A study in this month’s ICHE highlights the problems with using epidemiological definitions to designate MRSA as “nosocomial”. The study evaluated the impact of different numerators and denominators on the rate of apparent hospital-onset MRSA across 32 hospitals in California. The time that patients were hospitalized before being considered hospital-onset varied from 48 to more than three days and denominators were also variable. The particular combination of numerator and denominator used resulted in significant differences in the proportion of MRSA cases designated hospital-onset. This has clear implications for comparing rates of hospital-attributable MRSA in the era of public reporting.

The paper raises a wider problem of how to define healthcare- and community-associated MRSA in the era of CA-MRSA strains as a cause of healthcare-associated infections. A recent review in JHI (Otter & French 2012) made the case for a genotypic definition of CA-MRSA. Epidemiological definitions were useful for differentiating CA-MRSA and HA-MRSA strain types in the past. However, although HA-MRSA strain types are rarely transmitted in the community, CA-MRSA strains have now begun to be transmitted in healthcare facilities, so epidemiological definitions are breaking down. CA-MRSA are community strains of S. aureus that have acquired mecA. They are distinct from HA-MRSA and should be defined genetically. Carriage of the Panton-Valentine leukocidin (PVL) or antimicrobial susceptibly profiles can be useful indicators of CA-MRSA but should not be used to define them. For the full assessment of their epidemiology, MRSA infections should now be characterised as (1) caused by HA- or CA-MRSA strain types; (2) acquired in community or healthcare settings; and (3) onset in the community or healthcare facility. (This review made the 10 ten list of the JHI Editors choice and is freely available online here.)

Article citations:

Datta R, Kuo King M, Kim D et al. What Is Nosocomial? Large Variation in Hospital Choice of Numerators and Denominators Affects Rates of Hospital-Onset Methicillin-Resistant Staphylococcus aureus. Infect Control Hosp Epidemiol 2012; 33: 1166-9.

Otter JA, French GL. Community-associated meticillin-resistant Staphylococcus aureus: the case for a genotypic definition. J Hosp Infect 2012; 81: 143-8.