An up-to-date systematic review and meta-analysis by Karen Steingart and colleagues confirms that commercially available serological tests do not provide an accurate diagnosis of tuberculosis.

BackgroundThe global tuberculosis epidemic results in nearly 2 million deaths and 9 million new cases of the disease a year. The vast majority of tuberculosis patients live in developing countries, where the diagnosis of tuberculosis relies on the identification of acid-fast bacilli on unprocessed sputum smears using conventional light microscopy. Microscopy has high specificity in tuberculosis-endemic countries, but modest sensitivity which varies among laboratories (range 20% to 80%). Moreover, the sensitivity is poor for paucibacillary disease (e.g., pediatric and HIV-associated tuberculosis). Thus, the development of rapid and accurate new diagnostic tools is imperative. Immune-based tests are potentially suitable for use in low-income countries as some test formats can be performed at the point of care without laboratory equipment. Currently, dozens of distinct commercial antibody detection tests are sold in developing countries. The question is “do they work?”Methods and FindingsWe conducted a systematic review to assess the accuracy of commercial antibody detection tests for the diagnosis of pulmonary tuberculosis. Studies from all countries using culture and/or microscopy smear for confirmation of pulmonary tuberculosis were eligible. Studies with fewer than 50 participants (25 patients and 25 control participants) were excluded. In a comprehensive search, we identified 68 studies. The results demonstrate that (1) overall, commercial tests vary widely in performance; (2) sensitivity is higher in smear-positive than smear-negative samples; (3) in studies of smear-positive patients, Anda-TB IgG by enzyme-linked immunosorbent assay shows limited sensitivity (range 63% to 85%) and inconsistent specificity (range 73% to 100%); (4) specificity is higher in healthy volunteers than in patients in whom tuberculosis disease is initially suspected and subsequently ruled out; and (5) there are insufficient data to determine the accuracy of most commercial tests in smear microscopy–negative patients, as well as their performance in children or persons with HIV infection.ConclusionsNone of the commercial tests evaluated perform well enough to replace sputum smear microscopy. Thus, these tests have little or no role in the diagnosis of pulmonary tuberculosis. Lack of methodological rigor in these studies was identified as a concern. It will be important to review the basic science literature evaluating serological tests for the diagnosis of pulmonary tuberculosis to determine whether useful antigens have been described but their potential has not been fully exploited. Activities leading to the discovery of new antigens with immunodiagnostic potential need to be intensified.

The failure to diagnose tuberculosis (TB) accurately and rapidly is a key challenge in curbing the epidemic (45,88,116). Sputum microscopy, currently the sole diagnostic test in most areas where TB is endemic, has several limitations; in particular, the sensitivity compared with that of culture is variable (80,97,104,116), multiple patient visits are required (56,93,114), considerable technical training is necessary, and the procedure is labor-intensive (45, 65). Antibody detection tests (serological tests) are used for the diagnosis of many infectious diseases and could potentially improve the means of diagnosis of TB. These tests measure the presence of specific antibodies (most often immunoglobulin G [IgG]) directed against immunodominant antigens of the pathogen in question. Compared with microscopy, antibody detection methods may enable the rapid diagnosis of TB, as these tests have the advantages of speed (results can be available within hours), technological simplicity, and minimal training requirements. In addition, these tests can be adapted to point-of-care formats that can be implemented at lower levels of health services in low-and middle-income countries (21,22,57,65).Efforts to develop antibody detection tests for the diagnosis of TB have been under way for decades, and the performance of these tests has been well described (13,17,22,32,40,47,48,52,60,64,100,107). Several systematic reviews of these tests have been published (discussed below) (28,94,95).First-generation antibody detection tests were based on crude mixtures of constituents and products of Mycobacterium tuberculosis, for example, culture filtrate proteins and purified protein derivative, the preparation used in the tuberculin skin test. Several of these early tests had low specificities, as the tests contained antigens shared among different bacterial species (1,22,48,57). During the past two decades, an increased understanding of humoral immune responses to M. tuberculosis and the new tools of

SummaryESAT6 has recently been demonstrated to cause haemolysis and macrophage lysis. Our studies demonstrate that ESAT6 causes cytolysis of type 1 and type 2 pneumocytes. Both types of pneumocytes express membrane laminin, and ESAT6 exhibits dosedependent binding to both cell types and to purified human laminin. While minimal ESAT6 was detected on the surface of Mycobacterium tuberculosis grown in vitro, exogenously provided ESAT6 specifically associated with the bacterial cell surface, and the bacterium-associated ESAT6 retained its cytolytic ability. esat6 transcripts were upregulated~4-to~13-fold in bacteria replicating in type 1 cells, and~3-to~5 fold in type 2 cells. In vivo, laminin is primarily concentrated at the basolateral surface of pneumocytes where they rest on the basement membrane, which is composed primarily of laminin and collagen. The upregulation of esat6 transcripts in bacteria replicating in pneumocytes, the specific association of ESAT6 with the bacterial surface, the binding of ESAT6 to laminin and the lysis of pneumocytes by free and bacterium-associated ESAT6 together suggest a scenario wherein Mycobacterium tuberculosis replicating in pneumocytes may utilize surface ESAT6 to anchor onto the basolateral lamininexpressing surface of the pneumocytes, and damage the cells and the basement membrane to directly disseminate through the alveolar wall.

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SummaryMycobacterium tuberculosis ( M. tb ) uses the glyoxalate bypass for intracellular survival in vivo . These studies provide evidence that the M. tb malate synthase (MS) has adapted to function as an adhesin which binds to laminin and fibronectin. This binding is achieved via the unique C-terminal region of the M. tb MS. The ability to function as an adhesin necessitates extracellular localization. We provide evidence that despite the absence of a Sec-translocation signal sequence the M. tb MS is secreted/excreted, and is anchored on the cell wall by an undefined mechanism. The MS of Mycobacterium smegmatis is cytoplasmic but the M. tb MS expressed in M. smegmatis localizes to the cell wall and enhances the adherence of the bacteria to lung epithelial A549 cells. Antibodies to the C-terminal laminin/fibronectin-binding domain interfere with the binding of the M. tb MS to laminin and fibronectin and reduce the adherence of M. tb to A549 cells. Coupled to the earlier evidence of in vivo expression of M. tb MS during active but not latent infection in humans, these studies show that a housekeeping enzyme of M. tb contributes to its armamentarium of virulence promoting factors.

Conventional diagnostic tests for tuberculosis have several limitations and are often unhelpful in establishing the diagnosis of extrapulmonary tuberculosis. Although commercial serological antibody based tests are available, their usefulness in the diagnosis of extrapulmonary tuberculosis is unknown. A systematic review was conducted to assess the accuracy of commercial serological antibody detection tests for the diagnosis of extrapulmonary tuberculosis. In a comprehensive search, 21 studies that reported data on sensitivity and specificity for extrapulmonary tuberculosis were identified. These studies evaluated seven different commercial tests, with Anda-TB IgG accounting for 48% of the studies. The results showed that (1) all commercial tests provided highly variable estimates of sensitivity (range 0.00-1.00) and specificity (range 0.59-1.00) for all extrapulmonary sites combined; (2) the Anda-TB IgG kit showed highly variable sensitivity (range 0.26-1.00) and specificity (range 0.59-1.00) for all extrapulmonary sites combined; (3) for all tests combined, sensitivity estimates for both lymph node tuberculosis (range 0.23-1.00) and pleural tuberculosis (range 0.26-0.59) were poor and inconsistent; and (4) there were no data to determine the accuracy of the tests in children or in patients with HIV infection, the two groups for which the test would be most useful. At present, commercial antibody detection tests for extrapulmonary tuberculosis have no role in clinical care or case detection.