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New rapid point-of-care test detects TB resistance

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September 14, 2017

An investigational assay could accurately detect drug-resistant tuberculosis infections and could be used as a rapid point-of-care test to help clinicians determine the right treatment for their patients, researchers reported in The New England Journal of Medicine.

The assay, which was developed in the lab of David Alland, MD, chief of infectious diseases and associate dean of clinical research at Rutgers New Jersey Medical School, looks for resistance to isoniazid, a key front-line TB drug, plus the fluoroquinolones moxifloxacin and ofloxacin and the aminoglycosides amikacin and kanamycin — all important treatments for multidrug-resistant TB.

According to Alland, the assay is meant to be used as a reflex test only in patients who test positive for TB using one of two assays made by Cepheid. He said the new assay will speed up the detection of drug-resistant TB.

“This is important in getting TB patients on the right, potentially life-saving treatment, implementing infection control procedures that prevent the spread of drug-resistant TB [and] improving triage of TB patients into or out of short-course multidrug-resistant treatment that is highly beneficial to multidrug-resistant patients when appropriate,” Alland told Infections Disease News.

According to the study, multidrug-resistant TB occurs when an infection is resistant to isoniazid and rifampin. Extensively drug-resistant TB infections are additionally resistant to fluoroquinolones and second-line injectables such as amikacin and kanamycin.

Like Cepheid’s Xpert MTB/RIF assay — recently updated as the Xpert MTB/RIF Ultra — the new cartridge developed by Alland and colleagues can be run on GeneXpert software. Xpert MTB/RIF has been widely used to detect rifampin-resistant TB infections worldwide, but Alland and colleagues said it does not provide further information to help patients get the right treatment. In contrast, they said the investigational assay can detect 25 genetic mutations associated with resistance to isoniazid, fluoroquinolones and aminoglycosides, providing results from unprocessed sputum samples in just over 2 hours, with minimal hands-on technical time.

For their study, they enrolled more than 400 adults with symptoms of TB from June 2014 to June 2015 in South Korea and China. Of these patients, 308 had TB infections confirmed by a culture and were included in the main analysis for drug-susceptibility. The investigational assay detected 98% of TB infections, same as the Xpert MTB/RIF assay.

When using DNA sequencing as a reference standard for resistance, the investigational assay met WHO’s diagnostic sensitivity target of 95% for isoniazid, fluoroquinolones and amikacin but missed the target for kanamycin by approximately 2 percentage points, Alland and colleagues reported. They said the specificity was 99.6% or greater for all of the study drugs when compared with DNA sequencing, exceeding the specificity target of 98% set by WHO.

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The investigational assay did not compare as well with culture-based phenotypic testing — an unsurprising finding, Alland and colleagues said. In these tests, sensitivities of the assay for detecting resistance were 83.3% for isoniazid, 88.4% for ofloxacin, 87.6% for moxifloxacin at a critical concentration of 0.5 g/mL, 96.2% for moxifloxacin at a critical concentration of 2 g/mL, 71.4% for kanamycin and 70.7% for amikacin. The specificity of the assay for the detection of phenotypic resistance was 94.3% or greater for all drugs except moxifloxacin at a critical concentration of 2 g/mL, with a specificity of 84%.

Alland and colleagues said the investigational assay was effective at identifying patients who were microbiologically eligible or ineligible for a shortened treatment regimen for drug-resistant TB based on their infection’s response to fluoroquinolones or aminoglycosides.

“Among patients with rifampin-resistant TB, the predictive value of a positive investigational-assay result for resistance to fluoroquinolones, aminoglycosides or both was 94%,” Alland and colleagues wrote.

“Although we did not test this strategy, we speculate that a positive test result could be used to triage patients away from the new shortened treatment regimen for multidrug-resistant TB and toward treatment centers with the capacity for comprehensive drug-susceptibility testing and with experience treating highly drug-resistant TB.”

Conversely, Alland and colleagues reported a less-than-ideal predictive value of a test in which no resistance was detected.

“If the investigational assay were used as the sole test for triaging patients with rifampin-resistant TB onto or away from the shorter treatment regimen for multidrug-resistant TB, approximately 20% of patients who have TB with phenotypic resistance to fluoroquinolones, aminoglycosides, or both would be inappropriately triaged to receive the shorter regimen,” they wrote.

“However, this is predicted to be a shortcoming of any molecular test that interrogates the set of common resistance-associated genetic loci targeted by the investigational assay. Currently, there are no other rapid, point-of-care tests to facilitate evidence-based treatment selection for patients with rifampin-resistant TB.” – by Gerard Gallagher

Disclosure: Cartridges and other equipment used in the study were provided by Cepheid. Alland reports grants from Cepheid, the Foundation for Innovative New Diagnostics, the Henry M. Jackson Foundation, Johns Hopkins University School of Medicine and NIH, and having patents related to the work. Please see the full study for a list of all other authors’ relevant financial disclosures.

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