Comparison of the resistance ratio and proportion methods for antimicrobial susceptibility testing of Mycobacterium tuberculosis.

ABSTRACT

This study compared two in vitro antimicrobial susceptibility methods for determining drug susceptibilities of Mycobacterium tuberculosis isolated from newly diagnosed pulmonary tuberculosis patients to four front-line drugs. Of 250 strains of M. tuberculosis tested, 74.4 per cent were susceptible by the resistance ratio method, with 72.0 per cent by the proportion method. The results showed high agreement for both methods (P<0.0001) and agreement rates to streptomycin, isoniazid, rifampicin and ethambutol were 96.8, 98.0, 94.8 and 96.8 per cent, respectively. For drug resistance patterns, both methods showed the highest resistance to one drug, followed by two, three, and four drugs, respectively. Of the single drug resistance, both methods gave the highest resistance to streptomycin, followed by resistance to isoniazid, rifampicin and ethambutol, respectively. The correlation between both methods for determining susceptibility of M. tuberculosis to four drugs was not statistically significantly different by Mc Nemar chi2 (p>0.05). Thus, the resistance ratio method may be substituted. However, WHO recommended the use of the proportion method to be used for determining drug susceptibility of M. tuberculosis. The susceptibility testing result can be used as the guidance for proper treatment and is valuable for confirmation of drug resistance in patients showing unsatisfactory response to treatment, useful for identifying primary and acquired drug resistance trends in a community and for minimizing the spread of drug-resistant strains.