The Clinical and Laboratory Standards Institute (CLSI) recently published Susceptibility Testing of Mycobacteria, Nocardiae and Other Aerobic Actinomycetes; Approved Standard-Second Edition (M24-A2).
It is a revision of M24-A, which was published in 2003.
Based on comments from scientists involved with regular testing of mycobacteria and/or aerobic actinomycetes, M24-A2 was revised to address the susceptibility testing of Mycobacterium tuberculosis complex (MTBC), clinically significant slowly and rapidly growing mycobacterial species, Nocardia spp and other aerobic actinomycetes.
Supported by the most current data available, this standard includes recommendations for the selection of agents for primary and secondary testing, organism-group-specific methodologies, reporting recommendations and quality-control criteria for the above-listed organisms.
The standard includes new recommendations for testing initial isolates from all patients for susceptibility to isoniazid, rifampin, ethambutol and pyrazinamide, all of which are components of first-line therapy for tuberculosis.
For testing MTBC, M24-A2 recognises agar proportion as the primary methodology on which all other methodologies are essentially based.
This document also recommends the use of commercial broth susceptibility methods with shorter incubation times, which are now widely used in susceptibility testing of MTBC.
Laboratory tests for evaluating the susceptibility of mycobacteria and aerobic actinomycetes are important because they can confirm the choice of the initial course of chemotherapy and the emergence of drug resistance when a patient fails to show a satisfactory bacteriological response to treatment.
Additionally, they can guide the choice of further treatment with different drugs.
Susceptibility testing of MTBC can also be used to estimate the prevalence of primary and acquired drug resistance in a community.
For each of these purposes, CLSI claims that use of a reliable technique to perform the test is essential and that M24-A2 satisfies this demand.
