A Multisite Assessment of the Quantitative Capabilities of the Xpert MTB/RIF Assay

作者：Blakemore Robert; Nabeta Pamela; Davidow Amy L; Vadwai Viral; Tahirli Rasim; Munsamy Vanisha; Nicol Mark; Jones Martin; Persing David H; Hillemann Doris; Ruesch Gerdes Sabine; Leisegang Felicity; Zamudio Carlos; Rodrigues Camilla; Boehme Catharina C; Perkins Mark D; Alland David^*

来源：American Journal of Respiratory and Critical Care Medicine, 2011, 184(9): 1076-1084.

DOI：10.1164/rccm.201103-0536OC

• tuberculosis
• molecular diagnostics
• diagnostic techniques and procedures
• diagnosis
• clinical trial

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 X32 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.

• 出版日期2011-11-1
• 单位河北医科大学