GeneXpert Test For Tuberculosis: Introduction, Principle, Procedure, Result Interpretation and Advantages

GeneXpert Test For Tuberculosis: Introduction, Principle, Procedure, Result Interpretation and Advantages

Introduction of GeneXpert Test

GeneXpert test is a molecular test for tuberculosis diagnosis by detecting the presence of  Mycobacterium tuberculosis (bacteria), as well as testing for resistance to the drug rifampicin.

Principle of GeneXpert Test

GeneXpert test is a highly sensitive and specific test for tuberculosis(TB) diagnosis that detects DNA sequences specific for Mycobacterium tuberculosis and rifampicin resistance i.e. rpoB gene.  It is also called Gene Xpert because it works as its name.

Requirements for GeneXpert Test

1.  Kit contains-

  • Cartridge
  • Dropper
  • and reagent

2. Gene Xpert machine

3. Desktop

4. bar code reader/ bar code scanner

5. specimen: sputum, CSF, urine, and other body fluids like pleural fluid, synovial fluid, ascitic fluid, and so on

Procedure of GeneXpert Test

  1. Mix specimen (sputum) and reagent in equal volume.
  2. Wait for 15 minutes.
  3. Transfer reagent treated specimen into cartridge sample well by using a dropper. (Volume up to mark in dropper)
  4. Go to the desktop and click create test.
  5. Scan cartridge bar code by the scanner and fill the form mainly patient name and specimen type.
  6. Click the Start test button and finally load the cartridge on the holder as shown by default (A1/A2/A3/A4) also notice clicking orange signal.
  7. After that push the door and wait for the result (1 hour 51 minutes)

Result Interpretation of  Test

  • MTB not detected
  • MTB detected with grading either
  1. very low
  2. low
  3. medium
  4. or high  with
  • Rif Resistance NOT DETECTED or
  • Rif Resistance DETECTED

Advantages of GeneXpert Test

  1. GeneXpert test is highly sensitive comparatively AFB stain and culture of Mycobacterium tuberculosis. ( to be AFB positive load should be in sample  10000 bacteria/ml, in culture 10-100 whereas in gene Xpert only 1-10).
  2. Very little technical training is required to operate.
  3. Minimized bio hazard comparatively AFB stain and culture. ( reagent having 10% sodium hypochlorite not only dissolve the specimen but also minimizes contamination)
  4. Time is taken to  Mycobacterium culture 2-6 weeks and needs 3 weeks extra for drug resistance test that is minimized in this test within hours. We can dispatch the report within 2 half an hour.

Disadvantages of GeneXpert Test

  1. The shelf life of the cartridge is only 18 months.
  2. A very stable electricity supply is required.
  3. Calibrations need annually.
  4. Test cost is high.

Note: In various developing countries, test cost is free.

Further Reading

  1. https://www.cdc.gov/tb/publications/factsheets/testing/xpert_mtb-rif.htm
  2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5875728/
  3. https://tbfacts.org/genexper
  4. https://apps.who.int/iris/bitstream/handle/10665/112469/9789241506700_eng.pdf

 

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