Estimating the proportion of community-associated methicillin-resistant Staphylococcus aureus: two definitions used in the USA yield dramatically different estimates.

The objective of this retrospective study was to compare the prevalence of community-associated methicillin-resistant Staphylococcus aureus (CAMRSA) and healthcare-associated MRSA (HAMRSA) using healthcare risk factor exposure criteria with that obtained using Centers for Disease Control and Prevention (CDC) criteria. Cases were defined as CAMRSA or HAMRSA based on the general CDC guidelines for nosocomial infections, and then re-assessed with healthcare risk factor exposure criteria using a medical chart review. One hundred MRSA cases occurred at a mid-Western veterans affairs medical centre from November 2001 to November 2003. The proportion of these cases classified as CAMRSA differed dramatically when classified by healthcare risk factor exposure criteria (5%) compared with CDC nosocomial infection criteria (49%). Estimating the role of healthcare-related exposures and developing strategies to control MRSA can be markedly affected by the criteria used to determine CAMRSA and HAMRSA.