Comparison of The Resistance Ratio and proportion Methods for Drug Susceptibility testing of mycobacterium Tuberculosis Isolated from Patients Visiting National Tuberculosis Centre

Abstract

This CDM TU & NTC collaborative study was carried out based at National Tuberculosis Centre (NTC), Thimi, Bhaktapur, Nepal from September 2006 to June2007 with an objective to compare the two in vitro methods (viz, resistance ratio (RR)and proportion (PR) methods), used to determine antimicrobial susceptibilities of Mycobacterium tuberculosis against primary anti-tubercular drugs {Isoniazid (INH),Rifampicin (RFP), Streptomycin (SM) and Ethambutol (EMB)}. Among 221 isolates of M. tuberculosis analyzed during the study period, 56.56% and57.47 % of the isolates were resistant to at least one drug by resistance ratio and proportion method respectively. Among all resistant isolates detected, 5.88% were resistance to INH, 4.52% to SM and 1.81% to RFP by RR method, while 5.43% werer esistance to both INH and SM and 0.45% to RFP by PR method. No EMB resistant isolates could be detected by both methods. Multi-drug resistance (MDR) was observedin 28.96% and 29.86% isolates by RR and PR methods respectively. On correlation analysis using Mc Nemar chi-square test, no significant difference between the two tests were observed (p>0.05). The results showed high agreement between both the methods and agreement rates to INH, RFP, SM and EMB were 93.21,93.67, 93.21 and 94.12 % respectively. Similarly, the agreement rates between both the methods using kappa analysis showed kappa value of 0.864, 0.854, 0.861 and 0.838 for INH, RFP, SM and EMB respectively, which is believed to be good agreement between both methods(k= 0.80 to 1.00 :very good agreement). Thus both the resistance ratio and proportion methods are equally good for determining drug susceptibility of M. tuberculosis, so that we need not to depend only on tedious method like proportional method for Antimicrobial susceptibility testing.