title: Assessment of the sensitivity and specificity of Xpert MTB/RIF assay as an early sputum biomarker of response to tuberculosis treatment.
abstract: BACKGROUND
  An accurate biomarker is urgently needed to monitor the response to treatment in patients with pulmonary tuberculosis. The Xpert MTB/RIF assay is a commercially available real-time PCR that can be used to detect Mycobacterium-tuberculosis-specific DNA sequences in sputum samples. We therefore evaluated this assay with serial sputum samples obtained over 26 weeks from patients undergoing treatment for tuberculosis.
  
  
  METHODS
  We analysed sputum samples from 221 patients with smear-positive tuberculosis enrolled at two sites (Cape Town, South Africa, and Mbeya, Tanzania) of a multicentre randomised clinical trial REMoxTB of antituberculosis treatment on a weekly basis (weeks 0 to 8), then at weeks 12, 17, 22, and 26 after treatment initiation. The Xpert MTB/RIF results over time were compared with the results of standard smear microscopy and culture methods.
  
  
  RESULTS
  We obtained and analysed 2741 sputum samples from 221 patients. The reduction in positivity rates with Xpert MTB/RIF were slower than those with the standard methods. At week 8, positive results were obtained for 62 (29%) of 212 sputum samples with smear microscopy, 46 (26%) of 175 with solid culture (Löwenstein-Jensen medium), 77 (42%) of 183 with liquid culture (Bactec MGIT960 system), and 174 (84%) of 207 with Xpert MTB/RIF; at 26 weeks, positive results were obtained for ten (5%) of 199, four (3%) of 157, seven (4%) of 169, and 22 (27%) of 83 sputum samples, respectively. The reduction in detection of quantitative M tuberculosis DNA with Xpert MTB/RIF correlated with smear grades (ρ=-0·74; p<0·0001), solid culture grades (ρ=-0·73; p<0·0001), and time to liquid culture positivity (ρ=0·73; p<0·0001). Compared with the combined binary smear and culture results as a reference standard, the Xpert MTB/RIF assay had high sensitivity (97·0%, 95% CI 95·8-97·9), but poor specificity (48·6%, 45·0-52·2).
  
  
  CONCLUSIONS
  The poor specificity precludes the use of the Xpert MTB/RIF assay as a biomarker for monitoring tuberculosis treatment, and should not replace standard smear microscopy and culture.
  
  
  BACKGROUND
  Global Alliance for TB Drug Development, Bill & Melinda Gates Foundation, UK Medical Research Council, German Ministry of Science and Technology.
authors.affiliation: Division of Medical Physiology, Department of Biomedical Sciences, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town, South Africa.
authors.forename: Sven O
authors.initials: SO
authors.lastname: Friedrich
authors.affiliation: National Institute of Medical Research-Mbeya Medical Research Centre, Mbeya, Tanzania; Division of Infectious Diseases and Tropical Medicine, Medical Centre of the University of Munich (LMU), Munich, Germany; DZIF German Centre for Infection Research, Munich, Germany.
authors.forename: Andrea
authors.initials: A
authors.lastname: Rachow
authors.affiliation: Division of Infectious Diseases and Tropical Medicine, Medical Centre of the University of Munich (LMU), Munich, Germany.
authors.forename: Elmar
authors.initials: E
authors.lastname: Saathoff
authors.affiliation: Centre for Clinical Microbiology, Division of Infection and Immunity, University College London, London, UK.
authors.forename: Kasha
authors.initials: K
authors.lastname: Singh
authors.affiliation: National Institute of Medical Research-Mbeya Medical Research Centre, Mbeya, Tanzania.
authors.forename: Chacha D
authors.initials: CD
authors.lastname: Mangu
authors.affiliation: Division of Pulmonology, Department of Medicine, University of Cape Town Lung Institute, Groote Schuur Hospital, Cape Town, South Africa.
authors.forename: Rodney
authors.initials: R
authors.lastname: Dawson
authors.affiliation: Medical Research Centre (MRC) Clinical Trials Unit, London, UK.
authors.forename: Patrick Pj
authors.initials: PP
authors.lastname: Phillips
authors.affiliation: MRC Centre for Molecular and Cellular Biology, Department of Biomedical Sciences, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town, South Africa.
authors.forename: Amour
authors.initials: A
authors.lastname: Venter
authors.affiliation: Centre for Clinical Microbiology, Division of Infection and Immunity, University College London, London, UK.
authors.forename: Anna
authors.initials: A
authors.lastname: Bateson
authors.affiliation: Foundation of Innovative New Diagnostics, Geneva, Switzerland.
authors.forename: Catharina C
authors.initials: CC
authors.lastname: Boehme
authors.affiliation: Division of Infectious Diseases and Tropical Medicine, Medical Centre of the University of Munich (LMU), Munich, Germany.
authors.forename: Norbert
authors.initials: N
authors.lastname: Heinrich
authors.affiliation: Centre for Clinical Microbiology, Division of Infection and Immunity, University College London, London, UK.
authors.forename: Robert D
authors.initials: RD
authors.lastname: Hunt
authors.affiliation: Radboud University Nijmegen Medical Centre/UCCZ Dekkerswald, Nijmegen.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Boeree
authors.affiliation: Centre for Clinical Microbiology, Division of Infection and Immunity, University College London, London, UK.
authors.forename: Alimuddin
authors.initials: A
authors.lastname: Zumla
authors.affiliation: Centre for Clinical Microbiology, Division of Infection and Immunity, University College London, London, UK.
authors.forename: Timothy D
authors.initials: TD
authors.lastname: McHugh
authors.affiliation: University of St Andrews School of Medicine, North Haugh, St Andrews, Scotland, UK. Electronic address: shg3@st-andrews.ac.uk.
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Gillespie
authors.affiliation: Division of Medical Physiology, Department of Biomedical Sciences, Faculty of Medicine and Health Sciences, Stellenbosch University, Cape Town, South Africa.
authors.forename: Andreas H
authors.initials: AH
authors.lastname: Diacon
authors.affiliation: Division of Infectious Diseases and Tropical Medicine, Medical Centre of the University of Munich (LMU), Munich, Germany; DZIF German Centre for Infection Research, Munich, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Hoelscher
mesh_terms: D000328:Adult; D000995:Antitubercular Agents / Q000627:therapeutic use*; D001681:Biological Assay / Q000379:methods; D015415:Biomarkers / Q000378:metabolism; D004269:DNA, Bacterial / Q000302:isolation & purification; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009169:Mycobacterium tuberculosis / Q000235:genetics / Q000302:isolation & purification; D060888:Real-Time Polymerase Chain Reaction / Q000379:methods; D012008:Recurrence; D012680:Sensitivity and Specificity; D013183:Sputum / Q000382:microbiology*; D016896:Treatment Outcome; D014397:Tuberculosis, Pulmonary / Q000188:drug therapy*
chemical_list: D000995:Antitubercular Agents; D015415:Biomarkers; D004269:DNA, Bacterial