title: Discordance between GeneXpert assay and conventional drug-susceptibility testing in detecting rifampicin-resistant tuberculosis: A perspective of the line probe assay.
abstract: BACKGROUND
  Early detection for tuberculosis (TB) and rifampicin-resistant TB (RRTB) is crucial for proper control of this disease. WHO recommended the use of the GeneXpert assay at district level to cover these two public health demands? A study evaluated the diagnostic impact of the GeneXpert assay in detecting TB and RRTB. Odd results were observed in this study in the form of discordance between the GeneXpert assay and the conventional culture and drug-susceptibility testing (DST).
  
  
  OBJECTIVE
  To assess the molecular diagnostic validity of the GeneXpert assay when results do not match phenotypic results given by DST.
  
  
  METHODS
  Pulmonary TB patients with recently detected sputum positive for acid-fast bacilli (AFB) were recruited from random geographical clusters (18 out of 36 primary healthcare districts in the middle five governorates in Iraq) during a 1-year period (November 2013-October 2014). Sputum samples from all enrolled patients were sent for GeneXpert assay testing, culture, and DST. Genotype mycobacterium (GM) from Hain Lifescience (Nehren, Germany) was used to detect non-tuberculosis mycobacteria (NTM) whenever suspected. Those with discordant results regarding the status of RRTB between GeneXpert assay and DST were retested with the line probe assay (LPA). Simple frequency distribution was used to describe study results.
  
  
  RESULTS
  Four-hundred ten patients were enrolled, all of whom were culture positive. Only two patients were found negative for TB on GeneXpert assay who were then diagnosed as NTM by LPA (GM). Out of the 408 patients, discordance between GeneXpert and DST regarding the status of rifampicin susceptibility was observed in 17 cases (4%). Nine patients were RR on GeneXpert but rifampicin susceptible (RS) on DST. LPA agreed with GeneXpert assay for all nine cases. Eight patients were RS on GeneXpert but RR on DST. Here, LPA disagreed with GeneXpert assay only in one patient who was found to be RR by LPA.
  
  
  CONCLUSIONS
  GeneXpert assay is a valid molecular test for TB and RRTB regardless of its discordance with conventional culture and DST.
authors.affiliation: Veterinary Directorate, Baghdad, Iraq. Electronic address: rukia_mustafa19@yahoo.com.
authors.forename: Ruqaya M
authors.initials: RM
authors.lastname: Ali
authors.affiliation: Directorate of Public Health, National Specialized Center for Chest and Respiratory Disease, Baghdad, Iraq.
authors.forename: Ahmed A
authors.initials: AA
authors.lastname: Alsudani
keywords: Drug-susceptibility testing; GeneXpert assay; Line probe assay; Multidrug resistance