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Methicillin Resistance as a Risk Factor for Mortality in S. Aureus Bacteremia: Presented at ECCMID

By Chris Berrie

NICE, FRANCE -- April 5, 2006 -- Methicillin resistance was the most significant factor for 14-day mortality in patients with methicillin-resistant Staphylococcus aureus (MRSA) blood-stream infection (BSI) in Ireland in 2005, researchers reported here at the 16^th European Congress of Clinical Microbiology and Infectious Diseases (ECCMID).

Robert Cunney, MD, consultant microbiologist, Health Protection Surveillance Centre (HPSC), Dublin, Ireland, presented results of a multi-hospital analysis on behalf of the Irish European Antimicrobial Resistance Surveillance system (EARSS) Steering Group, part of the EARSS Enhanced Bacteraemia Surveillance in Ireland, 2005.

Although MRSA BSI has been shown to be associated with increased mortality, due to the need to adjust for other important confounding variables, international studies have proven to be too heterogeneous for the determination of local situations, Dr. Cunney said during his presentation on April 3^rd.

For this study, 5 hospital laboratories in Ireland provided data from 269 BSI episodes. To provide information specific to methicillin resistance, these included both MRSA patients (n = 116; male, 40%) and methicillin-sensitive Staphylococcus aureus (MSSA) patients (n = 153; male, 41%).

Of note, the MRSA patients showed significantly greater mean age (66.5 vs. 49.7 years for MSSA; P <.001), and mean length of stay in hospital (20.6 vs. 5.7 days; P <.001) before the detection of BSI.

Most of the patients showed a single risk factor (MRSA, 61%; MSSA, 52%; P =.185), and although the entry portal of the bacteremia via skin/soft tissues was significantly less prevalent in the MRSA patients (5% vs. 18%; P =.001), all of the other sources of infection were similarly distributed across the 2 groups. These included central venous catheter (CVC; 22% vs. 23%), respiratory tract (13% vs. 7%), surgical wound (5% vs. 4%), other (11% vs. 12%) and unknown (44% vs. 37%).

However, as Dr. Cunney initially stressed, the 14-day, 30-day and in-hospital mortalities were all significantly higher in the MRSA patients over the MSSA patients: 31% versus 14% (P =.001), 37% versus 18% (P =.001), and 42% versus 20% (P <.001), respectively.

At the univariate analysis level for in-hospital mortality, the significant factors were age (P <.001), length of stay (P <.001), respiratory tract (P =.01) and skin/soft tissue (P =.01) as entry portal, and methicillin resistance (P <.001).

When the in-hospital mortality analysis was taken to the multivariate level, age (P <.001) and length of stay (P <.001) remained, accompanied by surgical wound as entry portal (P =.03).

Dr. Cunney also noted that the age breakdown of this in-hospital mortality showed that older patients were more likely to acquire MRSA BSI and less likely to survive than younger patients and those with MSSA BSI.

The multivariate analysis of 14-day mortality showed significance for age (P <.001), CVC as entry portal (P =.04) and methicillin resistance (P =.04). When this was applied to the 30-day mortality, age remained significant (P <.001), along with length of stay (P =.05).

After controlling for other factors, methicillin resistance remained a significant risk factor for 14-day mortality for patients with MRSA BSI, Dr. Cunney stressed, although age, length of stay, and other factors become more significant for in-hospital and 30-day mortality.

By way of potential explanation, he said, "I suspect that what is happening is that once you go beyond the 14-day mortality, the other risk factors -- such as underlying illness, age and so forth -- become more important, and you would expect that if methicillin resistance has an impact on mortality, that impact would occur early, because it would be related to treatment options."

This study thus indicates the need for early identification of methicillin resistance in this population, and highlights the way in which such enhanced surveillance analyses can be used to determine the most appropriate measures for the control of MRSA infections, he concluded.


[Study title: Methicillin Resistance as a Risk Factor for Mortality in Staphylococcus Aureus Bacteraemia: an Analysis from the EARSS Enhanced Bacteraemia Surveillance in Ireland, 2005. Abstract P1011]

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