Immunologic Diagnosis of Active Tuberculosis

Abstract

Clinically evident active pulmonary or extrapulmonary tuberculosis (TB) does not pose a significant diagnostic challenge, but the diagnosis of subclinical or atypical forms of TB is much more troublesome. More accurate tests that can specifi cally diagnose active TB have been much awaited. Interferon-γ (IFN-γ) release assays (IGRAs) are recently developed tests recommended for the diagnosis of latent TB, together with the tuberculin skin test (TST), which had for a long time been the only diagnostic tool for this condition [1]. Although the original intent of IGRAs was for the diagnosis of latent TB, the tests have recently become attractive as a possible option in the diagnosis of active TB, as well. IGRAs measure the in vitro cellular immune responses to Mycobacterium tuberculosis-specifi c antigens, including early secreted antigenic target 6 (ESAT-6) and culture fi ltrate protein 10 (CFP-10), which are not present in any strain of Mycobacterium bovis BCG as well as in many nontuberculous mycobacteria. In contrast, the TST uses a nonspecifi c whole culture fi ltrate of tubercle bacilli containing over 200 antigens. Th is leads to its low specifi city. Two types of IGRAs are now commercially available. Th e fi rst is QuantiFERON-TB Gold (QFT-G; Cellestis), an assay that uses the patient’s whole blood to measure the level of IFN-γ from the stimulated supernatant. A variant method, QuantiFERON-TB Gold In-Tube (QFT-GIT), uses tubes prefi lled with antigens. The second type of IGRA, T-SPOT.TB (Oxford Immunotec), uses the enzyme-linked immunospot (ELISPOT) assay to measure the number of IFN-γ-secreting T cells on stimulation by M. tuberculosis-specifi c antigens. A fi xed number of peripheral mononuclear cells is used in this assay. Both types of IGRAs have internal positive and negative controls to guard against technical errors. Th e failure of the controls in the tests, defined as indeterminate test results (ITRs), means that the reliability of results cannot be guaranteed. Th e incubation step in IGRAs selectively amplifi es replication of eff ector memory T cells, since central memory T cells require a longer period of in vitro incubation than eff ector memory T cells. Central memory T cells are major components within the indurated skin produced by the TST. Th erefore, IGRAs are more likely to be positive in persons recently infected with M. tuberculosis [2]. Assessing the accuracy of IGRAs in diagnosing M. tuberculosis infection is difficult due to the absence of a gold standard to confirm a diagnosis of latent TB infection or culture-negative active tuberculosis. Th us, the performance of the tests has been measured based on the epidemiology of TB or by using cases of active TB. Studies regarding household contacts with active TB showed that the pooled sensitivity of IGRAs for the prediction of active TB after exposure was 80 to 90%, the specifi city 56 to 83%, the positive predictive value (PPV) 4 to 8%, and the negative predictive value (NPV) 99 to 100%. For the TST, the sensitivity was 90 to 100%, the specifi city 29 to 39%, the PPV 2.7 to 3.1%,