We evaluated risk factors for infection with HA-MRSA during the ICU stay by multivariate logistic regression analysis. Because we plan to introduce preemptive infection control measures for patients at high risk of HA-MRSA infection in the near future, only the factors obtained within 24 hours of ICU admission were sampled as possible prognostic variables in this study. To our knowledge, there are no previously published data on risk factors for MRSA infection analyzed from such a point of view. Our results showed that four risk factors were independently associated with HA-MRSA infection: intubation, open wound, treatment with antibiotics, and steroid administration, all occurring within 24 hours of admission to the ICU.
Several reports concerning the risk factors of HA-MRSA infection have been published. Ibelings and coworkers reported that patients in the ICU are at high risk of MRSA infection, and patients with MRSA infections are less likely to survive than those with methicillin-sensitive Staphylococcus aureus . In addition, several factors related to MRSA transmission and infection were reported, such as length of ICU stay , antibiotics administration [6, 7], previous hospital stay , history of surgery , trauma patients , burn patients , presence of CVCs , and steroid administration . Referring to these reports, we extracted 11 candidate prognostic variables that were possibly related to MRSA infection and that could be assessed within 24 hours of ICU admission. Although the length of ICU stay is strongly related to MRSA infection, this factor cannot be determined within the first 24 hours of ICU admission. Accordingly, we excluded this factor from the prognostic variables in this study. We also considered using a severity score, such as the Acute Physiology and Chronic Health Evaluation score or the Simplified Acute Physiology Score II score, as a prognostic variable. However, the complicated prediction calculation formulas of these scoring systems were not suitable in clinical practice, so severity scores were not considered in this study.
Although the risk factors detected in this study other than intubation did not contradict those of previous studies [6, 7, 10, 12], no previously published data, to our knowledge, has focused on intubation as a risk factor for HA-MRSA infection. There were several reasons why intubation was selected as one of the independent risk factors. First, the frequency of medical staff contact is high for the intubated patient compared with that for other patients. In an intubated patient, frequent nursing care such as suctioning of secretions, oral care, and postural change are necessary. As a result, the opportunity for MRSA transmission will increase. Second, patients who require ventilatory support may be severely ill. It has been shown that severely ill patients tend to acquire MRSA infection .
The costs of MRSA infection in terms of added morbidity, mortality, hospital days, and hospital charges are overwhelming [1, 2]. Although 'time-dependent bias' reportedly gives estimates that greatly overestimate the effect of nosocomial infection on the extra length of ICU stay , several studies have indicated that MRSA infections cause a significant additional length of stay or financial burden after adjustment for the factors that truly influence the length of stay [2, 14, 15]. The spread of MRSA occurs mainly from person to person . Therefore, control of MRSA transmission and infection is of serious concern. The findings of this study should be helpful in formulating and implementing several strategies for reducing the risk of MRSA infection. If we strengthen preemptive infection control in high-risk patients, we may be able to decrease the rate of healthcare-associated infection. We found that patients with 2 or 3 risk factors included nearly 100% of MRSA-infected patients, whereas those factors were limited to about one half of all patients admitted, as shown in Table 4. We believe that the two factors of intubation and open wound are the best combination to address. The main reason is that these two factors are quite simple and obvious to detect, so that it is easy for the medical staff to judge whether the patient is positive for these factors. Conversely, it is hard to identify visually whether the patient is being treated with antibiotics or steroids at ICU admission. In addition, there are a large variety of uses for antibiotics and steroids. Because prophylactic use of antibiotics [17–19] and MPSS therapy for spinal cord injury  are still controversial therapies, these indications may change in the future. Furthermore, even if treatment with antibiotics and steroid administration were evaluated along with the two factors of intubation and open wound, the sensitivity for acquisition of MRSA infection would be almost the same as that for the two factors alone.
We excluded the patients with preexisting MRSA on ICU admission because the purpose of this study was to identify the risk factors for healthcare-associated (nosocomial) MRSA infection in the patients who did not harbor MRSA at the time of ICU admission. Our goal was to identify the patients vulnerable to MRSA infection. We think this is very important from the viewpoint of nosocomial infection control. Patients with preexisting MRSA on ICU admission are an important group because they are a source of MRSA and of development of infection. We also separately investigated predictors of infection for patients with preexisting MRSA (data not shown). The number of patients in this study with MRSA on ICU admission was 19 patients, and of them, 9 patients (47%) developed infection during their ICU stay. Due to this small number of patients, we analyzed the risk factor of MRSA infection by univariate analysis alone, and only 'transferred from another hospital' was detected as a significant risk factor. We have already introduced contact precautions for transferred patients until their MRSA status can be proven to be negative by surveillance culture.
We acknowledge several limitations in this study. First, the sample size was small because the study duration was only 1 year. Second, this study was carried out in a single institution. Because there is epidemiologic variation in healthcare-associated infection among institutions, results potentially may not be universally applicable. Third, external validation was not performed in this study. It was difficult to divide the patients to developing and validating data set due to the small sample size of patients with MRSA infection (n = 30) in this study. In the future, it will be necessary to perform an external validation analysis or to examine whether preemptive infection control for patients with high-risk factors actually works effectively. Fourth, patients intubated after the first 24 hours from ICU admission were not investigated. Because those patients will also be at high risk for MRSA infection in terms of disease severity and contact incidence with medical workers, they should also be targeted for preemptive infection control. Our preliminary study warrants further multicenter investigation, and we are presently in the process of conducting a prospective multi-institutional cohort study to assess the effect of focused preemptive infection control according to risk factors for HA-MRSA infection revealed in the present study.