Abstract
Traditionally, smear microscopy has been used as a point-of-care measure of bacillary burden in tuberculosis patients to inform infection control and contact tracing. Xpert MTB/RIF has the potential to replace smear. However, data to support the use of its quantitative output [cycle threshold (CT)] as an alternate point-of-care measure of bacillary burden are limited. This study assessed the correlation (Spearman’s) between CT, smear, culture time-to-positivity (TTP), and clinical factors in patients with Xpert-positive sputum from Mozambique (n = 238) and South Africa (n = 462). Mean CT and smear grade correlated well (ρ0.72); compared to TTP and smear (ρ0.61); and mean CT and TTP (ρ0.50). In multivariate analyses, lower CT (higher bacillary load) was associated with negative HIV serostatus and low BMI. A smear positivity rule-out (95% sensitivity) CT cut-off of 28.0 was identified, with 54.1% specificity, 2.07 positive likelihood ratio, 0.09 negative likelihood ratio and 79.0% correctly classified. Cut-offs were higher for HIV positive compared to HIV negative individuals for any set sensitivity level. This study suggests Xpert CT values correlate well with smear, both in HIV positive and negative individuals, and that CT cut-offs might be broadly applicable to multiple settings. Studies to directly assess the association of CT with infectiousness are needed.
Highlights
IntroductionTime to positivity (TTP) of the mycobacterial growth indicator tube (MGIT) system (the amount of time in culture until mycobacterium tuberculosis (MTB) is detected) has been shown to predict the risk of transmission[9] and response to treatment[10,11] more reliably than smear status
(Spearman’s) between cycle threshold (CT), smear, culture time-to-positivity (TTP), and clinical factors in patients with Xpert-positive sputum from Mozambique (n = 238) and South Africa (n = 462)
This study has added to the evidence that Xpert MTB/RIF CT can be used as a measure of bacillary burden that correlates well with smear grade and moderately with Time to positivity (TTP) in liquid culture
Summary
Time to positivity (TTP) of the MGIT system (the amount of time in culture until mycobacterium tuberculosis (MTB) is detected) has been shown to predict the risk of transmission[9] and response to treatment[10,11] more reliably than smear status. Xpert provides results in under 2 hours and can detect down to 100 CFU per sample[8], improving the sensitivity for patients with low bacillary burden compared to smear microscopy[14]. It provides a cycle threshold (CT) result, which reflects the number of PCR cycles required to detect MTB.
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