Risk factors for methicillin resistant Staphylococcus aureus: a multi-laboratory study.

Background: The present study aimed to investigate the dose response relationship between the prescriptions of antimicrobial agents and infection/colonization with methicillin resistant Staphylococcus aureus (MRSA) taking additional factors like stay in a health care facility into account.

Methods: Multi-centre retrospective study on a cohort of patients that underwent microbiological diagnostics in Belgium during 2005. The bacteriological results retrieved from 17 voluntary participating clinical laboratories were coupled with the individual antimicrobial consumption patterns (July 2004-December 2005) and other variables as provided by pooled data of health insurance funds. Multivariate analysis was used to identify risk factors for MRSA colonization/infection.

Results: A total of 6844 patients of which 17.5% died in the year 2005, were included in a logistic regression model. More than 97% of MRSA was associated with infection (clinical samples), and only a minority with screening/colonization (1.59%). Factors (95% CI) significantly (p≤<0.01) associated with MRSA in the final multivariate model were: admission to a long term care settings (2.79-4.46); prescription of antibiotics via a hospital pharmacy (1.30-2.01); age 55+ years (3.32-5.63); age 15-54 years (1.23-2.16); and consumption of antimicrobial agent per DDD (defined daily dose) (1.25-1.40).

Conclusions: The data demonstrated a direct dose-response relationship between MRSA and consumption of antimicrobial agents at the individual patient level of 25-40% increased risk per every single day. In addition the study indicated an involvement of specific healthcare settings and age in MRSA status.