Abstract
With the spread of multidrug-resistant tuberculosis (MDR-TB) strains there is an increasing need for new accurate and cost-effective methods for a rapid diagnostic and drug susceptibility testing (DST), particularly in low-income countries where tuberculosis is hyperendemic. A colorimetric assay using 3-(4, 5-dimethylthiazol-2-yl)-2, 5- diphenyltetrazolium bromide (MTT) has been suggested as a promising method for DST, especially to rifampicin. In this study, we standardized and evaluated the MTT assay for a rapid direct detection of rifampicin and isoniazid resistant Mycobacterium tuberculosis strains from sputum specimens using Lowenstein-Jensen (LJ) culture medium as a gold standard. The MTT assay sensitivity, specificity, positive and negative predictive values for rifampicin were 100%, 86%, 100%, 99%, respectively. For isoniazid, the MTT assay had a 100% sensitivity, specificity, positive and negative predictive values. Interestingly, the MTT assay gave interpretable results within two weeks for 94% of the samples compared to 7–14 weeks for LJ media. Overall, an excellent agreement was observed between MTT assay and LJ proportion method (Kappa, 0.91 for rifampicin and 1.00 for isoniazid). In conclusion, the direct colorimetric MTT assay simultaneously detects susceptible and resistant strains of M. tuberculosis within three weeks. It significantly shortens the time required to obtain a DST result and could be a reliable alternative method for rapid detection of drug-resistant TB strains in high-TB-burden resource-limited settings.
Highlights
ObjectivesThe aim of this study was, to examine the MTT assay for detection of isoniazid resistance directly from acidfast bacilli smear-positive sputum and to evaluate the assay for direct detection of rifampicin and isoniazid resistant M. tuberculosis using proportion methods on Lowenstein Jensen (LJ) medium as a gold standard
Based on the polymerase chain reaction genomic deletion analysis, results of all our clinical isolates were confirmed as M. tuberculosis (Fig 2)
It requires 3–8 weeks for initial culture isolation and additional 4 to 6 weeks for the actual drug susceptibility test [5]. During this lag waiting period, the patient could suffer and the time for treatment is prolonged. During this period, the chance of spreading the drug-resistant TB strains to other healthy individuals increases
Summary
The aim of this study was, to examine the MTT assay for detection of isoniazid resistance directly from acidfast bacilli smear-positive sputum and to evaluate the assay for direct detection of rifampicin and isoniazid resistant M. tuberculosis using proportion methods on Lowenstein Jensen (LJ) medium as a gold standard
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