Abstract
BackgroundDetermining 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.ResultsExcluding 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.ConclusionOur 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.
Highlights
One of the World Health Organization (WHO) End TB strategies of reducing TB incidence by 80% can be achieved through early case detection[1]
The Xpert cycle threshold (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
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
Summary
One of the WHO End TB strategies of reducing TB incidence by 80% can be achieved through early case detection[1]. Culture is still not readily available due to the long turnaround time that results in patient treatment delays, is prone to contamination and requires specialized infrastructure Due to these limitations, Xpert has been endorsed by World Health Organization (WHO) as the initial TB diagnostic test among HIV infected individuals[6]. Xpert is a highly sensitive TB diagnostic test with a short turnaround time[7] It offers quantitative estimation of mycobacterial load in form of cycle threshold values (Ct) which inversely correlates with the concentration of TB bacilli. 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. A cut off Ct value which best predicts smear positivity was determined using the Receiver Operator Curve analysis method
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