Point-of-care assay zeros in on drug-resistant TB mutations

Results could guide therapeutic decisions for TB patients

This assay detected M.tuberculosis mutations associated with resistance to isoniazid, fluoroquinolones, and aminoglycosides with relatively high sensitivity and specificity, reported Yingda L. Xie, MD, of the National Institute of Allergy and Infectious Diseases (NIAID) in Bethesda, and colleagues.

However, both sensitivity and specificity were higher when comparing the investigational assay to DNA sequencing as opposed to culture-based drug susceptibility testing, the authors wrote in the New England Journal of Medicine.

They noted that improper diagnosis and treatment of drug-resistant tuberculosis (TB) can lead to increased morbidity and mortality, as well as a higher risk of transmission.

Recently, the World Health Organization (WHO) endorsed a regimen of fluoroquinolones and "second-line injectable drugs," such as aminoglycosides and capreomycin, for a "shortened regimen" of treatment for drug-resistant TB, but this treatment may only work in patients who are not resistant to these drugs and are thus "microbiologically eligible" for it.

Currently, the Xpert MTB/RIF cartridge-based system can identify if a patient has TB resistant to rifampin, but "provides no further information to guide the selection of appropriate antibiotics or to promptly identify and triage" patients with multidrug-resistant TB.

The authors noted that M.tuberculosis resistance to isoniazid, fluoroquinolones, and second-line injectables have been linked to approximately 25 mutations in six genes and promoter regions. This new cartridge is designed to detect these mutations.

Karen Lacourciere, PhD, also of NIAID, told MedPage Today that this study could be "very impactful clinically."

"The current test gives us partial drug sensitivity information, but oftentimes not enough -- it couldn't tell us what the right regimen to put [patients] on would be," she said. "This gives us a more complete picture of what the correct treatment regimen is, and with proper treatment, we can stop the transmission and curb the spread of disease."

The researchers examined sputum samples from 308 patients who were culture-positive for M. tuberculosis from South Korea and China. They were prospectively enrolled in either the drug-resistance risk-group or the case-detection group.

Phenotypic culture-based drug-susceptibility testing found 63% of participants had infections resistant to one or more drugs, including 55 with multidrug-resistant TB, 54 with multi-drug resistant TB also resistant to fluoroquinolones or aminoglycosides, and 39 with "extensively drug-resistant tuberculosis."

When using drug-susceptibility testing as the standard, sensitivity of the investigational assay was:

• 83.3% for isoniazid (95% CI 77.1-88.5)
• 88.4% for ofloxacin (95% CI 80.2-94.1)
• 87.6% for moxifloxacin (95% CI 79.0-93.7)
• 71.4% for kanamycin (95% CI 56.7-83.4)
• 70.7% for amikacin (95% CI 54.5-83.9)

However, when using DNA sequencing, sensitivity of the investigational assay was:

• 98.1% for isoniazid (95% CI 94.4-99.6)
• 95.8% for fluoroquinolones (95% CI 89.6-98.8)
• 92.7% for kanamycin (95% CI 80.1-98.5)
• 96.8% for amikacin (95% CI 83.3-99.9)

Compared with drug-susceptibility testing, specificity of the assay was 94.3% or greater for all drugs except one (moxifloxacin at a critical concentration of 2.0 μg per mL at 84.0%), but compared with DNA sequencing, specificity for all drugs was 99.6% (95% CI 97.9-100.0) or greater, the authors said.

They noted that the WHO targets for diagnostic sensitivity of 95% and specificity of 98% for "next-generation molecular drug-susceptibility tests with sequencing as the reference standard." The investigational assay met the target for isoniazid, fluoroquinolones, and amikacin and missed the target for kanamycin "by approximately two percentage points."

Moreover, the authors found that the investigational assay identified 48 of 53 patients (90.6%, 95% CI 79.3-96.9) who were microbiologically eligible for the shortened regimen, as well as 81 of 92 (88.0%, 95% CI 79.6-93.9) who were not microbiologically eligible.

Study limitations include the fact that participants had high sputum bacillary burdens; results will need to be confirmed in patients with smear-negative TB and "with a reference standard of multiple cultures." In addition, geographic representation of both participants and M. tuberculosis strains was limited.

Lacourciere said that the next step in this research would be to invest in a rapid drug susceptibility test to be used by clinicians in an endemic setting.

"This is more likely to be used in a lab or a hospital setting that requires the use of electricity, but we want to develop a point-of-care rapid drug susceptibility test assay that could be used in the field," she said.