GeneXpert MTB/RIF for Tuberculosis Diagnosis

GeneXpert MTB/RIF is a cartridge-based nucleic acid amplification test (NAAT) for simultaneous rapid tuberculosis diagnosis and rapid antibiotic sensitivity tests. It is an automated diagnostic test that can identify Mycobacterium tuberculosis (MTB) DNA and resistance to rifampicin (RIF).

It was co-developed by the laboratory of Professor David Alland at the University of Medicine and Dentistry of New Jersey (UMDNJ). Cepheid Inc. and Foundation for Innovative New Diagnostics, with additional financial support from the US National Institutes of Health (NIH).

In December 2010, the World Health Organization (WHO) endorsed the Xpert MTB/RIF for use in TB endemic countries. This followed 18 months of assessment of its field effectiveness in TB, MDR-TB and TB/HIV co-infection. This test and others that are likely to follow could have the potential to improve the diagnosis of TB in those that are likely to be missed by traditional tests.

The CDC said in 2015 that the Xpert MTB/RIF test was "revolutionizing tuberculosis (TB) control by contributing to the rapid diagnosis of TB disease and drug resistance. The test simultaneously detects Mycobacterium tuberculosis complex (MTBC) and resistance to rifampin (RIF) in less than 2 hours. In comparison, standard cultures can take 2 to 6 weeks for MTBC to grow and conventional drug resistance tests can add 3 more weeks."

Background of the GeneXpert  test

Tuberculosis is one of the deadliest public health threats today. Traditionally, tuberculosis is mostly being diagnosed by a combination of chest X-rays, the staining of sputum with special dyes followed by microscopy, the growth of Mycobacterium tuberculosis in culture and the Mantoux test.

The sputum smear microscopy is easy to do and is very cheap and combined with chest X-rays has been used for a long time by TB control agencies worldwide. However, the sputum smear microscopy(sputum AFB ) test has some problems in HIV-positive patients and children, as well as patients with a low bacterial load.

GeneXpert  MTB/RIF test exhibits high sensitivity and specificity for detecting pulmonary TB disease. An in vitro study demonstrated a limit of detection of as few as 131 colony-forming units/mL of MTB, compared with approximately 10,000 colony-forming units/mL with conventional smear microscopy.

Drug susceptibility could only be diagnosed from the growth of Mycobacterium tuberculosis in a culture which can take as long as six weeks and needs high biosafety labs and is costly.

The determination of drug susceptibility is particularly relevant because Mycobacterium tuberculosis becomes increasingly resistant to two of the major anti-tuberculosis drugs, isoniazid, and rifampicin. This form of tuberculosis is called multi-drug-resistant tuberculosis (MDR-TB) is rapidly on the rise globally.

MDR-TB cases need different antibiotics and are more difficult to treat owing to higher costs of drugs and longer regimens.
How the test works

The GeneXpert MTB/RIF detects DNA sequences specific for Mycobacterium tuberculosis and rifampicin resistance by a polymerase chain reaction. It is based on the Cepheid GeneXpert system, a rapid, simple-to-use nucleic acid amplification test (NAAT).

The GeneXpert MTB/RIF purifies and concentrates Mycobacterium tuberculosis bacilli from sputum samples, isolates genomic material from the captured bacteria by sonication and subsequently amplifies the genomic DNA by PCR. The process identifies most of the clinically relevant Rifampicin resistance inducing mutations in the RNA polymerase beta (rpoB) gene in the Mycobacterium tuberculosis genome in a real-time format using fluorescent probes called molecular beacons.

Results are obtained from unprocessed sputum samples in 90 minutes, with minimal biohazard and very little technical training required to operate.

Credits to:

gene Xpert MTB/RIF