Accuracy of reporting of methicillin-resistant Staphylococcus aureus in a provincial quality control program: a 9-year study.

We report the results of a province-wide quality control program in which five methicillin-resistant Staphylococcus aureus strains were circulated to all Ontario laboratories (hospital, private, and public health laboratories) on nine occasions between 1980 and 1989. The level of expression of methicillin resistance in each of the isolates was determined by performing viable colony counts on serial dilutions of methicillin in agar, and each isolate was assigned to an expression class according to previous published criteria (A. Tomasz, S. Nachman, and H. Leaf, Antimicrob. Agents Chemother. 35:124-129, 1991). Over this time there was an improvement in the performance of laboratories in the recognition of three strains that were relatively easy to detect (strains B, C, and E). These strains were of expression class II, and 98% of laboratories reported correct identifications in 1986. Performance in identifying two strains (strains A and D) of expression class I remained poor. Strain A was circulated in two surveys in 1987 and 1989, and laboratories were sent a questionnaire requesting details of the methods used in those two surveys. The methods used by the laboratories were classified into three categories: disk diffusion, single-plate screening by agar incorporation, and automated methods, which included premanufactured MIC panels. Between the 1987 and 1989 surveys, there was no change in the performance of the disk diffusion test (60% correct on both occasions), but there was improvement in the sensitivity of the agar incorporation test (36% correct in 1987 and 84% correct in 1989) and in automated methods (43% correct in 1987 and 79% correct in 1989). Over a decade, there was overall improvement in the performance of laboratories in detecting easy-to-detect strains, but there were difficulties in detecting organisms of low expression class, and an organism of very low expression class should be designated as a control organism for routine testing of methicillin-resistant s. aureus isolates.