Methicillin-resistant Staphylococcus aureus: how reliable is laboratory reporting?

Microbiology laboratories in Oregon were surveyed in 1981 to determine how often methicillin-resistant Staphylococcus aureus (MRSA) was being reported to physicians in the state. Results of this survey were surprising in three respects. First, the mean percent of S. aureus isolates reported by laboratories as methicillin-resistant was 8.3%. Second, a significant inverse correlation was found between the percent of reported MRSA and laboratory size (p = 0.0001). Finally, laboratories which retested initially resistant isolates reported significantly less MRSA (mean 3.3%, median 1%) than those laboratories which accepted initial results (mean 20%, median 12%) (p = 0.0001). Independent testing by the Centers for Disease Control of isolates reported to be MRSA confirmed that some misidentification was occurring. Participating laboratories were notified of our initial findings and their potentially serious clinical ramifications. Laboratories were resurveyed in 1982, and a significant decrease in the reported MRSA to a mean percentage of 3.4% was observed (p = 0.002). It is unlikely that this situation is unique to Oregon, and similar misreporting of MRSA to physicians in other areas of the country may be compromising the safe and effective treatment of S. aureus infections.