Diagnosis of Active Tuberculosis by Enzyme-Linked Immunospot Assay for Interferon-γ in HIV-Infected Patients

To the Editors: An enzyme-linked immunospot (ELISPOT) assay, like QuantiFERON-TB Gold In-Tube (QFT-GIT), was developed to detect interferon-γ produced by activated T cell after exposure to 2 specific antigens of Mycobacterium tuberculosis, that is, early secretory antigenic target 6 and culture filtrate protein 10 (1). Several studies have demonstrated that ELISPOT are also useful for diagnosis of active pulmonary and extrapulmonary tuberculosis (TB).1-4 The previous prospective analysis of the usefulness of a whole-blood interferon-γ release assay-QFT-GIT assay in the detection and prediction of active TB in HIV-1-infected individuals suggested that QFT-GIT assay may be a sensitive tool for the detection and prediction of active TB in HIV-1-infected patients in a low prevalence area for TB.5 This study indeed provides an important basis for more future studies to improve our diagnostic capability and efficiency in TB among immunocompromised hosts. We hereby postulate that the ELISPOT assay may be another sensitive tool for the diagnosis of active TB in patients with HIV infection. From March 2007 to February 2009, we carried out a study to evaluate the performance of ELISPOT assay in 39 HIV-infected patients who presented with symptoms suggestive of active TB at a referral hospital for HIV care in Taiwan, where the TB incidence was 63.2 cases per 100,000 population with 14,480 newly diagnosed TB cases in year 2007.6 The assay was performed by using the ELISPOT kit (T SPOT-TB, Oxford Immunotec Ltd, Oxford, UK) according to the manufacturer's instruction as previously described,2 with the result readily available within 24 hours. Nine patients (30%) have confirmed TB by cultures, all were males with a mean age of 39.6 years (range 30-52; Table 1). Eight patients had pulmonary TB and 1 patient had peripheral TB lymphadenitis. There were no significant differences in CD4+ T-cell count and the proportion of patients with CD4 counts of <200 cells/μL between the patients with and those without active TB. Eight of the 9 patients with confirmed TB were tested positive by ELLISPOT assay. The CD4+ T-cell count for the patient with false-negative result was 53 cells/μL. There were 2 false-positive results, one in a patient with disseminated Mycobacterium avium-intracellulare complex infection and the other in a patient with septic arthritis. The CD4+ T-cell count for the 2 patients with false positive ELISPOT was 9 and 182 cells/μL, respectively. No TB developed in these 2 patients after follow-up for 221 and 245 days, respectively. Those that have no confirmed TB have a median follow-up time of 157 days (range 18-621).TABLE 1: The Clinical Characteristics of 39 HIV-Infected Patients With Suspected Active TBThe overall sensitivity, specificity, positive predictive value, and negative predictive value for ELISPOT to diagnose confirmed TB in our HIV-1-infected individuals with moderate to severe immunosuppression, was 88.9% (8/9), 93.3% (28/30), 80.0% (8/10), and 96.6% (28/29), respectively, whereas acid-fast smear had a sensitivity of 77.8% and a specificity of 90%. Both the sensitivity and specificity of ELISPOT were higher than acid-fast smear. The influence of CD4+ T-cell counts on the performance of interferron-γ release assays is of special interest for HIV-infected patients. In our study, we found that the ELISPOT assay seem not affected by the CD4+ T-cell counts, which was consistent with previous study.7 In the other hand, the performance of QFT-IT test is impaired by low CD4-cell count in HIV-positive patients and possibly by other factors in both HIV-positive and HIV-negative patients.8 Higher rates of undetermined results were noted among patients with advanced immunosuppression.5 Therefore, the ELISPOT assay might be a better diagnostic tool for the detection of active TB in patients with reduced CD4+ T-cell counts. In conclusion, besides QFT-GIT assay, the ELISPOT assay may be another fast and promising diagnostic tool for active TB in HIV-infected patients. ACKNOWLEDGMENTS Supported partly by the Institute for Biotechnology and Medicine Industry, Taiwan (DOH97-DC-1501). Che-Kim Tan, MD* Chien-Ching Hung, MD† Chih-Cheng Lai, MD‡ Chun-Hsing Liao, MD§ Chien-Hong Chou, MD† Yu-Tsung Huang, MD† Po-Ren Hsueh, MD† *Department of Intensive Care Medicine, Chi-Mei Medical Center, Taiwan †Department of Internal Medicine, National Taiwan University Hospital, National Taiwan University College of Medicine, Taipei, Taiwan ‡Department of Internal Medicine, Cardinal Tien Hospital, Taipei, Taiwan §Department of Internal Medicine, Far Eastern Memorial Hospital, Taipei County, Taiwan