Molecular Tuberculosis Diagnosis

More than 9 million new M tuberculosis (MTB) cases occur annually worldwide. Tuberculosis (TB) is responsible for 1.7 million deaths per year - the vast preponderance in resource limited settings. Globally, TB is an extraordinarily common cause of illness and death among HIV infected patients.
Multiple approaches to improve TB diagnosis are in development. A single test, recently endorsed by the World Health Organization (WHO), has the potential to lead a revolution in the diagnosis of active TB disease and multidrug resistant (MDR) TB: GeneXpert® MTB/RIF (Cepheid-USA).

The GeneXpert MTB/RIF test, which is based on nucleic acid amplification and detection of an MTB specific region of the rpoB gene, uses real-time polymerase chain reaction with molecular beacons. The test also detects mutations associated with rifampin resistance, which serves as a proxy for MDR-TB. This fully automated system integrates sputum processing, DNA extraction, and amplification to diagnose TB.

The Xpert 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. 

The WHO convened an expert group, stating (in December 2010): "XPert should be used as the initial diagnostic test in individuals suspected of having MDR-TB or HIV associated TB (strong recommendation)," and "Xpert may be used as a follow-on test to microscopy where MDR and/or HIV are of lesser concern, especially in smear negative specimens (conditional recommendation).

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