Diagnosis of Tuberculous Meningitis by ESAT-6 in CSF
Condition: Tuberculous Meningitis
MeSH terms: Tuberculosis; Tuberculosis, Meningeal; Meningitis
Study type: Observational

Summary: 
      Early and reliable diagnosis of tuberculous meningitis (TBM) still poses a great challenge.
      One of the underlying difficulties is due to the fact that tubercle bacilli are mainly not
      present in the cerebrospinal fluid (CSF) but in the phagocytotic macrophages. The present
      study was designed to demonstrate early secretory antigenic target 6 (ESAT-6), a
      mycobacterium-specific antigens, in the macrophages in infected CSF samples and compare the
      efficiency of this antigen in the laboratory diagnosis of TBM.
    

Detailed description: 
      Tuberculosis is a disease that carries a global health burden, and tuberculous meningitis
      (TBM) is one of the most serious clinical manifestations of extrapulmonary tuberculosis. The
      failure to promptly recognize and treat patients with TBM enables the disease to exact a
      devastating toll around the globe despite the availability of effective chemotherapy.
      Therefore, accurate and timely identification of infected individuals and subsequent prompt
      initiation of effective anti-tuberculosis chemotherapy are essential in reducing the
      mortality of TBM.1 However, the disease is often difficult to diagnose with certainty,
      especially at early stages,2 as the presenting clinical and cerebrospinal fluid (CSF)
      features of TBM are non-specific and have to be differentiated from a plethora of other
      infectious meningitides such as viral, bacterial or cryptococcal meningitis, which frequently
      result in diagnostic confusion. Consequently, TBM is often diagnosed when irreversible
      neurologic damages have already taken place and immediate anti-tuberculosis therapy is
      therefore recommended regardless of the results of individual tests if TBM is seriously
      contemplated.

      The confirmation of TBM depends on the demonstration of Mycobacterium tuberculosis in the CSF
      by bacteriological methods. Smear acid-fast bacillus microscopy is rapid and inexpensive, but
      has a very low sensitivity (10-20%),3 and culture also lacks sensitivity and is
      time-consuming. A number of strategies have been attempted to improve the laboratory
      diagnosis of TBM. Nucleic acid amplification tests (NAAT) show potential roles in confirming
      the diagnosis of TBM, but they, too, suffer from the problem of overall low sensitivity
      (0.46-0.66), partly due to a low bacillary load in the CSF.4-5 Immunodiagnostic methods have
      also been evaluated, but the heterogeneity of immune responses in TBM patients at the
      different stages of the infection poses a major obstacle to the detection of mycobacterial
      antibodies in CSF samples.6-7 In general, patients at the chronic stages of TBM have a myriad
      of antibody responses to all major antigens of Mycobacterium tuberculosis, while patients at
      the early stages have scarcely any detectable antibody response. In addition, interpretation
      of mycobacterial antibodies in the CSF must take into account the contribution of antibodies
      from the plasma, therefore, jeopardizing the use of these antibody-based immunodiagnostic
      methods in the location where they are most needed.7 One approach to provide direct evidence
      of existing infection is the detection of the presence of specific antigens in the
      circulating CSF. The first generation of these tests employed non-specific antigens as
      exemplified by bacille Calmette-Guerin (BCG) or purified protein derivative (PPD).8-10
      Genomic analysis and antigen mining of Mycobacterium tuberculosis have yielded novel, more
      specific antigens, such as early secretory antigenic target 6 (ESAT-6), 38-kD antigen, and
      Ag85 complex.11 Nevertheless, the diagnostic efficiency of this antigen-based approach is
      still unsatisfactory.5-6 Mycobacterium tuberculosis belongs to the group of intracellular
      bacteria, which replicate within resting macrophages. During the early stages of the central
      nervous system (CNS) infection, the tubercle bacilli in the CSF are immediately phagocytosed
      by the macrophages,12-13 leading to the scarcity of mycobacterial markers in the circulating
      CSF. Thus, no such tests have yet become available for early diagnosis of active TBM with the
      requisite sensitivity and specificity.

      The investigators hypothesized that, in contrast to the scarcity of mycobacterial markers in
      the circulating CSF, macrophages in infected CSF may carry a high load of mycobacterial
      antigens, which could lend themselves to the development a cell-based diagnostic approach for
      TBM. Here, the investigators sought to develop a smear immunocytochemical method to improve
      the accuracy of early diagnosis of TBM by examining early secretory antigenic target 6
      (ESAT-6), a mycobacterium-specific antigens, in the macrophages in CSF specimens from
      patients with TBM and patients with infectious meningitis other than TBM. The investigators
      also compared the efficiency of this cell-based approach for detecting mycobacterial antigens
      with microbiological and NAAT method in the laboratory diagnosis of TBM. Further, on the
      basis of the clinical presentations, CSF and etiological findings in this study, the
      investigators proposed novel diagnostic criteria for TBM.
    

Observational model: Case-Control
Time perspective: Prospective
Sampling method: Probability Sample
Study population: 
        All the patients described in this study were admitted to the Department of Neurology,
        Xijing Hospital of the Fourth Military Medical University, Xi'an, Shaanxi, China.
      
Keywords: tuberculous meningitis; early secretory antigenic target 6; diagnosis