EVIDENCE THAT THE NATIONAL-COMMITTEE-FOR-CLINICAL-LABORATORY-STANDARDS DISK TEST IS LESS SENSITIVE THAN THE SCREEN PLATE FOR DETECTION OF LOW-EXPRESSION-CLASS METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS

A low-expression-class methicillin-resistant Staphylococcus aureus strain (strain 9302-2) was sent to 207 laboratories as part of the bacteriology component of the Laboratory Proficiency Testing Program of Ontario. An incorrect (susceptible) result was reported by 16 of 76 (21%) of laboratories that used the National Committee for Clinical Laboratory standards disk test, whereas 1 of 104 laboratories that used other methods reported an incorrect result (P < 0.05). Experiments showed discrepancies in the disk test results given by Mueller-Hinton agars from three manufacturers. We advise that laboratories should use a low expression-class methicillin-resistant S. aureus isolate as a control for the National Committee for Clinical Laboratory Standards disk test.