Aids for the early diagnosis of tuberculous meningitis (TBM)
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Mortality and morbidity rates associated with tuberculous meningitis (TBM) are substantial. The average duration of the untreated disease from onset to death is about 17 days. The prognosis of TBM is known to correlate with the stage of the disease at the time of diagnosis and commencement of chemotherapy. Early diagnosis improves the chances of recovery without neurological sequelae. Early diagnosis is a problem because the presenting symptoms are non-specific and the onset of the disease is typically insidious. To date no single test is available that is totally reliable and specific for TBM. I have attempted to develop a reliable and easily applicable test for the diagnosis of TBM. In fulfilling this objective, the work undertaken may be divided into three major sections:- 1. Detection of soluble Mycobacterium tuberculosis antigens in the cerebrospinal fluid (CSF) of patients with TBM and in control groups by using Mycobacterium bovis BCG antigens. The technique used was that of inhibition enzyme-linked immunosorbent assay (ELISA). The principle of this technique is illustrated in Fig. 5. 2. Detection of soluble M. tuberculosis antigens in the CSF of tuberculous and control groups of patients by using antibodies raised against M.bovis BCG. The technique used was that of the double antibody sandwich ELISA. An outline of this ELISA is given in Fig. 6. 3. Correlation of chloride levels in the blood and CSF of patients with tuberculous and other forms of meningitis. It has been established that the SERUM/CSF ratio of bromide tends towards unity in patients with TBM because the permeability of the blood-brain barrier is impaired. Since both bromide and chloride are chemically similar (both being halides), it was thought that a similar pattern may exist for BLOOD/CSF chloride ratios; and this was investigated. The method used for the INHIBITION ELISA had to be standardized before the samples could be tested. This involved investigating the acceptability of various microtitre plates; determination of the optimal working dilutions for the coating solution and conjugate; and determination of optimal conditions for the various incubation periods, both in terms of time and temperature. A total of 70 specimens was tested. These consisted of 25 normal CSF controls; 25 pleural and ascitic fluid samples; 10 TBM samples, and 10 bacterial meningitis CSF samples. It was found that a distinction existed between the absorbance values obtained from positive TBM CSF samples (Mean 0,658 + 0,043) and that from normal CSF samples (Mean 1,089 + 0,224). The mean absorbance of the culture-positive bacterial CSF's also differed significantly from the other 2 groups (Tables VII; IX). Some overlap occurred amongst the absorbance values of bacterial culture positive CSF's (Range 0,975-0,879) and normal CSF's (Range 1,486-0,934). The mean absorbance value for bacterial positive CSF samples (0,920 _+ 0,029) differed significantly (p <0,01) from those of normal CSF (1,089 + 0,224) and TBM CSF's (0,658 + 0,043). The difference between the mean values obtained with tuberculous and non-tuberculous groups of pleural and ascitic fluid was also significant (p < 0,01). The method used for the DOUBLE ANTIBODY SANDWICH ELISA was that of Sada et al. (1983). Before the samples could be tested, the method had to be standardized and similar investigations to those for the INHIBITION ELISA were performed. In addition, antibodies raised against M.bovis BCG were conjugated to alkaline phosphatase since no commercial preparation was available. Unfortunately no distinction was recorded between negative and positive test specimens, even on repetition of the entire procedure. Measurement of chloride was done by a fully automated procedure using the BECKMAN ASTRA-8. A total of 149 samples were tested. Of these 10 were tuberculous, 34 were viral, and the remainder were bacterial meningitis. No pattern was established that could differentiate TBM from viral or bacterial meningitis. The results obtained are tabulated in Table III and illustrated in Figures 9, 10, and 11. In summarizing, the use of the INHIBITION ELISA technique for the accurate diagnosis of TBM seems promising. However, its validity in the clinical situation will have to be assessed further and with greater numbers of specimens before it can be adopted as a diagnostic procedure for TBM. OBJECTIVE. To determine 1. The ability and reliability of the INHIBITION ELISA1 technique to detect mycobacterial antigens in pleural, ascitic, and cerebrospinal fluids. 2. The accuracy and reproducibility of the double antibody sandwich ELISA in the detection of mycobacterial antigens in CSF of patients with tuberculous meningitis (TBM). 3. Whether a correlation exists between blood and CSF chloride levels in patients with tuberculous and other forms of meningitis.