GeneXpert: What, When and How?

What is Genexpert TB Test?
  • The Xpert MTB/RIF detects DNA sequences specific for Mycobacterium tuberculosis and Rifampicin resistance by polymerase chain reaction
  • It is based on the Cepheid GeneXpert system, a platform for rapid and simple-to-use nucleic acid amplification tests (NAAT)
  • The Xpert® MTB/RIF purifies and concentrates Mycobacterium tuberculosis bacilliform sputum samples, isolates genomic material from the captured bacteria by sonication and subsequently amplifies the genomic DNA by PCR
  • The process identifies all the clinically relevant Rifampicin resistance inducing mutations in the RNA polymerase beta(rpoB) gene in the Mycobacterium tuberculosis genome
  • Results are obtained from unprocessed sputum samples in 90 minutes
  • It has a sensitivity of 89% and specificity of 99%
Advantages of the GeneXpert TB test:
  • The test can be done on AFB smear positive and AFB smear negative samples
  • The test can be done on pulmonary and extra pulmonary samples
  • The results are available the same day
  • The specificity of the test is 99%, so very few false positive cases
  • The test detects Rifampicin resistance (rpoB gene) which is the surrogate maker for MDRTB
Utility of the GeneXpert test:

GeneXpert is recommended forrapid and confirmatory molecular diagnosis of tuberculosis

  • Prevents delays in diagnosis and treatment initiation
  • Detects Rifampicin resistance (detects rpoBgene mutation)
  • Helps in selecting the most appropriate antitubercular drugs
  • Can be performed on all samples: pulmonary & extra pulmonary, smear positive or negative
  • Highly sensitive (88%) and specific (98%)
  • Recommended as an early confirmatory test by WHO for suspected MDRTBand HIV-TBco-infection
TEST NAMEGeneXpert
SAMPLE TYPESputum, BAL, CSF, Tissue
TATSame day reporting, Cut off by 5 pm
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