GeneXpert Test For Tuberculosis: Introduction, Principle, Test Requirements, Test Procedure, Result -Interpretation, Advantages, Disadvantage, and Keynotes

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 volumes.
2. Wait for 15 minutes.
3. Transfer the reagent-treated specimen into a cartridge sample well by using a dropper. (Volume up to mark in dropper)
4. Go to the desktop and click create test.
5. Scan the cartridge bar code with 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 the orange signal.
7. After that push the door and wait for the result (1 hour 51 minutes)

Result and Interpretation of the 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 to 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 and a half an hour.

Disadvantages of  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. The test cost is high.

Keynotes

• People infected with M. tuberculosis (etiological agent of Tuberculosis) have a 5–10% lifetime risk of falling ill with tuberculosis. Those with compromised immune systems like people living with HIV, malnutrition, or diabetes, or people who are using tobacco, have a higher risk of falling ill.
• 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