Abstract
The World Health Organization has recommended use of molecular-based tests MTBDRplus and GeneXpert MTB/RIF to diagnose multidrug-resistant tuberculosis in developing and high-burden countries. Both tests are based on detection of mutations in the Rifampin (RIF) Resistance-Determining Region of DNA-dependent RNA Polymerase gene (rpoB). Such mutations are found in 95–98% of Mycobacterium tuberculosis strains determined to be RIF-resistant by the “gold standard” culture-based drug susceptibility testing (DST).We report the phenotypic and genotypic characterization of 153 consecutive clinical Mycobacterium tuberculosis strains diagnosed as RIF-resistant by molecular tests in our laboratory in Port-au-Prince, Haiti. 133 isolates (86.9%) were resistant to both RIF and Isoniazid and 4 isolates (2.6%) were RIF mono-resistant in MGIT SIRE liquid culture-based DST. However the remaining 16 isolates (10.5%) tested RIF-sensitive by the assay.Five strains with discordant genotypic and phenotypic susceptibility results had RIF minimal inhibitory concentration (MIC) close to the cut-off value of 1 µg/ml used in phenotypic susceptibility assays and were confirmed as resistant by DST on solid media. Nine strains had sub-critical RIF MICs ranging from 0.063 to 0.5 µg/ml. Finally two strains were pan-susceptible and harbored a silent rpoB mutation.Our data indicate that not only detection of the presence but also identification of the nature of rpoB mutation is needed to accurately diagnose resistance to RIF in Mycobacterium tuberculosis. Observed clinical significance of low-level resistance to RIF supports the re-evaluation of the present critical concentration of the drug used in culture-based DST assays.
Highlights
The World Health Organization (WHO) recommends using rapid molecular tests MTBDRplus and GeneXpert Mycobacterium tuberculosis (MTB)/RIF to diagnose tuberculosis (TB) and multi-drug resistant tuberculosis (MDR-TB) in developing and high-burden countries [1,2]
Two cases detected with MTBDRplus, and 7 cases detected with GeneXpert MTB/RIF directly in primary specimens did not produce cultures
89,5% of MTB isolates initially found RIF-resistant by molecular tests in our laboratory in Haiti between March 2008 and July 2012 were confirmed to be resistant to RIF by MGIT SIRE assay, the method recommended by the WHO for automated culture-based drug susceptibility testing (DST) [5]
Summary
The World Health Organization (WHO) recommends using rapid molecular tests MTBDRplus and GeneXpert MTB/RIF to diagnose tuberculosis (TB) and multi-drug resistant tuberculosis (MDR-TB) in developing and high-burden countries [1,2]. Molecular tests dramatically shorten time to diagnosis from months to days (MTBDRplus) or hours (GeneXpert MTB/RIF) Both tests are based on PCR amplification of the beta subunit of mycobacterial DNA-dependent RNA Polymerase (rpoB) followed by detection of mutations in its 81 bp Rifampin ResistanceDetermining Region (RRDR). Such mutations are found in 9598% of all Mycobacterium tuberculosis (MTB) strains resistant to Rifampin (RIF) [3,4]. Haiti is representative of countries where the TB burden is high but resources and laboratory facilities are very limited It has the highest rate of TB in the Western Hemisphere, with an estimated prevalence of 331 per 100 000 population [7]. The TB laboratory of the Groupe Haıtien d’Etude du Sarcome de Kaposi et des Infections Opportunistes (GHESKIO) in the capital Port-auPrince is the only laboratory in a country with population of over 10 millions with the capacity to perform mycobacterial culture and Drug Susceptibility Testing (DST) and serves as a national reference facility
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
