Gene Xpert MTB/RIF assay for the diagnosis of intra-ocular tuberculosis from vitreous fluid samples
Introduction
The Gene Xpert MTB/RIF assay (GX) has been described as ‘a major step forward in tuberculosis diagnostics’ [1], [2]. Although GX has proved its worth in pulmonary [3] and various extra-pulmonary forms of tuberculosis [4], there is no data regarding its use in vitreous fluid (VF) samples from patients suspected of intra-ocular tuberculosis (IOTB). The present study investigated the potential of GX in an otherwise challenging diagnosis of presumed IOTB.
Section snippets
Methods
After approval by the Institute's Ethics committee, GX was performed on 165 VF samples collected from eyes undergoing pars plana vitrectomy. Study group (n = 85) had patients of presumed ocular tuberculosis based on the criteria described previously [5] while control group had patients with either non-tubercular intra-ocular inflammation (n = 50) or non-inflammatory vitreoretinal disorders (n = 30). Approximately 1 ml of VF was collected, 500μl of which was subjected to GX in accordance with
Results
GX detected M. tuberculosis complex in 19/85 (22.3%) cases from study group and none from the control group. The overall sensitivity, specificity, positive predictive value and negative predictive value of GX was 22.3%, 100%, 100% and 43.1%, respectively. 16/19 (84.2%) positive cases were rifampicin sensitive and 3/19 (15.8%) were rifampicin resistant by GX. rpoB gene sequencing reported 17 cases as rifampicin-sensitive while rifampicin resistance was revealed in two cases, at codons 516 and
Discussion
GX showed a sensitivity and specificity of 22.3% and 100%, respectively. While studies on CSF have documented the sensitivity of GX to range from 33% [4] to 59% [10], the sensitivity of GX for pleural fluid ranges from 5% [11] to 33% [12]. This lower sensitivity in sterile body fluids can be attributed to the low number of organisms, poor lysis of bacteria and interference by proteinaceous compounds in the DNA extraction step. Moreover, among the various sterile body fluids, the vitreous fluid
Author's contribution
Concept of study – KS, AS, AG; Plan of study KS, AG, VG, RS; Collection of data KS, AG, VG, RS, KA, RB, SP; Analysis of results KS, AG, RS, MS; Drafting of manuscript KS, MS, AS, PF; Study supervision AG.
Conflict of interest
None.
Acknowledgement
We acknowledge the financial support provided by DBT.
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