False-susceptible results from the MS-2 system used for testing resistant Pseudomonas aeruginosa against two third-generation cephalosporins, moxalactam and cefotaxime.

The MS-2 system (Abbott Laboratories) was used for cefotaxime and moxalactam susceptibility testing of 111 Pseudomonas aeruginosa isolates collected from our burn treatment center. The frequency of very major errors was 52% for moxalactam and 32% for cefotaxime for all isolates requiring a minimal inhibitory concentration of greater than or equal to 64 micrograms/ml. For isolates requiring a minimal inhibitory concentration of 64 micrograms/ml, the very major errors were 78% for moxalactam and 57% for cefotaxime. Testing of an additional 44 isolates collected from another hospital confirmed that these results were not unique to nosocomial strains from within the burn center. The ratio of errors did not change when MS-2 results were compared with disk diffusion results. Colony count testing done on cuvettes which produced false-susceptible results indicated that little numerical change occurred during the 5- to 6-h test cycle, whereas microscopy of cuvette contents revealed metabolically active filamentous bacilli. The high frequency of false-susceptible results indicated that the MS-2 should not be used for testing clinical P. aeruginosa isolates for resistance to the two third-generation cephalosporins currently available for the MS-2. A comparison of the disk diffusion results indicated that neither moxalactam nor cefotaxime could be reliably used for predictive, class-concept testing of P. aeruginosa.