Distinguishing tuberculosis from Mycobacterium avium complex disease using an interferon-gamma release assay

SETTING: The QuantiFERON (R)-TB Gold (QFT-G) test can be used to distinguish between tuberculosis (TB) and non-tuberculous mycobacterial disease, but a high background TB infection rate may pose a problem. Although the QuantiFERON (R)-TB (QFT) test, the first-generation QFT-G test, employs a non-specific PPD antigen, avium sensitin is also used as a stimulating antigen. OBJECTIVE: To evaluate the utility of these two interferon gamma release assays (IGRAs), QFT-G and QFT, and the tuberculin skin test (TST), to differentiate TB from Mycobacterium avium complex (MAC) disease in an intermediate TB burden country. METHODS: We compared the diagnostic performance of these three tests in 38 prospectively enrolled patients with TB and 40 with MAC lung disease. RESULTS: The TST yielded positive results in 70.6% of TB and 47.5% of MAC patients; the proportions were respectively 89.5% and 34.3% for QFT-G and 86.8% and 35.3% for QFT. The three tests were of similar accuracy, sensitivity and specificity in diagnosing TB. CONCLUSION: Our findings indicate that the TST and IGRAs could not discriminate between active TB and MAC disease or latent TB infection in a TB-endemic area.