Examination of major disagreements in susceptibility test results by Autobac-1 and MS-2

Major disagreements between two automated susceptibility test instruments, Autobac-1 and MS-2, generated from 4,213 tests on 630 bacterial isolates, were analyzed. The major disagreement rate between the instruments was only 7.4%. The highest major disagreement rates for bacteria were with Staphylococcus epidermidis, enterococci, gram-positive bacilli, and Providencia species, and those for antibiotics were with ampicillin and penicillin G. With most other bacteria and antibiotics, the instruments disagreed at a rate less than 10% and frequently at a rate less than 4%. However, 32 specific bacterium-antibiotic combinations exceeded a 10% rate. Possible reasons for some of these higher disagreement rates are discussed. Reconciliation of instrument results with the standard agar disk diffusion result for each major disagreement revealed that about 15% were irreconcilable and, of the remaining 259, 177 agreed with MS-2 and 82 agreed with Autobac-1. Major disagreements between instruments seemed random, but there appeared to be tendencies, with certain bacteria and certain antibiotics, for MS-2 to detect susceptible reactions more consistently than resistant ones and for Autobac-1 to detect resistant reactions more consistently than susceptible ones.