Gene Xpert

The Gene Xpert is a new test for tuberculosis. It can find out if a person is infected with TB, and also if the TB bacterium of the person has resistance to one of the common TB drugs, rifampicin.

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The four-cartridge Gene Xpert

Contrary to the tests that exist at the moment, it works on a molecular level to identify mycobacterium tuberculosis. This means that it does not use microscopy but a kind of chemical test to look for the TB bacterium. The GeneXpert is a small machine, about the size of a microwave oven, that can fit easily on a small table. It has been developed in the United States by a company called Cepheid and an organisation called the Foundation for Innovative New Diagnostics (FIND), supported by the American National Institutes of Health. The test is also called the Xpert MTB/RIF (for mycobacterium tuberculosis and rifampicin).

How it works

The Gene Xpert is a machine that can detect mycobacterium tuberculosis in a sample of sputum. A person suspected of having TB needs to give a sputum sample, which the health care worker than places in a small tube. From the tube, the sample is fed into the machine, and then biochemical reactions are started to see if the sample contains the TB bacterium. The machine looks for the DNA specific to the TB bacterium. If there are TB bacteria in the sample, the machine will detect their DNA and automatically multiply it. This technique is called PCR (polymerase chain reaction), and allows the machine to also look at the structure of the genes. This is important to detect if a TB bacterium has developed resistance to drugs. The DNA of the TB bacterium is, in a way, like a long string of different colours. If one or more of the colours change (if there is a mutation in the DNA), then the bacterium can become resistant to certain TB drugs. The Gene Xpert can test for resistance to one of the most common TB drugs, rifampicin. This means that it can tell us two things: first, whether or not a person has TB, and second, whether or not the TB that the person has can be treated with rifampicin. The test is very quick and only takes about two hours – much faster than the other TB tests, which usually take at least a few days.

The Gene Xpert comes in various sizes. The smallest machine has capacity for four cartridges (in other words four tests can be run at a time). The largest has capacity for 100 cartridges.

Pros and Cons

A big advantage of the Gene Xpert is that it can give very fast results. A person can know on the same day whether or not she has TB. It is also very important and good that the Gene Xpert can detect if the TB of the person is resistant to rifampicin. This is a good indicator that the patient might have multi-drug resistant (MDR) TB, which is defined as resistance to both rifampicin and isoniazid. If the health care worker knows from the start that the TB of a person is resistant to rifampicin, she can choose other drugs to treat the TB effectively. The Gene Xpert is also more sensitive than most other TB tests (up to 98%) – this means it can detect TB better than these tests. This is important because it is often very difficult to detect TB; often people with compromised immune systems, especially if they have AIDS, test negative for TB even though they are sick with this disease.

Unfortunately, the Gene Xpert is more expensive than the standard sputum-smear tests. It needs a thorough check at least once a year. It also requires an uninterrupted supply of electricity. The machine needs to be connected to a computer. Therefore, it is important that all clinics and hospitals where the Gene Xpert is used have proper and reliable infrastructure and security measures in place.

Price

The market price of the Gene Xpert is approximately $60,000.

FIND and Cepheid have entered an agreement that fixes the pricing of the device and the cartridges for the public sectors of 120 low- and middle-income countries, as well as various organisations providing health services in those countries. For these institutions, the price in 2011 is $17,000 for the 4-cartridge machine and approximately $17 per cartridge.

The Gene Xpert in South Africa

In 2010, the World Health Organisation decided that the Gene Xpert test should, in many settings, be the first test conducted to find out if a person has TB. In South Africa, the Gene Xpert has been tested in facilities in the Western Cape, Kwazulu-Natal and the Gauteng. About 30 machines have been bought by the state. There is an urgent need to have the Gene Xpert in all clinics in South Africa.

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By Alex Müller

Published: Oct. 3, 2011, 1:34 p.m.

Last updated: Oct. 3, 2011, 1:51 p.m.

Tags: Diagnostics

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