The usefulness of microscopic observation for drug susceptibility of Mycobacterium tuberculosis complex in routine clinical microbiology laboratory

Abstract

Abstract: Clinical management of tuberculosis (TB) cases in developing countries is hampered by the lack of a simple and effective diagnostic test. Correct diagnosis of TB is needed to improve treatment, reduce transmission, and control development of drug resistance. This study was undertaken to establish microscopic observation for drug susceptibility (MODS) in clinical microbiology routine. Thirty Mycobacterium tuberculosis
isolates and four smear positive sputum specimens were tested for susceptibility to isoniazid, rifampicin, ethambutol and streptomycin using MODS. Results were compared to gold standard methods used at a TB
reference laboratory. The median turn around time (TAT) was six days for both direct and indirect assays. Results for rifampicin were 100% concordant with the reference laboratory and those of ethambutol, streptomycin and isoniazid were 97%, 94% and 94% concordant, respectively. In all discordant cases MODS categorize the isolates as resistant. Using SYBR Green to detect growth, there was clear increase in fluorescence for cultures of drug-resistant strains when compared to culture of sensitive strains. The discrepancy in these cases can be explained by the fact that in MODS any growth in drug-containing wells is labelled as resistance and can be resolved using SYBR Green. MODS can therefore, be considered as a reliable and fast method which could be used as routine in Clinical Microbiology Laboratory in a TB endemic area.