Comparison of the Diagnostic Usefulness of Two Whole-Blood Interferon-Gamma Assays for Extrapulmonary Tuberculosis

Han, Song Yee; Lee, Hyuck; Jung, Dong Sik; Kim, Kyeong Hee; Woo, Su Mi; Park, So Young; Seo, Jeong Min; Jung, Jin Kyu; Yoon, Neul Bom; Lee, Sung Woo

Abstract

BACKGROUND/AIMS
The QuantiFERON-TB Gold (QFT-G) and QuantiFERON-TB Gold in tube (QFT-IT) assays have been studied primarily for the use of diagnosing active pulmonary tuberculosis (TB) or latent TB. The clinical usefulness of these assays for the detection of active extrapulmonary (EP) TB has not been fully defined. The aim of this study was to compare the diagnostic value of these two interferon-gamma assays for EP-TB.
METHODS
From June 2007 to August 2010, we evaluated the usefulness of QFT-G (n = 56) and QFT-IT (n = 48) in patients (n = 104) with suspected EP-TB. The diagnostic sensitivity, specificity, postive predictive value (PPV), and negative predictive value (NPV) of QFT-G and QFT-IT, and the cut-off value of QFT-IT were analyzed.
RESULTS
EP-TB was diagnosed in 55 (53%) patients. The overall sensitivity, specificity, PPV, and NPV of the QFT-IT assay were 96%, 42%, 62%, and 91%, respectively, and those of the QFT-G test were 81%, 52%, 68%, and 68%, respectively. In subgroup analyses according to infection site, the sensitivity and NPV of QFT-IT were higher than those of QFT-G. Analysis confirmed that the manufacturer's recommended test cut-off value fell within our cut-off value range (0.30-0.45 IU/mL; 95.8% sensitivity, 41.7% specificity).
CONCLUSIONS
The QFT-IT assay showed superior sensitivity and NPV, and equivalent specificity, as comparison with the QFT-G test for the detection of Mycobacterium tuberculosis infection. The logistic benefits of the QFT-IT test format should facilitate the diagnosis of EP-TB.