseases (No. PXM2014_014226_000011), and the Beijing Outstanding Talent Training Project (No. 2013D003034000038).
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