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Understanding the Basics of Tuberculosis
Hello everyone! Big Grin As promised earlier, here’s an article on GeneXpert technology which is used as a confirmatory diagnosis test for Tuberculosis. Let us have a look at what it is and how it works.

GeneXpert is a nucleic acid amplification based test. It can not only identify M. tuberculosis DNA but also effectively detects any Rifampicin resistance shown by the bacteria. The name of this test - Xpert MTB/RIF is therefore deduced from the name of the causative organism of TB and Rifampicin.

It’s a cartridge based automated PCR test.

GeneXpert MTB/RIF was developed by Professor David Alland’s lab at University of Medicine and Dentistry of New Jersey in association with the Cepheid Inc. and Foundation for Innovative New Diagnostics(FIND), sponsored by the US National Institutes of Health (NIH).
It was officially launched in 2004.

As described earlier, there are many tests used to confirm TB cases, like Sputum test, Mantoux test, X-rays etc. But GeneXpert holds an edge over the others as it has :

·        Better sensitivity.
·        Highly TB specific testing.
·        Quicker than the traditional mycobacterial culture and drug susceptibility testing.
·        More efficient for detection of MDR-TB and HIV related TB.

Here's a picture of the cartridge loading:
            [Image: Gene_Xpert_USAID.jpg]

Steps of the test:
Probable patient’s sputum collected
Purification and concentration of M. tuberculosis from sputum
Isolation of bacterial genome (Sonication)
PCR to amplify genomic DNA
 Rifampicin resistant mutations searched by the device
Identification and target characterisation by six colour laser detector/beacons.

Here's a diagram showing the steps:

[Image: xpert_mtb_rif_procedure.jpg]

The entire process is fairly quick and gives us the result under 2 hours.

A shortcoming of this test is that it detects both live and dead bacteria. Hence, it can’t be used for monitoring on-going treatment. Also, the cost of each test as well as the cartridges and related devices are quite high. The shelf life of the cartridges is low. Moreover, the devices have limited throughput per day.

Cepheid Inc. continues developing new and improved versions of the test device. GeneXpert is being promoted worldwide by WHO in association with MIND.
Let’s hope for the betterment and further globalisation of this technique. Let’s hope for a TB free world!

Fun While Learning Visit here for an interactive map by WHO on the GeneXpert roll-out!


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