The development of the Xpert MTB/RIF assay has been the most important breakthrough for the diagnosis of tuberculosis in decades.
This molecular-based test provides a rapid and sensitive method for the diagnosis of active TB and detection of organisms that are resistant to rifampin from the sputum of infected patients. It is superior to acid fast bacilli (AFB) smear microscopy, the relatively insensitive standard method for the initial detection of TB in most parts of the world.
In 2010, after WHO endorsed the use of the Xpert MTB/RIF assay (Cepheid) for the diagnosis of HIV-related TB and suspected drug-resistant TB, there was a huge scale up with widespread implementation of the test in many TB control programs in resource-limited settings following the purchase or provision of the platform and sputum cartridges with appropriate training. Indeed, in South Africa, with the highest incidence rate of TB and largest burden of cases of HIV-related TB in the world, Xpert has replaced smear microscopy as the initial test for diagnosis of pulmonary TB. By the end of 2016, this change also occurred in 27 other high-burden countries, along with the procurement of 6.9 million Xpert cartridges in dozens of countries! Surprisingly, in subsequent studies evaluating clinical outcomes, including randomized trials comparing Xpert with AFB smears, and despite the more rapid initiation of treatment of both drug-sensitive and drug-resistant TB with the use of Xpert, an impact on TB-related morbidity or mortality was not demonstrated.
To improve the diagnostic accuracy of this assay — specifically to enhance the sensitivity of the test in patients with paucibacillary disease (ie, smear-negative pulmonary TB and extrapulmonary TB), and to improve the reliability of detecting mutations associated with rifampin resistance — Cepheid developed the next-generation test, Xpert MTB/RIF Ultra. In a study by Dorman and colleagues from the NIAID-sponsored Tuberculosis Clinical Diagnostics Research Consortium and the Foundation for Innovation of New Diagnostics, multiple international sites were involved, all of which were in high-burden countries and heterogeneous with respect to rates of HIV coinfection and multidrug-resistant TB. In all subsets of patients with culture-positive TB as the gold standard, Xpert Ultra was more sensitive than Xpert, with the greatest difference between the two tests for smear-negative cases (17%), followed by patients with HIV infection (13%), and then all patients (5.4%). In contrast, there was a drop-off in specificity. Some patients were Ultra-positive and culture-negative, especially in those with prior TB. These patients are undergoing further follow-up to better understand the findings and whether retreatment of such patients is required. For more information about the study, click here.
As a result of this study, and other parallel studies performed in children and patients with extrapulmonary TB, in March WHO recommended that Xpert Ultra can be used as an alternative to the existing Xpert MTB/RIF test for the diagnosis of TB and detection of rifampin resistance in all settings. On balance, the considerable increase in sensitivity outweighs the small reduction in specificity in selected patients but will vary in different settings. Xpert Ultra will be especially useful in countries with high rates of TB/HIV coinfection given the higher prevalence of smear-negative TB, including those patients who are most immunocompromised with high rates of mortality. When Xpert was first rolled out, widespread implementation was facilitated by Cepheid providing cartridges at lower costs to low- and middle-income countries and in many sites that had no prior experience in molecular diagnostics. Because most of these sites still have the platforms, implementation with a different set of cartridges and new software should be a relatively smooth transition, with the knowledge that as with Xpert, Cepheid will continue to offer Xpert Ultra at a preferential price to low- and middle-income countries.
Ongoing and future studies will continue to evaluate the performance characteristics of other molecular tests licensed or in the pipeline, including GenoType MDRTBplus (Hain Lifescience), NTM + MDRTB (Nipro), Determine TB LAM (Alere), FluoroType MTBDR (Hain Lifescience), MTB Complex (BioGX), TB-LAMP (Eiken), RealTime MTB and MTB RIF/INH (Abbott), Truenat MTB (Molbio Diagnostics), and Xpert XDR (Cepheid). In addition, a great deal of research is being conducted to identify and evaluate various biomarkers in urine and blood for the diagnosis of active TB and latent TB infection, to monitor efficacy in the treatment of TB, and in vaccine studies. For diagnosis, biomarkers under investigation are both pathogen-specific (eg, lipoarabinomannan and Mycobacterium tuberculosis antigen 85 complex) and host specific (eg, interferon-inducible protein 10, microRNA, and transcriptional and metabolic signatures). It remains to be seen which of these tests will find their niche in the increasing and improving landscape of TB diagnostics. Stay tuned.
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David L. Cohn, MD, is a professor of medicine in the division of infectious diseases at the University of Colorado School of Medicine and an Infectious Disease News Editorial Board member.
Disclosure: Cohn reports no relevant financial disclosures.