Evaluation of Colorimetric Nitrate Reductase Assay in Liquid Medium for Detection of Resistance to First-line Antitubercular Drugs

BACKGROUND

On a global scale, India holds the distinction of having the greatest number of tuberculosis (TB) cases caused by Mycobacterium tuberculosis (MTB) complex. The study aimed at evaluating the sensitivity, specificity, accuracy, cost, rapidity, and feasibility of the performance of the colorimetric nitrate reductase-based antibiotic susceptibility (CONRAS) test against the indirect proportion method (IPM) on Lowenstein-Jensen media as the gold standard.

METHODS

A comparative cross-sectional study was performed on 51 MTB isolates. Fresh subcultures were used for drug susceptibility testing by IPM on the Lowenstein-Jensen medium and the CONRAS method in liquid medium. Quality control for drug susceptibility testing was done using a known sensitive strain of MTB (H37Rv) and strains resistant to both isoniazid (INH) and rifampicin (RIF) - multidrug-resistant (MDR), mono-resistant to RIF, streptomycin (STM), and ethambutol (EMB). Statistical analysis was performed using MedCalc software (Version 20.027).

RESULTS

CONRAS, carried out in microfuge tubes, was cost-efficient and easy to perform/interpret with most results being available in 10 days compared to 42 days in the case of IPM. The sensitivity, specificity, and accuracy of RIF and INH were 100%, 97.37%, and 98.04 and 93.33%, 97.59%, and 96.08%, respectively, which translates into an almost perfect agreement between the two methods as indicated by κ value of 0.905 and 0.949, respectively, for the two drugs. The performance of CONRAS was less satisfactory for STM and EMB when compared to IPM.

CONCLUSIONS

CONRAS may serve as a useful test for the detection of MDR-TB because of its accuracy, low cost, ease of performance/interpretation, and rapidity when compared to IPM on LJ medium. It does not involve the use of expensive reagents and equipment, as is the case with molecular methods like GeneXpert and line probe assay, making it a suitable option for the detection of MDR-TB in resource-poor settings.

Evaluation of BACTEC MGIT 960 system for testing susceptibility of Mycobacterium tuberculosis to first-line drugs in China

BACKGROUND

The purpose of this study was to evaluate the performance of the BACTEC MGIT 960 (M960) system compared with the proportion method (PM) on Löwenstein-Jensen (L-J) medium in a peripheral laboratory in China for the testing of Mycobacterium tuberculosis (MTB) susceptibility to streptomycin (SM), isoniazid (INH) rifampicin (RIF) and ethambutol (EMB) a combination known as SIRE.

METHODS

The susceptibility of 205 clinical isolates of MTB to SM, INH, RIF and EMB was performed with the M960 system. The drugs were tested at the following concentrations: 1.0 µg/ml for SM, 0.1 µg/ml for INH, 1.0 µg/ml for RIF, and 5.0 µg/ml for EMB. The results were compared with those obtained by the L-J PM. The L-J PM at an arbiter site was used to resolve any discordant results.

RESULTS

The overall consistency was 96.6% and concordance values were 95.6% for SM, 97.6% for INH, 98.0% for RIF and 95.1% for EMB. The overall sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of the M960 system for PM (the standard method) was 95.6%, 97.3%, 96.2% and 96.9% respectively, and the sensitivity were 93.3% for SM, 96.9% for INH, 97.4% for RIF and 94.6% for EMB, the specificity were 96.9% for SM, 98.2% for INH, 98.4% for RIF and 95.5% for EMB, the PPV were 94.6% for SM, 97.9% for INH, 97.4% for RIF and 94.6% for EMB, the NPV were 96.2% for SM, 97.3% for INH, 98.4% for RIF and 95.5% for EMB. The turnaround time with the M960 system (median 8.0 days, ranged from 5 to 14 days) was significantly shorter than that with the PM (28 days or 42 days).

CONCLUSION

There was a substantial degree of agreement between the two methods. The M960 system was a reliable and rapid method for SIRE susceptibility testing of tuberculosis in China.

Comparative evaluation under routine conditions of the nitrate reduction assay, the proportion assay and the MGIT 960 assay for drug susceptibility testing of clinical isolates of Mycobacterium tuberculosis

The performance of the nitrate reductase assay (NRA) was compared with the proportion method (PM) on Lowenstein-Jensen medium and the BACTEC MGIT960 assay under routine conditions using 160 clinical isolates of Mycobacterium tuberculosis with a high proportion of resistant strains. The mean time to obtain results was 8.8 days and the overall agreements between NRA and PM and NRA and M960 were 95% and 94%, respectively. NRA was easy to perform and represents a useful tool for the rapid screening of drug-resistant M. tuberculosis strains in low-resource countries.

Current developments and future perspectives for TB diagnostics

TB persists as a global epidemic with high morbidity and mortality, especially in low-income countries. It is the only infectious disease ever declared as a global emergency by the WHO. The HIV pandemic and the emergence of drug resistance represent two additional obstacles to better control of the disease. Important progress has been made in the last decade in TB diagnostics. Major needs still exist, such as the availability of a real point-of-care test, a better diagnosis of TB in immune-compromised populations and in children, and the possibility to predict progression to disease in latently infected people. This review will summarize the current developments in TB diagnostics and the perspectives for future developments in the field.

Direct nitrate reductase assay versus microscopic observation drug susceptibility test for rapid detection of MDR-TB in Uganda

The most common method for detection of drug resistant (DR) TB in resource-limited settings (RLSs) is indirect susceptibility testing on Lowenstein-Jensen medium (LJ) which is very time consuming with results available only after 2–3 months. Effective therapy of DR TB is therefore markedly delayed and patients can transmit resistant strains. Rapid and accurate tests suitable for RLSs in the diagnosis of DR TB are thus highly needed. In this study we compared two direct techniques - Nitrate Reductase Assay (NRA) and Microscopic Observation Drug Susceptibility (MODS) for rapid detection of MDR-TB in a high burden RLS. The sensitivity, specificity, and proportion of interpretable results were studied. Smear positive sputum was collected from 245 consecutive re-treatment TB patients attending a TB clinic in Kampala, Uganda. Samples were processed at the national reference laboratory and tested for susceptibility to rifampicin and isoniazid with direct NRA, direct MODS and the indirect LJ proportion method as reference. A total of 229 specimens were confirmed as M. tuberculosis, of these interpretable results were obtained in 217 (95%) with either the NRA or MODS. Sensitivity, specificity and kappa agreement for MDR-TB diagnosis was 97%, 98% and 0.93 with the NRA; and 87%, 95% and 0.78 with the MODS, respectively. The median time to results was 10, 7 and 64 days with NRA, MODS and the reference technique, respectively. The cost of laboratory supplies per sample was low, around 5 USD, for the rapid tests. The direct NRA and MODS offered rapid detection of resistance almost eight weeks earlier than with the reference method. In the study settings, the direct NRA was highly sensitive and specific. We consider it to have a strong potential for timely detection of MDR-TB in RLS.