[Multidrug-resistant tuberculosis. 1. Drug susceptibility testing and its standardization]

Drug susceptibility testing is one of the most difficult procedures to standardize in mycobacteriology laboratories. International standardization of the test is needed for comparative evaluation of controlled chemotherapeutic trials, for epidemiological surveys on the prevalence of drug resistance, and for guidance in the treatment of tuberculosis patients. In 1996, a new procedure for drug susceptibility testing was proposed by an ad hoc committee of the Japanese Society for Tuberculosis. The proposal is for a test with the proportion method using an Ogawa egg medium similar to those recommended by the WHO. Resistance is expressed as the percentage of colonies that grow on critical concentrations of the drugs, i.e. 0.2 microgram/ml for isoniazid, 40 micrograms/ml for rifampicin, 10 micrograms/ml for streptomycin and 2.5 micrograms/ml for ethambutol. Strains of tubercle bacilli which are grown on drug-containing media represents more than 1% of the number of colonies that develop on drug-free media are considered to be clinically resistant to that agent. The results are recorded as susceptible (S) or resistant (R) on the laboratory forms. The proportion method using agar- and liquid-based media is used worldwide. In the near future a test with agar- or liquid-based media will be discussed for standardization of the test in Japan, as described by the National Committee for Clinical Laboratory Standards of the United States.