Evaluation of the BBL MGIT (Mycobacterial Growth Indicator Tube) AST SIRE system for antimycobacterial susceptibility testing of Mycobacterium tuberculosis to 4 primary antituberculous drugs

Objective.-To evaluate the performance of the BBL MGIT (Mycobacterial Growt h Indicator Tube) AST SIRE system for the antimycobacterial susceptibility testing of Mycobacterium tuberculosis to isoniazid (at a concentration equi valent to the lower concentration used for testing by the method of proport ion), rifampin, ethambutol, and streptomycin. Design.-Thirty-one clinical isolates and 30 challenge strains provided by t he Centers for Disease Control and Prevention (CDC) were tested by MGIT AST SIRE using 2 methods of inoculum preparation, and results were compared wi th those of the method of proportion, which was considered the reference me thod. Clinical isolates for which the results of the 2 methods were discord ant also were tested at 2 reference laboratories. Results.-Based on data from our site and the reference laboratories, agreem ent rates between initial MCIT AST SIRE results and the method of proportio n for the clinical isolates with the inoculum prepared from a McFarland equ ivalent and from a positive MGIT tube, respectively, were 100 % and 96.8 % for isoniazid, 100 % and 100% for rifampin, 96.8% and 100% for ethambutol, and 100% and 100 % for streptomycin, excluding the isolate for which the di scordant strepiomycin result could not be resolved. For the 30 challenge is olates, agreement rates between MGIT AST SIRE and expected results and betw een method of proportion and expected results, respectively, were 96.7 % an d 93.3 % for isoniazid, 93.3 % and 100 % for rifampin, 83.3 % and 100 % for ethambutol, and 93.3 % and 100 % for streptomycin, For the clinical isolat es, the mean time to an MGIT AST SIRE result of susceptible was 6.15 +/- 0. 13 days (range, 5-8 days). For a result of resistant, the mean time overall was 5.00 +/- 0.24 days (range, 3-8 days). Conclusion.-These data suggest that the MGIT AST SIRE system, using either method of inoculum preparation, is an acceptable alternative to the BACTEC 460 TB method of susceptibility testing of clinical isolates of M tuberculo sis to isoniazid, rifampin, ethambutol, and streptomycin. Reasons for the l ower agreement with the CDC challenge isolates should be investigated. Furt her evaluation of the MGIT AST SIRE system using a concentration of isoniaz id equivalent to the higher concentration tested by the method of proportio n would be useful, because the decision concerning use of this agent genera lly is based on the susceptibility test result at the higher concentration.