title: Three year evaluation of Xpert MTB/RIF in a low prevalence tuberculosis setting: A Scottish perspective.
abstract: Xpert MTB/RIF (Cepheid) is a rapid molecular assay shown to be sensitive and specific for pulmonary tuberculosis (TB) diagnosis in highly endemic countries. We evaluated its diagnostic performance in a low TB prevalence setting, examined rifampicin resistance detection and quantitative capabilities predicting graded auramine microscopy and time to positivity (TTP) of culture.
  
  
  
  Xpert MTB/RIF was used to test respiratory samples over a 3 year period. Samples underwent graded auramine microscopy, solid/liquid culture, in-house IS6110 real-time PCR, and GenoType MTBDRplus (HAIN Lifescience) to determine rifampicin and/or isoniazid resistance.
  
  
  
  A total of 2103 Xpert MTB/RIF tests were performed. Compared to culture sensitivity was 95.8%, specificity 99.5%, positive predictive value (PPV) 82.1%, and negative predictive value (NPV) 99.9%. A positive correlation was found between auramine microscopy grade and Xpert MTB/RIF assay load. We found a clear reduction in the median TTP as Xpert MTB/RIF assay load increased. Rifampicin resistance was detected.
  
  
  
  Xpert MTB/RIF was rapid and accurate in diagnosing pulmonary TB in a low prevalence area. Rapid results will influence infection prevention and control and treatment measures. The excellent NPV obtained suggests further work should be carried out to assess its role in replacing microscopy.
authors.affiliation: Aberdeen Royal Infirmary, Aberdeen, AB25 2ZN, United Kingdom. Electronic address: b.parcell@nhs.net.
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Parcell
authors.affiliation: Ninewells Hospital and Medical School, Dundee, DD1 9SY, United Kingdom. Electronic address: anna.jarchow-macdonald@nhs.net.
authors.forename: Anna A
authors.initials: AA
authors.lastname: Jarchow-MacDonald
authors.affiliation: Scottish Mycobacteria Reference Laboratory SMRL- Royal Infirmary of Edinburgh, 51 Little France Crescent, Old Dalkeith Road, Edinburgh, EH16 4SA, United Kingdom. Electronic address: Louise.Seagar@nhslothian.scot.nhs.uk.
authors.forename: Amie-Louise
authors.initials: AL
authors.lastname: Seagar
authors.affiliation: Scottish Mycobacteria Reference Laboratory SMRL- Royal Infirmary of Edinburgh, 51 Little France Crescent, Old Dalkeith Road, Edinburgh, EH16 4SA, United Kingdom. Electronic address: Ian.Laurenson@nhslothian.scot.nhs.uk.
authors.forename: Ian F
authors.initials: IF
authors.lastname: Laurenson
authors.affiliation: The Institute of Applied Health Sciences, University of Aberdeen, Aberdeen, United Kingdom. Electronic address: gordon.prescott@abdn.ac.uk.
authors.forename: Gordon J
authors.initials: GJ
authors.lastname: Prescott
authors.affiliation: Ninewells Hospital and Medical School, Dundee, DD1 9SY, United Kingdom. Electronic address: michael.lockhart@nhs.net.
authors.forename: Michael
authors.initials: M
authors.lastname: Lockhart
mesh_terms: D000904:Antibiotics, Antitubercular / Q000494:pharmacology*; D042241:Early Diagnosis; D006801:Humans; D058889:Molecular Typing / Q000379:methods*; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000235:genetics*; D011237:Predictive Value of Tests; D012189:Retrospective Studies; D012293:Rifampin / Q000494:pharmacology*; D012606:Scotland; D014376:Tuberculosis / Q000175:diagnosis* / Q000382:microbiology
chemical_list: D000904:Antibiotics, Antitubercular; D012293:Rifampin
keywords: Early diagnosis; Molecular testing; Mycobacterium tuberculosis; Nucleic acid amplification; Tuberculosis; Xpert MTB/RIF