Risk Factors for Methicillin-resistant Staphylococcus Aureus (MRSA) Urinary Tract Infection in a Geriatric Hospital: A Matched Case-control Study

The first strains of methicillin-resistant Staphylococcus aureus (MRSA) were reported in the United Kingdom in 1961, only two years after the introduction of methicillin1). Since then, similar strains have been isolated in other parts of the world 2,3), including Japan4). Since MRSA represents a high level of resistance for all antibiotics except for a few such as vancomycin and arbekacin5), the development of MRSA strains has become a serious clinical problems as a causative pathogen of nosocomical infections. The elderly patients have been reported to be one of the high risk groups for MRSA infections). It is, therefore, an urgent problem to reduce the incidence of MRSA infection in geriatric hospitals. Our previous study7) showed that the disability to perform the activities of daily living (ADL) is one of the important risk factors for MRSA infection in the elderly. However, the analysis by the nutritional status such as hypoalbuminemia was not presented in that paper. In addition, most of materials, from which MRSA was isolated, were sputums and pus from decubital ulcers. The

The first strains of methicillin-resistant Staphylococcus aureus (MRSA) were reported in the United Kingdom in 1961, only two years after the introduction of methicillin1). Since then, similar strains have been isolated in other parts of the world 2,3), including Japan4). Since MRSA represents a high level of resistance for all antibiotics except for a few such as vancomycin and arbekacin5), the development of MRSA strains has become a serious clinical problems as a causative pathogen of nosocomical infections. The elderly patients have been reported to be one of the high risk groups for MRSA infections). It is, therefore, an urgent problem to reduce the incidence of MRSA infection in geriatric hospitals.
Our previous study7) showed that the disability to perform the activities of daily living (ADL) is one of the important risk factors for MRSA infection in the elderly. However, the analysis by the nutritional status such as hypoalbuminemia was not presented in that paper. In addition, most of materials, from which MRSA was isolated, were sputums and pus from decubital ulcers. The purpose of this study is to evaluate the effect of the hypoalbuminemia, the use of antibiotics, the use of the third generation cephems and the indwelling of catheter on the occurrence of MRSA urinary tract infection in the elderly after controlling the influence of age and ADL. maximum number of antibiotics administered in MRSA negative patients. The ADL score was the sum of the abilities to perform ADL, ranging from 0 to 3. The patients got one point for each ability to perform ADL, which was the ability to take meals, to walk or to urinate and have bowel movements. The diagnosis of MRSA was made by Showa monodisc8). S. aureus, which showed resistance to both methicillin and ceftizoxime, was diagnosed as MRSA. Serum albumin was measured by autoanalyzer (Olympus, Tokyo).
A statistical analysis was performed using the Statistical Analysis System package (SAS Institute Inc.). Significance was determined by the Wilcoxon test and the chi-square test. A logistic regression analysis was used to control for the possible confounding effects of serum albumin and the use of antibiotics or the use of the third generation cephems on the occurrence of MRSA infection. The odds ratios (ORs) and their 95% confidence intervals (95% CIs) were then calculated for each factor on the basis of the logistic regression coefficient and its standard error.

RESULTS
Gender, age and ADL score did not differ between MRSA group and non-MRSA group. Compared to the non-MRSA group, the number of antibiotics used prior to the bacterial culture was significantly greater for the MRSA group (p<0.05) and the third generation cephems were used more commonly for the MRSA group (p<0.05) ( Table 1). In contrast, serum albumin level was significantly lower in the MRSA group than in the non-MRSA group (p<0.05). The indwelling of catheter was more common in the MRSA group than in the non-MRSA group (25.0% vs 8.3%), but failed to show significant difference.
As shown in Tables 2, 3, the patients with hypoalbuminemia had an increased risk for MRSA infection after  Matsumoto1) described that the common use of the third generation of cephems, which have a weak antibacterial effect for Staphylococcus aureus, must have induced the development of MRSA. In the present study, the third generation cephems were twelve times more commonly used in the MRSA group than in the non-MRSA group (50.0% vs 4.2%, p<0.05). The result confirms that the administration of the third generation cephems is an important risk factor for MRSA infection.
Hypoalbuminemia is suggested to associate with limitations in ADL in the elderly population13,14). In the present study, however, the level of serum albumin was significantly lower in the MRSA group than in the non-MRSA group without the influence of ADL (Table 1). In addition, the average of serum albumin level in the MRSA group was lower compare to the reference range of serum albumin level (3.5-5.0 g/dl) 15) Either antibiotics induced liver damages or long lasting infections may cause hypoalbuminemia in the MRSA group patients, who had been suffered from long lasting infections in spite of antibiotics administration and received a great number of antibiotics before the isolation of MRSA.
In the present study, hypoalbuminemia increased the risk of MRSA infection after the controlling the effect of use of antibiotics or the administration of the third generation cephems which were used only in the cases whose bacterial infections were resistant to other kinds of antibiotics (Table  2). These results suggest that hypoalbuminemia is an independent risk factor for MRSA urinary tract infection.
Konnols) described that the indwelling of catheter is a risk factor for MRSA infection. In the present study, the rate of patients with the indwelling of catheter in the MRSA group was three times as great as the non-MRSA group (25.0% vs 8.3%), but failed to show the significant difference. A small number of MRSA positive cases in the present study may explain this result.
In summary, hypoalbuminemia, the use of many kinds of antibiotics and the use of the third generation cephems may be risk factors for MRSA urinary tract infection among the elderly patients. Physicians should avoid any unnecessary administration of antibiotics in treating the elderly patients. Furthermore, physicians should avoid the third generation cephems when they can use antibiotics other than the third generation cephems.