What is the Genexpert test?
What is usually referred to as the Genexpert test, has now been developed into a series of molecular tests. Depending on the cartridge/assay used, the Genexpert can test for a number of infectious diseases including TB & COVID.
What is the CB-NAAT?
In India, where the Genexpert is starting to be widely used, it is known as the CB-NAAT. The full name of the CB-NAAT test is the Cartridge Based Nucleic Acid Amplification Test.
How does the Genexpert work in testing for TB?
The Genexpert diagnoses TB by detecting the presence of TB bacteria. The test is a molecular TB test which detects the DNA in TB bacteria. It uses a sputum sample and can give a result in less than 2 hours.
The Genexpert can also detect the genetic mutations associated with resistance to the drug Rifampicin.1"Two hour detection of MTB and resistance to rifampicin", Cepheid International, 2011, www.cepheidinternational.com
Genexpert for drug resistant TB
In July 2020 it was announced that a new test had been developed which could detect resistance to both first line and second line drugs. This means that clinicians can find out in 90 minutes which drugs a patient is resistant to, and prescribe the correct treatment for both MDR-TB and XDR-TB.
By providing test results in just a few hours, the Xpert MTB/XDR test will save lives2New Rapid Molecular Test for Tuberculosis Can Simultaneously Detect Resistance to First- And Second-Line Drugs, 2020, PR Newswire, https://www.prnewswire.com/news-releases/new-rapid-molecular-test-for-tuberculosis-can-simultaneously-detect-resistance-to-first--and-second-line-drugs-301094563.html
Who developed the Genexpert test?
The Genexpert has been developed by the Foundation for Innovative New Diagnostics (FIND), who have partnered with the Cepheid corporation and the University of Medicine and Dentistry of New Jersey. Some of the funding for the development of the Genexpert was provided by the NIH.3“Policy statement: Automated real-time nucleic acid amplification technology for rapid and simultaneous detection of tuberculosis and rifampicin resistance: Xpert MTB/RIF system”, WHO, Geneva, 2011
When did the Genexpert test first become available?
In December 2010 WHO endorsed the Genexpert technology and released a recommendation and guidance for countries to incorporate the new test into their programs.
Alternatives to GeneXpert
The TrueNat test was endorsed by the World Health Organisation in January 2020. The TrueNat test and CB-NAAT (Genexpert) are designed to be used in different situations.
What is the cost of the Genexpert TB test?
FIND originally negotiated discounts for the Genexpert test in 120 high burden and in all low and middle income countries and the initial cost was approximately US$17 per cartridge.
In June 2012 the organisation UNITAID approved funding of $30 million with the aim of scaling up access to the Genexpert test. It was hoped that the roll out of the test would bring the price down from US$17 to US$10. However the roll out proceeded slower than expected and it is believed that many customers delayed ordering as they expected the price to be reduced following the UNITAID announcement.
It was subsequently announced in August 2012 that Cepheid had entered into an agreement with the Bill and Melinda Gates Foundation, PEPFAR, and USAID to further bring down the cost of the test. It was agreed that the test would immediately be available for US$9.98 with the difference between this and the earlier US$16.86 being paid by the collaborators with an initial US$3.5 million being provided by the Bill and Melinda Gates Foundation.4Siddiqi, Kamran “Clinical diagnosis of smear-negative pulmonary tuberculosis in low-income countries: the current evidence”, The Lancet Infectious Diseases, Vol 3, May 2003, 288
www.thelancet.com/journals/ The concessionary cost is however only available for the public sector and for NGOs. This causes problems for countries such as India, where initial diagnosis often takes place in the private sector.
In addition to the cost of each test there is the cost of the Genexpert device. The device comes in different sizes providing for a different number of samples to be tested simultaneously. The commonly used GX4 which has 4 modules, has a cost of approximately US$17,000. There are in addition costs for delivery and installation, and there is an annual servicing cost.5Abdurrahman, S “The hidden costs of installing xpert machines in a tuberculosis high-burden country: experiences from Nigeria”, Pan Afr Med J, 2014
The use of the Genexpert test
WHO recommended that the test should be used as the initial diagnosis test in individuals suspected of having MDR TB, or HIV associated TB. They also suggested that it could be used as a follow on test to microscopy in settings where MDR TB and/or HIV is of lesser concern, especially in smear negative specimens, because of the lack of accuracy of smear microscopy. They did however say that they recognised the major resource implications of using it in this second way.6“Roadmap for rolling out Xpert MTB/RIF for rapid diagnosis of TB and MDR TB”, WHO, Geneva, 8 December 2010 www.who.int/tb/laboratory/roadmap_xpert_mtb-rif.pdf
WHO did also emphasize that the test does not eliminate the need for conventional microscopy culture and drug sensitivity testing, as these are still required to monitor treatment progress and to detect other types of drug resistance. The Genexpert MTB/RIF cannot be used for treatment monitoring, as it detects both live and dead bacteria.
Further guidance was subsequently provided by WHO on the rapid implementation of the new test.7“Xpert MTB/RIF - rapid TB test - WHO publishes policy and guidance for implementers”, WHO, Geneva, May 2011
www.who.int/tb/features_archive/ 8“Rapid implementation of the Xpert MTB/RIF diagnostic test. Technical and operational ‘How-to’. Practical considerations, WHO, Geneva, May 2011 apps.who.int/iris/bitstream/10665/44593/1/9789241501569_eng.pdf
Advantages and disadvantages of the test
WHO have been particularly keen on the use of the test and in 2010 said that:
“This new test represents a major milestone for global TB diagnosis and care. It also represents new hope for the millions of people who are at the highest risk of TB and drug resistant disease.”
Dr Mario Ravihlione, Director of WHO Stop TB Department 9“WHO endorses new rapid tuberculosis test”, WHO, Geneva, 8 December 2010
The test does however have disadvantages as well as advantages.
The disadvantages include:
- The shelf life of the cartridges is only 18 months;
- A very stable electricity supply is required;
- The instrument needs to be recalibrated annually;
- The cost of the test;
- The temperature ceiling is critical.10Trebucq, A “Xpert MTB/RIF for national tuberculosis programmes in low income countries: when, where and how?”, Int J Tuberc Lung Dis, October 2011 1567-1571
The main advantages of the test are, for diagnosis, reliability when compared to sputum microscopy and the speed of getting the result when compared with the culture test. For diagnosis of TB, although sputum microscopy is both quick and cheap, it is often unreliable. It is particularly unreliable when people are HIV positive. Although culture gives a definitive diagnosis, to get the result usually takes weeks rather than the hours of the Genexpert test.
The main advantage in respect of identifying rifampicin resistance, is again the matter of speed. Normally to get any drug resistance result takes weeks rather than hours.
A considerable number of articles have been written reviewing the use of the Genexpert test, and the WHO maintains a list of some of the major articles.11“Published evidence and commentary on the Xpert MTB/RIF assay”, WHO, 5 April 2013 www.stoptb.org/wg/gli/assets/documents/map/XpertPublications.pdf
Generally the views given are extremely variable, with some people considering that the test is extremely useful, as well as cost effective, and should be used in as many places as possible as soon as possible. Other people consider that it is not really suitable and practical at the present time for major use in low and middle income countries.12Trebucq, A “Xpert MTB/RIF for national tuberculosis programmes in low income countries: when, where and how?”, Int J Tuberc Lung Dis, October 2011 1567-1571 www.ncbi.nlm.nih.gov/pubmed/2200511013Singh, J “The ethics of national tuberculosis programmes in low income countries not rolling out Xpert MTB/RIF”, Int J Tuberc Lung Dis, October 2011 1563 www.ncbi.nlm.nih.gov/pubmed/22118160 14Evans, C “GeneXpert - A game changer for tuberculosis control?”, PLoS Med, July 2011 www.plosmedicine.org/article/
In October 2013 a multi centre trial comparing the use of the Genexpert to microscopy, concluded that using the Genexpert meant that more patients had a same day diagnosis and same day treatment initiation, but the benefits did not translate into lower TB morbidity.15Theron, G “Feasibility, accuracy, and clinical effect of point-of-care Xpert MTB/RIF testing for tuberculosis in primary-care settings in Africa: a multicentre, randomised, controlled trial”, The Lancet 28 October 2013 www.thelancet.com
“Earlier diagnosis by the Genexpert test did not reduce overall severity of TB related illness.” Study leader, Keertan Dheda, University of Cape Town 16“Sputum test may not hold key to TB eradication, study says”, Medical Express 27 October 2013 www.medicalxpress.com
The global roll-out of the Genexpert
The global rollout of the test was initially slower than organisations such as WHO and FIND would probably have wished. By the end of 2011 a total of 460 Genexpert machines (comprising 2,401 modules) and 591,450 Genexpert test cartridges had been bought for 47 countries, using the special pricing arrangements arranged by FIND with Cepheid.
Over half of the Genexpert cartridges (330,540 cartridges) had been bought for use in South Africa. Other countries who had bought cartridges were Pakistan (21,440), Kenya (20,140), the Philippines (17,440) and Swaziland (16,600).17“Update on Xpert MTB/RIF roll out Xpert MTB/RIF February 2012”, WHO, February 2012
www.stoptb.org/wg/gli/xpert.asp There is more about TB in South Africa and about TB in Pakistan. By the end of September 2012 a total of 898 Genexpert machines (comprising 4,660 modules) and 1,482,550 cartridges had been purchased in 73 of the 145 countries eligible for concessional pricing. There was an increase in the purchase of cartridges in the third quarter of 2012, which it is likely was associated with the drop in cartridge price around that time.18“Update on Xpert MTB/RIF roll out Xpert MTB/RIF December 2012”, WHO, December 2012
The UNITAID TBXpert Project
In September 2013 UNITAID and WHO started the largest rollout of GeneXpert with UNITAID investing US$25.9 million to purchase over 220 GeneXpert machines and 1.4 million test cartridges for 21 countries in Africa, Eastern Europe and Asia.19“Largest ever roll-out of GeneXpert rapid TB test machines September 2013”, UNITAID, September 2013
www.unitaid.eu/en/resources/press-centre/releases/ However, it is still a challenge as the machines need a continuous power supply and air conditioning is also needed where there are high temperatures.20“Largest ever push to diagnose tuberculosis within two hours”, Citizen News Service, September 14 2013 www.tbonline.info/posts/2013/9/20/largest-ever-push-diagnose-tuberculosis-within-two/
By 2014 the use of the GeneXpert had expanded significantly. In total 4.8 million cartridges were bought in 2014 by 116 low and middle income countries at concessional prices. By 2015 69% of the countries recommended using Genexpert as the initial diagnostic test for people at risk of developing drug-resistant TB and 60% recommended it as the initial diagnostic test for people living with HIV. Between 2011 and 2016 more than 16 million tests had been carried out in 122 countries.21“Development, roll-out and impact of Xpert MTB/RIF for tuberculosis: what lessons have we learnt and how can we do better?”, European Respiratory Journal 2016 48: 516-525 erj.ersjournals.com/content/48/2/516
The use of the GeneXpert test in India
The GeneXpert test first started to be used to test for TB in India when it was used in reference laboratories as part of a demonstration project. Subsequently a two year project took place to evaluate the use of the test at 18 sites across India.22Sayyed-Pathan, N “2 hour test pilot to start at Dharavi”, DNA, Mar 23 2012 www.dnaindia.com/mumbai/ In March 2012 it was announced which hospitals were going to benefit by being the first sites where the GeneXpert will be installed.23“Largest ever push to diagnose tuberculosis within two hours”, Citizen News Service, September 14 2013 www.tbonline.info/posts/2013/9/20/largest-ever-push-diagnose-tuberculosis-within-two/
It has been said that it might not be as effective in India as elsewhere as the heat and humidity could affect the equipment. The result could be that:
“The technique cannot be used for routine testing. We have pilot projects at 18 sites going on to test its feasibility in the Indian situation where the climate and temperature conditions are different from the western countries. … Solid culture/sputum testing remains the golden standard for Indian conditions.”
Ashok Kumar, Deputy Director General, Central TB control Division, Ministry of Health and Family Welfare 24“90-minute TB test not a game changer for India”, Indo Asian News Service, 29 August 2011 news.bioscholar.com/2011/08/
Subsequently though the CB-NAAT test, as it is known in India, has started to be widely used. There is more about the diagnosis of TB in India.
The CB-NAAT is also now starting to be used in India to diagnose TB in Children.25K.S.Sahana, "Usage of Cartridge Based Nucleic Amplification Test (CB-NAAT/GeneXpert) test as diagnostic modality for pediatric tuberculosis; case series from Mangalore, South India", Journal of Clinical Tuberculosis and other Mycobacterial disease, 2017, https://www.sciencedirect.com/science/article/pii/S2405579417300347
New developments Ultra & Omni
In 2017 the Xpert Ultra assay was launched. It was found to be non-inferior to the standard Xpert MTB assay for the diagnosis of TB and the detection of rifampicin resistance and it can be used as an alternative to the latter in all settings. Cepheid plans to gradually phase out and replace the current Xpert MTB assay with the Ultra assay.26“WHO meeting report of a technical expert consultation: Non-inferiority analysis of Xpert MTB/RIF Ultra compared to Xpert MTB/RIF”, Geneva, World Health Organisation; 2017, www.who.int/tb/areas-of-work/laboratory/
A new device called the GeneXpert Omni is currently under development. It is intended for point of care testing for TB and rifampicin resistance, using the same cartridges as those used in the current Genexpert machine. It is expected that it will be smaller, lighter and less expensive than the current Genexpert. It will also come with a built-in 4 hour battery.
This page was last updated in May 2021.
Author Annabel Kanabus
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