QUALITY ASSURANCE PROGRAM FOR DRUG SUSCEPTIBILITY TESTING OF MYCOBACTERIUM-TUBERCULOSIS IN THE WHO IUATLD SUPRANATIONAL LABORATORY NETWORK - FIRST ROUND OF PROFICIENCY TESTING/

SETTING: Quality assurance of the WHO/IUATLD global tuberculosis drug resistance surveillance programme. OBJECTIVE: To perform a proficiency test of drug susceptibility procedures within the WHO/IUATLD network of supranational reference laboratories (SRL). DESIGN: Identical cultu re panels consisting of 20 clinical isolates of Mycobacterium tubercul osis containing both drug susceptible and drug resistant cultures were tested by the 16 laboratories of the network for resistance to strept omycin, isoniazid, rifampicin and ethambutol. The drug susceptibility testing procedures included the proportion, absolute concentration and resistance ratio methods as well as their variants, including the rad iometric BACTEC 460 method. RESULTS: The first round of proficiency te sting has shown that the specificity of drug susceptibility testing wi thin the SRL network was significantly higher than its sensitivity. Th e testing of isoniazid and rifampicin shows a high degree of agreement between the labs, but discordant results can be obtained with strepto mycin and ethambutol. CONCLUSION: Drug susceptibility procedures for t he testing of isoniazid and rifampicin, the two anti tuberculosis drug s which define multidrug-resistant tuberculosis, are highly reliable w ithin the SRL network. Procedures for drug susceptibility testing of s treptomycin and ethambutol are still in need of standardization.