Diagnosis of smear-negative TB is greatly improved by Xpert MTB/RIF
Abstract
Background
Diagnosis of pulmonary
(PTB) and extra-pulmonary tuberculosis (EPTB) in smear-negative
patients can be difficult. We assessed retrospectively the
performance of Xpert MTB/RIF system (Xpert, Cepheid) in
diagnosing smear-negative tuberculosis (TB), which represents
the most common form of TB in a low incidence setting.
Methods
Performance of Xpert was
compared to acid-fast microscopic examination using
Ziehl-Neelsen (ZN) stain in patients with culture-confirmed TB.
Results
386 Mycobacterium
tuberculosis (MTB) culture-positive samples were detected out of
5170 specimens tested with smear microscopy, Xpert and culture:
323 were both culture- and Xpert-positive, and 63
culture-positive only. Of these, 234 (60.6%) were
smear-negative. In addition Xpert detected 40 probable TB cases,
based on clinical findings, which were culture-negative.
Compared
to culture, Xpert showed an overall sensitivity of 83.7% and a
specificity of 99.1%; sensitivity was higher for respiratory
samples (86.5%) than for non-respiratory samples (76.8%). Xpert
sensitivity for smear-negative culture-confirmed TB was 73.1%
and was not influenced by TB localization. As sensitivity of
microscopy alone was poor (39.4%), Xpert improved both diagnosis
of pulmonary TB (Δ = 36.5%) and extra-pulmonary TB
(Δ = 63.4%).
Conclusions
Xpert MTB/RIF is a
sensitive method for rapid diagnosis of TB compared to the
conventional ZN staining. Xpert can serve as a sensitive and
time-saving diagnostic method for microbiological diagnosis of
smear-negative TB in countries with a low TB prevalence.
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Source:
PLOS ONE
