Microscopic Observation Drug Susceptibility Assay, a Rapid, Reliable Diagnostic Test for Multidrug-Resistant Tuberculosis Suitable for Use in Resource-Poor Settings

Abstract

There is an urgent need for new tools to improve our ability to diagnose tuberculosis (TB) and multidrugresistant TB (MDR-TB) in resource-poor settings. In a retrospective analysis undertaken in a region with a high incidence of TB, we evaluated the performance of the microscopic observation drug susceptibility assay (MODS), a novel assay developed in Perú which uses an inverted light microscope and culture in Middlebrook 7H9 broth to detect mycobacterial growth. MODS detected 94.0% of 1,908 positive sputum cultures, whereas Löwenstein-Jensen (LJ) culture detected only 86.9% (P < 0.001). The median time to culture positivity was 8 days (compared to 16 days for the same 208 samples by LJ culture; P < 0.001, Wilcoxon signed rank test). The results obtained by direct susceptibility testing using MODS demonstrated excellent concordance for isoniazid and rifampin and the detection of multidrug resistance with those obtained by indirect colorimetric methods: the microplate Alamar Blue assay (MABA) and the tetrazolium microplate assay (TEMA) (agreement, 95, 98, and 94%; kappa values, 0.8, 0.7, and 0.7, respectively). The concordance of the susceptibility testing results for ethambutol and streptomycin was poor. MODS is a novel assay which can detect the organisms responsible for TB and MDR-TB directly from sputum inexpensively, rapidly, and effectively. A comprehensive prospective evaluation of MODS is under way in Perú, and independent validation in nonresearch laboratories should be undertaken at the earliest opportunity.Every single day at least 6,000 people die of tuberculosis (TB), a curable respiratory disease. The diagnosis of TB by sputum smear microscopy is an integral feature of the World Health Organization DOTS (direct observation of treatmentshort-course chemotherapy) strategy for global TB control (25). Low cost, simplicity, and inherent detection of the most infectious cases are the three principal advantages of microscopy for acid-fast bacilli. However, the sensitivity of microscopy for the detection of all cases is low, even when the optimum sensitivity of microscopy is achieved (approximately half of all culture-positive cases are smear negative), and the performance of microscopy is highly variable. Furthermore, the contribution of transmission of infection by smear-negative culture-positive patients (which, by definition, pass undetected when the sole mode of diagnosis is sputum smear) is not inconsiderable (2), and the potential impact of the detection and treatment of these patients is significant (19). Moreover, in this era of emerging drug resistance (9), the lack of information on drug susceptibility threatens the continuing role of the sputum smear as the sole tool for the diagnosis of the majority of cases of TB worldwide. The development of new, low-cost diagnostic tools offers the possibility of future TB control on the basis of culture-based diagnosis and more widespread, targeted susceptibility testing.The simple microscopic observation drug susceptibility assay (MODS) (5), developed in o...

Training Laboratory Technicians from the Ethiopian Periphery in the MODS Technique Enables Rapid and Low-Cost Diagnosis of Mycobacterium tuberculosis Infection

“…6,9 MODS, Microscopic Observation Drug Susceptibility assay, is a novel method for the diagnosis of TB infection and determination of susceptibility to anti-TB medications. This technique has been previously validated and shown to be quick and reliable [23][24][25][26] and has been implemented in the regional laboratories of the national TB program in Lima-Peru, 30,31 but otherwise has not been tested in the "field," in regional or district laboratories of low resource settings, for which it is intended. Demonstrating the ability to train laboratory technicians outside of research settings in developing countries to validly perform MODS, has been identified as a research priority.…”

“…Utilization of MODS requires neither expensive equipment nor prepared special reagents. [22][23][24] The use of simple and available reagents addresses the problem of delivery of prepared supplied reagents to remote areas with a final cost for culture and susceptibility for one specimen of~2-3 US$. 21 We obtained positive culture results in 5-10 days (Figure 1).…”

“…20 Microscopic observation drug susceptibility assay (MODS), is a new laboratory technique that was developed by researchers from Johns Hopkins in collaboration with researchers from Universidad Peruana Cayetano Heredia and AB PRISMA in Lima, Peru. [21][22][23][24] This inexpensive and low tech method enables rapid culture of M. tuberculosis and concomitantly provides a reliable sensitivity profile of isolated bacteria to the anti-TB medications isoniazid and rifampin. 25,26 These advantages make this approach an attractive new option for TB diagnosis in developing countries.…”

Rapid culture-based methods for drug-resistance detection in Mycobacterium tuberculosis

“…Growth in the drug-containing wells and in the growth control is interpreted as resistance. MODS has been evaluated against first-line drugs although better results have been obtained for RIF and INH (Moore et al, 2004(Moore et al, , 2006. Although fairly accurate to detect resistance, especially to RIF, the requirement of an inverted microscope to read the plates and biosafety concerns related to the use of liquid medium in 24-well plates may hamper, however, its application outside large reference laboratories with the necessary logistic facilities.…”

Rapid Diagnosis of Mycobacterium tuberculosis Infection and Drug Susceptibility Testing

“…[2][3][4][5][6] This method is based on the use of microtiter plates containing Middlebrook 7H10 liquid medium, with wells for controls as well as growth and AST of any isolates that grow. Specimens are processed prior to inoculation and the microtiter plates inoculated and incubated.…”