Comparison of the Resistance Ratio and Proportion Methods for Antimicrobial Susceptibility Testing of Mycobacterium tuberculosis

UNCHALEE T ANSUPHASIRI, M.Sc. *, SIRIMART SUBPAIBOON, M.Sc.**, SOMSAK RIENTHONG, M.Sc. ***

Affiliation : * Department of Microbiology, Faculty of Public Health, Mahidol University, Bangkok 10400, ** Bang Khla Hospital, Chachoengsao 24110, *** TB Division, Department of Communicable Disease Control, Ministry of Public Health, Nonthaburi 11000, Thailand.

This study compared two in vitro antimicrobial susceptibility methods for determining drug susceptibilities of Mycobacterium tuberculosis isolated from newly diagnosed pulmonary tuber- culosis 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 (P0.05). Thus, the resis- tance 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.

Keywords : Mycobacterium tuberculosis, Antimicrobial Susceptibility Testing, Resistance Ratio, Proportion Method

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