Quality assurance programme 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 culture panels consisting of 20 clinical isolates of Mycobacterium tuberculosis containing both drug susceptible and drug resistant cultures were tested by the 16 laboratories of the network for resistance to streptomycin, 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 radiometric BACTEC 460 method.

Results: The first round of proficiency testing has shown that the specificity of drug susceptibility testing within the SRL network was significantly higher than its sensitivity. The testing of isoniazid and rifampicin shows a high degree of agreement between the labs, but discordant results can be obtained with streptomycin and ethambutol.

Conclusion: Drug susceptibility procedures for the testing of isoniazid and rifampicin, the two anti tuberculosis drugs which define multidrug-resistant tuberculosis, are highly reliable within the SRL network. Procedures for drug susceptibility testing of streptomycin and ethambutol are still in need of standardization.