title: A multisite assessment of the quantitative capabilities of the Xpert MTB/RIF assay.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  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.
  
  
  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.
authors.affiliation: Division of Infectious Disease, Department of Medicine, University of Medicine and Dentistry of New Jersey, Newark, USA.
authors.forename: Robert
authors.initials: R
authors.lastname: Blakemore
authors.forename: Pamela
authors.initials: P
authors.lastname: Nabeta
authors.forename: Amy L
authors.initials: AL
authors.lastname: Davidow
authors.forename: Viral
authors.initials: V
authors.lastname: Vadwai
authors.forename: Rasim
authors.initials: R
authors.lastname: Tahirli
authors.forename: Vanisha
authors.initials: V
authors.lastname: Munsamy
authors.forename: Mark
authors.initials: M
authors.lastname: Nicol
authors.forename: Martin
authors.initials: M
authors.lastname: Jones
authors.forename: David H
authors.initials: DH
authors.lastname: Persing
authors.forename: Doris
authors.initials: D
authors.lastname: Hillemann
authors.forename: Sabine
authors.initials: S
authors.lastname: Ruesch-Gerdes
authors.forename: Felicity
authors.initials: F
authors.lastname: Leisegang
authors.forename: Carlos
authors.initials: C
authors.lastname: Zamudio
authors.forename: Camilla
authors.initials: C
authors.lastname: Rodrigues
authors.forename: Catharina C
authors.initials: CC
authors.lastname: Boehme
authors.forename: Mark D
authors.initials: MD
authors.lastname: Perkins
authors.forename: David
authors.initials: D
authors.lastname: Alland
mesh_terms: D000465:Algorithms; D001426:Bacterial Proteins / Q000032:analysis / Q000235:genetics; D004269:DNA, Bacterial / Q000235:genetics; D006801:Humans; D008853:Microscopy; D015337:Multicenter Studies as Topic; D009169:Mycobacterium tuberculosis / Q000235:genetics / Q000302:isolation & purification*; D021141:Nucleic Acid Amplification Techniques; D011237:Predictive Value of Tests; D016032:Randomized Controlled Trials as Topic; D060888:Real-Time Polymerase Chain Reaction; D012107:Research Design; D012680:Sensitivity and Specificity; D013183:Sputum / Q000382:microbiology*; D014376:Tuberculosis / Q000175:diagnosis* / Q000382:microbiology
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial