Molecular assays should be used to distinguish between ineffective tuberculosis therapy and TB reinfection, according to a new report. Annika Kruuner and colleagues at Tartu University in Tartu, Estonia and the Karolinska Institute in Stockholm, Sweden, "investigated the means by which drug resistance emerges among drug-susceptible Mycobacterium tuberculosis strains during antituberculosis therapy" in their study, published in Clinical Infectious Diseases ("Use of Molecular Techniques to Distinguish Between Treatment Failure and Exogenous Reinfection with Mycobacterium Tuberculosis," July 15, 2002; 35;(2)146-155). Apparent drug resistance is often the result of exogenous reinfection, which can be detected using restriction fragment-length polymorphism (RFLP) analysis, Kruuner and coauthors found.

The researchers reviewed data from 11 pulmonary TB patients whose cases were recorded in the Estonian National Reference Laboratory archive. Each of the patients had ultimately failed to respond to TB therapy despite carrying initially drug-susceptible TB strains, according to the report. RFLP analysis using the IS6110 polymorphism, which acts as a genetic TB fingerprint, showed RFLP pattern shifts in all 11 patients, study data showed. These shifts indicated new infections with different M. tuberculosis strains.

In six patients, the TB strain responsible for the original infection remained drug-susceptible. "Exogenous reinfection with drug-resistant M. tuberculosis may be misinterpreted as the emergence of drug resistance if molecular testing techniques are not used," Kruuner and colleagues warned.

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