A Comparative Study of Drug Susceptibility Testing Techniques for Identification of Drug Resistant TB in a Tertiary Care Centre, South India

Abstract

India tops the global list for Drug resistant Tuberculosis, but inadequate and expensive laboratory culture techniques have led to delay in the diagnosis and treatment. We studied the potential of an alternative method which could be cost-effective by combining the drugs in the same tube for identification of drug resistance. Drug Susceptibility Test (DST) results of 1000 sputum samples are got from suspected TB patients against INH (isoniazid) and Rifampicin by two techniques: a) a modified technique with both drugs in the same MGIT tube and b) a standard technique with the antibiotics in separate MGIT tubes for the diagnosis of MDR-TB (Multidrug Resistant). 39 samples were contaminated and were excluded from final analysis. 198 were smear positives by the concentrated Ziehl-Neelsen’s staining method. 219 were found to be culture positive out of which 195 were identified as M. tuberculosis complex. 40 (20.5%) strains were identified as MDR-TB by the conventional method and 39 were picked up by the modified DST. INH and Rifampicin mono-resistance accounted for 32 (16.4%) and 4 (2%) respectively. 99% concordance was observed between the two tests in categorizing MDR-TB. Similarly modified technique with combination of the second line Antibiotics-Ofloxacin, Kanamycin and Capreomycin was applied on the identified MDR strains in a stepwise manner. 6 (15%) were identified as Pre-XDR strains and 2 (5%) were found to be XDR-TB strains. This study implies that combining drugs in the same tube may be an equivalent and possibly a cost-effective alternative which needs to be explored further.