Clinical Microbiology Newsletter 29:17,2007 © 2007 Elsevier 0196-4399/00 (see frontmatter) 129 Introduction and Background Establishing the diagnosis of TB, particularly latent TB infection (LTBI), can often be quite challenging for clin- icians and laboratory professionals. It is estimated that about one-third of the world’s population have LTBI, and 10% of those infected will develop active disease over their lifetimes (1-4). One of the first lines in establishing the diagnosis of latent tuberculosis has been the tuberculin skin test (TST), also known as the intradermal Mantoux test or the purified protein derivative (PPD). The TST, originally derived from Kochs’ tuberculin (old tuberculin), was intro- duced in 1890, has been in routine use since 1910, and is perhaps the oldest diagnostic medical test still routinely used (5-9). Despite its longevity, how- ever, the TST has several important disadvantages. First, a positive or inter- mediate reaction can be produced by non-tuberculous mycobacterial (NTM) infections or by vaccination with bacil- lus Calmette-Guérin (BCG). Secondly, sensitivity for the detection of LTBI is mediocre and is particularly problem- atic in immunocompromised individuals (6). Third, the TST typically requires two visits with a health professional (10), which is both inconvenient and costly. A number of alternative testing stra- tegies have been developed in order to address some of the TST’s disadvan- tages. Many of these are gamma inter- feron (IFN-γ) release assays (IGRA) (described below), and among the most promising and readily available is the QuantiFERON-TB GOLD test (QFT-G) (Cellestis Incorporated, Carnegie, Australia). Cleared in 2005 by the U.S. Food and Drug Administration (FDA), the QFT-G is a second-generation, whole-blood test for use in diagnosing LTBI and TB disease. It is commer- cially available in the U.S., and the CDC released guidelines for its use (6,11,12). Principles of the Assay The QuantiFERON (QFT) test uti- lizes an enzyme-linked immunosorbent assay (ELISA) to measure IFN-γ pro- duced by sensitized T cells stimulated by Mycobacterium tuberculosis anti- gens. (see Fig. 1 for a summary of the assay). The QFT-G test is based on a whole-blood ELISA developed in the late 1980s. Early assays used PPD as the stimulating antigen, and the first widely utilized QFT assay, also referred to as the “first generation,” was one such assay. Newer IGRA, such as the QFT-G, which is also referred to as the “second-generation” QFT assay or QFT-2G, use selected M. tuberculosis antigens or peptide-simulating antigens, such as early secreted antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP-10) in the case of the QFT-G (6,7,10,12). QuantiFERON-TB Gold Assay for Tuberculosis Infection Alton B. Farris, M.D. and John A. Branda, M.D., Department of Pathology, Massachusetts General Hospital, Boston, Massachusetts Abstract Latent Mycobacterium tuberculosis infections can be difficult to diagnose, and diagnosis has formerly relied primarily on the tuberculin skin test (TST), an in vivo assay with inherent limitations. Recently, in vitro gamma interferon (IFN- γ) release assays have been developed that measure IFN-γ after stimulation with tuberculosis (TB) antigens such as PPD, CFP-10, and ESAT-6. The QuantiFERON-TB GOLD test (QFT-G) was the first such assay to garner U.S. Food and Drug Administration clearance. Potential uses of the QFT-G include screening for latent TB, testing for active TB infection, and confirmation of a positive or intermediate PPD test. The advantages of QFT-G over the TST include potentially easier follow-up and more objec- tive test administration and measurement; however, multiple clinical studies have shown that the QFT-G has its own limitations, such as indeterminate results in immunocompromised patients (e.g., HIV and chemotherapy patients), sensitivity and specificity concerns, and the requisite infrastructure to collect and analyze specimens. Implementation of “in tube” QFT-G and T-SPOT TB assays may offer further promise over the QFT-G. Vol. 29, No. 17 www.cmnewsletter.com September 1, 2007 Mailing address: Alton B. Farris, M.D., Department of Pathology, Massachusetts General Hospital, 55 Fruit St., GRJ-245, Boston, MA 02114. Pager 617-724-5700, #20245. E-mail: afarris@partners.org Clinical Microbiology Newsletter