title: The Xpert® MTB/RIF assay evaluation in South Korea, a country with an intermediate tuberculosis burden.
abstract: METHODS
  A central hospital laboratory in South Korea.
  
  
  OBJECTIVE
  To evaluate the usefulness of the Xpert® MTB/RIF assay in a country with an intermediate tuberculosis burden.
  
  
  METHODS
  A total of 71 real-time polymerase chain reaction-positive sputum sediments were tested within  24 h by the Xpert MTB/RIF assay. Mycobacterium tuberculosis detection was compared with smear microscopy and culture. Rifampicin (RMP) resistance was compared with a culture-based method and rpoB gene sequencing. We also assessed the limit of detection for mutant proportions  and time savings in diagnosis.
  
  
  RESULTS
  The Xpert MTB/RIF assay detected M. tuberculosis in 71 (100%) specimens (32 smear-positive, 39 smear-negative). This assay showed 100% (62/62) concordance with drug resistance confirmed by culture and 98.4% (61/62) concordance with sequencing.  A specimen containing approximately 50% of mutant p.His526Tyr was falsely interpreted as wild-type bacilli by this assay. The minimal detection ratio was 5:1 of mutant vs. wild-type cells. The median time saved was 18.5 days (range 9-30) for the diagnosis of M. tuberculosis and  81.5 days (65-136) for RMP susceptibility in smear-negative, culture-positive patients.
  
  
  CONCLUSIONS
  The Xpert MTB/RIF assay showed high sensitivity in detecting M. tuberculosis with information on RMP resistance, and had a more rapid time to diagnosis compared to conventional  tests; however, the location and amount of mutation may affect test sensitivity.
authors.affiliation: Department of Laboratory Medicine, Seoul National University Hospital, Seoul, South Korea.
authors.forename: S Y
authors.initials: SY
authors.lastname: Kim
authors.forename: H
authors.initials: H
authors.lastname: Kim
authors.forename: S Y
authors.initials: SY
authors.lastname: Kim
authors.forename: E K
authors.initials: EK
authors.lastname: Ra
authors.forename: S-I
authors.initials: SI
authors.lastname: Joo
authors.forename: S
authors.initials: S
authors.lastname: Shin
authors.forename: M-W
authors.initials: MW
authors.lastname: Seong
authors.forename: C-G
authors.initials: CG
authors.lastname: Yoo
authors.forename: E-C
authors.initials: EC
authors.lastname: Kim
authors.forename: S S
authors.initials: SS
authors.lastname: Park
mesh_terms: D000995:Antitubercular Agents / Q000494:pharmacology; D001426:Bacterial Proteins / Q000235:genetics; D001483:Base Sequence; D012321:DNA-Directed RNA Polymerases; D024881:Drug Resistance, Bacterial; D006801:Humans; D057230:Limit of Detection; D008826:Microbial Sensitivity Tests; D008853:Microscopy / Q000379:methods; D008969:Molecular Sequence Data; D009169:Mycobacterium tuberculosis / Q000187:drug effects / Q000302:isolation & purification*; D021141:Nucleic Acid Amplification Techniques / Q000379:methods*; D060888:Real-Time Polymerase Chain Reaction; D056910:Republic of Korea / Q000453:epidemiology; D012293:Rifampin / Q000494:pharmacology*; D012680:Sensitivity and Specificity; D013183:Sputum / Q000382:microbiology; D013997:Time Factors; D014376:Tuberculosis / Q000175:diagnosis* / Q000453:epidemiology / Q000382:microbiology
chemical_list: D000995:Antitubercular Agents; D001426:Bacterial Proteins; C505805:rpoB protein, Mycobacterium tuberculosis; D012321:DNA-Directed RNA Polymerases; D012293:Rifampin