Rapid Tests Match Culture Gold Standard for Diagnosing Drug-Resistant Tuberculosis

By LabMedica International staff writers
Posted on 22 Sep 2015

A recent paper compared three "rapid" tests for diagnosis of drug-resistant tuberculosis (TB) to the standard culture technique, which can take up to a month to yield results.

Investigators at the University of California, San Diego (USA) evaluated samples from 1,128 M/XDR-TB suspects. MDR-TB is defined as TB resistant to both isoniazid (INH) and rifampin (RIF) while XDR-TB is MDR-TB with additional resistance to any fluoroquinolone, including moxifloxacin (MOX) or ofloxacin (OFX), and one or more of the injectables: amikacin (AMK), kanamycin (KAN), or capreomycin (CAP).

Image: Mycobacterium tuberculosis (stained purple) in a tissue specimen (blue) (Photo courtesy of the CDC – [US] Centers for Disease Control and Prevention).

Sputum samples were examined by Line Probe Assay (LPA), Pyrosequencing (PSQ), and Microscopic Observation of Drug Susceptibility (MODS) and compared to the Becton, Dickinson and Company (Franklin Lakes, NJ, USA) BACTEC MGIT960 reference standard to detect M/XDR-TB directly from patient samples collected at TB clinics in India, Moldova, and South Africa.

The Line Probe Assay is a DNA strip test that allows simultaneous molecular identification of tuberculosis and the most common genetic mutations causing resistance to rifampicin and isoniazid. This technology can diagnose MDR-TB directly from smear-positive sputum samples, providing results in just five hours—an enormous improvement on the one to two months needed for conventional culture.

Pyrosequencing (PSQ) is a rapid sequencing technique used in several commercially available platforms; however, it is still under development for use as a TB diagnostic. The noncommercial PSQ M/XDR-TB assay used in this study involved sequencing and interpreting targeted, resistance-conferring regions of the Mtb (Mycobacterium tuberculosis) genome. PSQ has shown high sensitivity and specificity for detection of M/XDR-TB isolates in controlled laboratory settings, but has not been evaluated in a large clinical trial.

Microscopic Observation of Drug Susceptibility (MODS) was carried out with a noncommercial assay where drug-containing wells were inoculated directly with concentrated sputum and subsequent Mtb growth was observed microscopically.

Results showed that the specificity for all three assays was excellent: 97%–100% for isoniazid (INH), rifampin (RIF), moxifloxacin (MOX) and ofloxacin (OFX) and 99%–100% for amikacin (AMK), capreomycin (CAP) and kanamycin (KAN) resistance. Sensitivities were lower, but still very good: 94%–100% for INH, RIF, MOX and OFX, and 84%–90% for AMK and CAP, but only 48%–62% for KAN. In terms of agreement, statistically significant differences were only found for detection of RIF (MODS outperformed PSQ) and KAN (MODS outperformed LPA and PSQ) resistance. Mean time-to-result was 1.1 days for LPA and PSQ, 14.3 days for MODS, and 24.7 days for MGIT.

"Our study shows that TB testing that once took two to three months can now be done in as little as a day," said contributing author Dr. Richard Garfein, professor in the division of global public health at the University of California, San Diego. "This means we can put people on the right medications sooner, spare them the toxic effects of drugs that are ineffective and prevent the development of drug resistant forms of TB that can occur when the wrong medications are given."

The study was published in the August 31, 2015, online edition of the journal PLOS ONE.

Related Links:
University of California, San Diego
Becton, Dickinson and Company