Diagnostic usefulness of T-cell based assays for tuberculous meningitis in HIV-uninfected patients
Introduction
Tuberculous meningitis (TBM) is the most severe form of tuberculosis, causing substantial morbidity and mortality. Early diagnosis and treatment is essential to save lives and reduce long-term morbidity.1, 2 However, TBM is sometimes difficult to diagnose with certainty, especially in its early phase, because the initial clinical features are non-specific and the laboratory tests are insensitive. Cerebrospinal fluid (CSF) acid-fast bacilli (AFB) smear microscopy, although rapid and inexpensive, has very low sensitivity in routine clinical practice (less than 20%).3, 4, 5 Culture is the gold standard for diagnosing TBM, but it is not useful for deciding whether to treat for TBM due to the time required for a positive result and suboptimal sensitivity.
Efforts to identify new diagnostic markers for TBM in CSF have been ongoing, mostly in HIV-infected populations, and some of the markers identified may be used as aids in diagnosing TBM.6, 7, 8, 9, 10, 11, 12, 13, 14 Currently, there is no single test with optimal sensitivity in patients with suspected TBM. New molecular-based microbiological tests, as well as AFB smear microscopy and mycobacterial culture, had lower sensitivity in HIV-uninfected patients because the bacterial loads may have been lower than in HIV-infected patients.8, 10 A recently developed fully automated real-time polymerase chain reaction (PCR), Xpert MTB/RIF, for the diagnosis of TBM was less sensitive among HIV-uninfected patients (0–48%) than among HIV-infected patients (67–79%).8, 10 Furthermore, its diagnostic utility was hampered by a very poor sensitivity for probable or possible TBM, reported to be as low as 0–3%.8, 10
Where the bacterial burden is low, an immunodiagnostic tool may be more useful than microbiological tools for detecting Mycobacterium tuberculosis at infection sites. Recently, interferon-gamma release assays (IGRAs) using mononuclear cells that are compartmentalized at the site of infection, such as pleural fluid,15, 16 peritoneal fluid,17, 18 pericardial effusion,19 and CSF,6 have yielded promising results in the diagnosis of active TB. In our earlier preliminary studies undertaken among HIV-uninfected patients (with 50–82 evaluable CSF samples) we concluded that the enzyme-linked immunosorbent spot (ELISPOT) assay using CSF-mononuclear cells (CSF-MC) appeared to be a promising adjunct to current tests for diagnosing TBM.13, 20 This study builds on those earlier publications.13, 20 In it we evaluated on a larger number of subjects, the clinical utility of peripheral blood mononuclear cells (PBMC) and CSF-MC ELISPOT assays for diagnosing TBM. The study was conducted in a country with an intermediate TB burden and a low prevalence of HIV infection (<0.1%).21
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Patients and study design
Adult patients with suspected TBM admitted to Asan Medical Center, a 2700-bed tertiary hospital in Seoul, South Korea, were prospectively enrolled from April 2008 through March 2014. We enrolled adult patients (≥16 years) with suspected TBM who agreed to additional sampling for PBMC and CSF-MC ELISPOT assays. CSF and blood samples were collected before staring anti-TB treatment. We excluded HIV-infected patients with suspected TBM. Decisions regarding anti-TB therapy were made by the attending
Patient characteristics
Two-hundred and ninety-two patients with meningeal illness who agreed to sampling for PBMC and/or CSF-MC ELISPOT assays were prospectively enrolled in the study. Of these, 9 patients with uncertain diagnosis, 4 without CSF samples, and 3 HIV-infected patients were excluded. The remaining 276 patients with evaluable samples were included in the final analysis. Of these, 90 (33%) were classified as having TBM (30 definite TBM, 19 probable TBM, and 41 possible TBM), and 186 (67%) as non-TBM (Fig. 1
Discussion
This study is the largest study of a T-cell based assay for TBM undertaken among HIV-uninfected patients. The CSF-MC ELISPOT assay appears to be a rapid and accurate rule-in test for the diagnosis of TBM among HIV-uninfected patients. When the CSF-MC ELISPOT assay was incorporated into clinical decision-making based on clinical characteristics and laboratory data, it improved diagnostic accuracy for TBM.
Recently, several studies have reported some successful use of diagnostic markers in CSF for
Financial support
This study was supported by the National Research Foundation of Korea funded by the Ministry of Education (grant NRF-2015R1D1A1A01059315).
Potential conflicts of interest
The authors declare no conflicts of interest.
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