Sputum quality and diagnostic performance of GeneXpert MTB/RIF among smear-negative adults with presumed TB in Uganda
Abstract
Background
Introduction of GeneXpert
MTB/RIF (Xpert) assay has constituted a major breakthrough for
tuberculosis (TB) diagnostics. Several patient factors may
influence diagnostic performance of Xpert including sputum
quality.
Objective
We carried out a
prospective, observational, cross-sectional study to determine
the effect of sputum quality on diagnostic performance of Xpert
among presumed TB patients in Uganda.
Methods
We collected clinical and
demographic information and two sputum samples from
participants. Staff recorded sputum quality and performed LED
fluorescence microscopy and mycobacterial culture on each
sample. If both smear examinations were negative, Xpert testing
was performed. We calculated diagnostic yield, sensitivity,
specificity, and other indicators for Xpert for each stratum of
sputum quality in reference to a standard of mycobacterial
culture.
Results
Patients with salivary
sputum showed a trend towards a substantially higher proportion
of samples that were Xpert-positive (54/286, 19%, 95% CI
15–24) compared with those with all other sputum sample
types (221/1496, 15%, 95% CI 13–17). Blood-stained sputum
produced the lowest sensitivity (28%; 95% CI 12–49) and
salivary sputum the highest (66%; 95% CI 53–77).
Specificity didn’t vary meaningfully by sample types.
Salivary sputum was significantly more sensitive than mucoid
sputum (+13%, 95% CI +1 to +26), while blood-stained sputum was
significantly less sensitive (-24%, 95% CI -42 to -5).
Conclusions
Our findings demonstrate
the need to exercise caution in collecting sputum for Xpert and
in interpreting results because sputum quality may impact test
yield and sensitivity. In particular, it may be wise to pursue
additional testing should blood-stained sputum test negative
while salivary sputum should be readily accepted for Xpert
testing given its higher sensitivity and potentially higher
yield than other sample types. These findings challenge
conventional recommendations against collecting salivary sputum
for TB diagnosis and could inform new standards for sputum
quality.
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Source:
PLOS ONE
