Interferon- Enzyme-Linked Immunospot Assay in Patients with Tuberculosis and Healthy Adults

Background: Interferon-${\gamma}$ assays based on tuberculosis (TB)-specific antigens have been utilized for diagnosing and ruling out latent TB and active TB, but their utility is still limited for TB incidence countries. The aim of this study is to understand the clinical utility of enzyme-linked immunospot (ELISpot) assays among patients with clinically suspected TB and healthy adults in clinical practices and community-based settings. Methods: The ELISpot assays (T SPOT.TB, Oxford Immunotec, UK) were prospectively performed in 202 patients. After excluding those with indeterminate results, 196 were included for analysis: 41 were TB patients, 93 were non-TB patients, and 62 were healthy adults. Results: The sensitivity and negative predictive values of the T SPOT.TB assays for the diagnosis of TB were 87.8% and 89.1%, respectively, among patients with suspected TB. The agreement between the tuberculin skin test (10-mm cutoff) and the T SPOT.TB assay was 66.1% (kappa=0.335) in all participants and 80.0% (kappa=0.412) in TB patients. Among those without TB (n=155), a past history of TB and fibrotic TB scar on chest X-rays were significant factors that yielded positive T SPOT.TB results. There was a significant difference in the magnitude of T SPOT.TB spot counts between TB patients and non-TB patients or healthy adults. Conclusion: The T SPOT.TB assay appeared to be a useful test for the diagnostic exclusion of TB. A positive result, however, should be cautiously interpreted for potential positives among those without active TB in intermediate TB incidence areas.