Attributable mortality in critically ill patients with bacteremia involving methicillin susceptible (MSSA) and methicillin resistant Staphylococcus aureus (MRSA)

Background

Staphylococcus aureus bacteremia carry high fatality rates. Outcome comparisons between MRSA and MSSA bac-teremic patients are difficult to perform because of important differences in severity of illness.

Methods

In a retrospective study (01/1992-12/1998), attributable mortality for MSSA and MRSA bacteremia was investigated and compared in critically ill patients. Two independent case-control studies were performed. Matching (1:2-ratio) was based upon APACHE II-score and admission diagnosis. As expected mortality can be derived from these two variables, this matching procedure resulted in an equal expected mortality rate for cases and controls. Attributable mortality is determined by subtracting the in-hospital mortality rate of the controls from the in-hospital mortality rate of the cases.

Results

During the study period 22,431 patients were admitted to the ICU. In 85 patients a microbiologically documented S. aureus bacteremia was diagnosed. In the MSSA case-control study, an attributable mortality of 1.3% was found: mortality in cases (n = 38) and controls (n = 76) was respectively 23.7% and 22.4% (P = 0.937). In the MRSA case-control study an attributable mortality of 23.4% was found: mortality rates for cases (n = 47) and controls (n = 94) were respectively 63.7% and 40.4% (P = 0.017). The difference between both attributable mortality rates (22.1%) was statistically significant (95% CI: 8.8–35.3%).

Conclusion

In critically ill patients, MRSA bacteremia have a significantly higher attributable mortality than MSSA bacteremia.

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