Comparison of GeneXpert cycle threshold values with smear microscopy and culture as a measure of mycobacterial burden in five regional referral hospitals of Uganda- A cross-sectional study
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Date
2019
Authors
Najjingo, Irene
Muttamba, Winters
Kirenga, Bruce J.
Nalunjogi, Joanitah
Bakesiima, Ritah
Olweny, Francis
Lusiba, Pastan
Katamba, Achilles
Joloba, Moses
Ssengooba, Willy
Journal Title
Journal ISSN
Volume Title
Publisher
PLoS ONE
Abstract
Determining mycobacterial burden is important in assessing severity of disease, evaluating
infectiousness and predicting patient treatment outcomes. Mycobacterial burden assessed
by smear microscopy grade and time to culture positivity is clearly interpretable by most physicians.
GeneXpert (Xpert) has been recommended by WHO as a first line tuberculosis (TB)
diagnostic test as an alternative to smear microscopy. Xpert gives cycle threshold (Ct) values
as a potential measure for mycobacterial burden. For physicians to clearly interpret Ct
values as measures of mycobacterial burden, this study compared the Xpert quantification
capabilities with those of smear microscopy and culture. The study also determined a linear
relationship between Xpert Ct values and MGIT culture time to positivity (MGIT-TTP) and
associated factors. A cut off Ct value which best predicts smear positivity was also determined
using the Receiver Operator Curve analysis method.
Results
Excluding missing results and rifampicin resistant TB cases, a moderately strong correlation
of 0.55 between Xpert Ct value and smear grade was obtained. A weak correlation of
0.37 was obtained between Xpert Ct values and MGIT time to positivity while that
between Xpert Ct values and LJ culture was 0.34. The Xpert Ct values were found to
increase by 2.57 for every unit increase in days to positive and HIV status was significantly
associated with this relationship. A cut off Ct value of 23.62 was found to best predict
smear positivity regardless of HIV status. Conclusion
Our study findings show that GeneXpert Ct values are comparable to smear microscopy
as a measure of M. tuberculosis burden and can be used to replace smear microscopy.
However, given the low correlation between Xpert Ct value and culture positivity, Xpert
Ct values cannot replace culture as a measure of M. tuberculosis burden among TB
patients.
Description
Keywords
GeneXpert cycle threshold, Smear microscopy, Culture, Mycobacterial burden
Citation
Najjingo I, Muttamba W, Kirenga BJ, Nalunjogi J, Bakesiima R, Olweny F, et al. (2019) Comparison of GeneXpert cycle threshold values with smear microscopy and culture as a measure of mycobacterial burden in five regional referral hospitals of Uganda- A cross-sectional study. PLoS ONE 14(5): e0216901. https://doi.org/10.1371/ journal.pone.0216901