C-202. Evaluation of Clarithromycin Drug Susceptibility Testing for Mycobacterium avium/intracellulare complex (MAC) using Commercially Available MAISLOW Sensititre Plates and Custom JustOne Strips

L. Hall1, A. T. Abbenyi1, J. J. Eisberner1, B. A. Brown-Elliott2, C. J. Pratt1, K. Beierle2, M. McGlasson2, S. L. Wohlfiel1, S. M. Deml1, R. J. Wallace, Jr.2, N. L. Wengenack1;
1Mayo Clin., Rochester, MN, 2The Univ. of Texas Hlth. Ctr., Tyler, TX.

Background: MAC is the most commonly isolated nontuberculous mycobacteria in the clinical laboratory. The ATS/IDSA Statement on Diagnosis, Prevention and Treatment of Nontuberculous Mycobacterial Diseases and the CLSI recommend that MAC be tested only against clarithromycin. We evaluated two new products for clarithromycin susceptibility testing, the custom JustOne strip and the MAISLOW Sensititre plate (TREK Diagnostic Systems, Cleveland, OH), using a variety of media for inoculum preparation including solid media, Middlebrook 7H9 broth and VersaTREK™ Myco Broth (TREK Diagnostic Systems, Cleveland, OH). Methods: Eighty-three MAC isolates with known susceptibility profiles were tested using the MAISLOW and JustOne systems and results were compared to those obtained using the BACTEC™ 460 (BD, Sparks, MD) or broth microdilution (The University of Texas Health Science Center, Tyler, TX) methods. More than one-third of the isolates were known to be resistant to clarithromycin. An additional 25 isolates were tested prospectively by the MAISLOW, JustOne, and BACTEC™ 460 methods. Clarithromycin susceptible and resistant strains were run as quality controls. Discordant results were repeated using both methods to confirm the results.
Results: 480 of 539 clarithromycin results (89%) with the MAISLOW and JustOne products were concordant with BACTEC™ 460 or broth microdilution results. There were 14 (2.6%) very major errors (test is susceptible and the comparative method is resistant) and 45 (8.3%) minor errors (comparative method is intermediate; test method is resistant or susceptible). All 25 isolates in the prospective study, including 4 resistant strains, had concordant results with BACTEC™ 460. Conclusions: Clarithromycin testing for MAC by JustOne strip and MAISLOW plate methods are easy to set up, read and incorporate in the clinical laboratory. These systems offer advantages over the BACTEC™ 460 system which include the lack of need for radioactive substrates, sharps, or instrumentation. Both JustOne and MAISLOW compared very well with microbroth dilution and BACTEC™ 460.