Abstract
Rationale
: The Xpert MTB/RIF is an automated molecular test for
Mycobacterium tuberculosis
that estimates bacterial burden by measuring the threshold-cycle (Ct) of its
M. tuberculosis
–specific real-time polymerase chain reaction. Bacterial burden is an important biomarker for disease severity, infection control risk, and response to therapy.
Objectives
: Evaluate bacterial load quantitation by Xpert MTB/RIF compared with conventional quantitative methods.
Methods
: Xpert MTB/RIF results were compared with smear-microscopy, semiquantiative solid culture, and time-to-detection in liquid culture for 741 patients and 2,008 samples tested in a multisite clinical trial. An internal control real-time polymerase chain reaction was evaluated for its ability to identify inaccurate quantitative Xpert MTB/RIF results.
Measurements and Main Results
: Assays with an internal control Ct greater than 34 were likely to be inaccurately quantitated; this represented 15% of
M. tuberculosis
–positive tests. Excluding these, decreasing
M. tuberculosis
Ct was associated with increasing smear microscopy grade for smears of concentrated sputum pellets (
r
s
= −0.77) and directly from sputum (
r
s
= −0.71). A Ct cutoff of approximately 27.7 best predicted smear-positive status. The association between
M. tuberculosis
Ct and time-to-detection in liquid culture (
r
s
= 0.68) and semiquantitative colony counts (
r
s
= −0.56) was weaker than smear. Tests of paired same-patient sputum showed that high-viscosity sputum samples contained ×32 more
M. tuberculosis
than nonviscous samples. Comparisons between the grade of the acid-fast bacilli smear and Xpert MTB/RIF quantitative data across study sites enabled us to identify a site outlier in microscopy.
Conclusions
: Xpert MTB/RIF quantitation offers a new, standardized approach to measuring bacterial burden in the sputum of patients with tuberculosis.