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Usefulness of Whole-Blood Interferon- Assay and Interferon- Enzyme-Linked Immunospot Assay in the Diagnosis of Active Pulmonary Tuberculosis^*

^* From the Division of Pulmonary and Critical Care Medicine (Drs. Kang, Um, Han, Shim, and Yim, and Ms. Lee), Department of Internal Medicine and Lung Institute, Seoul National University College of Medicine, Seoul; and the National Cancer Control Research Institute (Dr. Hwang), National Cancer Center, Goyang, South Korea.

Correspondence to: Jae-Joon Yim, MD, Department of Internal Medicine, Seoul National University College of Medicine, 28 Yongon-Dong, Chongno-Gu, Seoul, 110–744, South Korea; e-mail: yimjj{at}snu.ac.kr

Abstract

Purpose: The aim of this study was to evaluate the usefulness of the whole-blood interferon- assay (enzyme-linked immunosorbent assay [ELISA]) and interferon- enzyme-linked immunospot assay (ELISPOT) based on early secretory antigenic target 6 and culture filtrate protein 10 in the diagnosis of active pulmonary tuberculosis (TB) in routine clinical practice.

Method: We conducted a prospective study enrolling 144 participants with suspected pulmonary TB in a tertiary referral hospital in Seoul, South Korea, to investigate the diagnostic sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of these tests. Clinical assessment, tuberculin skin test (TST), whole-blood interferon- ELISA (QuantiFERON-TB Gold [QFT-G]; Cellestis Ltd; Victoria, Australia), and an ELISPOT assay (T SPOT.TB; Oxford Immunotec; Oxford, UK) were performed. Test results were compared with the final confirmed diagnoses.

Results: Active pulmonary TB was diagnosed in 67 of 144 participants (47%). Sensitivities of QFT-G and T SPOT.TB for active pulmonary TB were 89% (95% confidence interval [CI], 79 to 96%) and 92% (95% CI, 83 to 97%), respectively; and specificities were 49% (95% CI, 37 to 61%) and 47% (95% CI, 36 to 59%). NPVs of QFT-G (84%; 95% CI, 69 to 93%) and T SPOT.TB (87%; 95% CI, 73 to 96%) were higher than that of TST (64%; 95% CI, 51 to 76%) [p = 0.001 and p < 0.001, respectively].

Conclusion: High NPVs of QFT-G and T SPOT.TB for the diagnosis of active TB suggest the supplementary role of these tests for the diagnostic exclusion of active TB, although the low PPV limits their usefulness in routine clinical practice in South Korea, where the prevalence of latent TB infection is considerable.

Key Words: culture filtrate protein 10 • diagnosis • early secretory antigenic target 6 • interferon- • tuberculosis

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