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Figure 1. Sensitivity of TB diagnostic assays in 84 HIV patients with culture-confirmed TB. LAM: Lipoarabinomannan; MTB/RIF: Mycobacterium tuberculosis /rifampicin. Data taken from [2].
(Figure omitted. See article PDF.)
Discussion
Need for new TB diagnostic tests
The search for a reliable, accurate and low-cost diagnostic test for TB has accelerated in the past few years. The need is especially great for HIV-infected individuals living in TB-endemic countries, as TB is still the most common cause of mortality and morbidity, and where the diagnosis is often delayed or missed [1]. Lawn et al. recently reported on the diagnostic accuracy of Determine® TB-LAM (Alere, MA, USA), which measures the urine antigen lipoarabinomannan (LAM) as a point-of-care screening assay for HIV-associated TB before antiretroviral therapy (ART) [2]. In 2010, of the 8.8 million incident TB cases, 1.1 million (13%) were among people living with HIV and 0.35 million (range: 0.32-0.39 million) deaths occurred from HIV-associated TB [101]. The limitations of the existing TB diagnostic tools that utilize sputum or a specimen from the diseased site include low sensitivity of sputum microscopy, long turnaround time of the gold standard culture and high cost coupled with technology requirements for molecular methods [3]. Higher rates of smear-negative, extrapulmonary TB in HIV-infected individuals, especially in those with advanced disease and immunosuppression, make diagnosis more challenging, contributing to increased mortality rates [3]. This emphasizes the need for a low-cost, rapid, point-of-care test for HIV-associated TB, preferably one that could work on blood, urine or saliva, thus covering both pulmonary and extrapulmonary TB. Research on new diagnostics has explored two avenues: identification of mycobacterial products (protein, antigen, DNA, RNA or isolation of bacterium) and host immune response as a surrogate marker of infection or disease.
Antigen-based tests for TB diagnosis: types, performance & limitations
Detection of Mycobacterium tuberculosis (MTB) antigen has the potential to provide rapid, direct evidence of TB disease. Both in-house and commercial antigen-detection tests have been evaluated in various clinical specimens such as serum, sputum, urine, cerebrospinal fluid (CSF) and pleural fluid for the diagnosis of TB [3-9]. The antigens that have been studied include LAM, 30 kDa, purified protein derivative, 45/47 kDa, 19 kDa, 38 kDa, 58 kDa, 20 kDa, Ag85 complex, Rv1656 and 65 kDa, among others...