Trends and Characteristics of Culture-Confirmed Staphylococcus aureus Infections in a Large U.S. Integrated Health Care Organization

Abstract

Infections due to Staphylococcus aureus present a substantial health problem in the United States (20, 23). S. aureus is a major cause of hospital-acquired pneumonia and lower respiratory tract infections (37) and the primary cause of surgical-site infections (36) and skin and soft tissue infections (SSTIs) (29) and is now likely the leading cause of invasive bacterial disease (20,23). Throughout the 1990s, S. aureus infections in hospitalized patients were increasingly caused by methicillin-resistant S. aureus (MRSA), making treatment of these infections more difficult (20,21,23,40). Moreover, between 1990 and 2005, there was an even more dramatic increase in community-associated MRSA (CA-MRSA) infections (1,5,10,11,19,27,32 In general, surveillance activity for MRSA has been limited to bloodstream or invasive infections and to health care-associated and hospital onset disease, and there have been few populationbased studies. As the epidemiology of S. aureus disease changes, inclusion of community-associated, community onset, and noninvasive disease is important for assessing the magnitude of the burden of disease in the population, for setting priorities for prevention and control, and for creating guidelines for empirical antibiotic treatment.In this retrospective study covering 12 years, we describe laboratory-confirmed S. aureus infections in a large cohort of persons of all ages, using isolates from sterile and nonsterile sites in both ambulatory and inpatient settings. We describe trends in methicillin resistance of laboratory-confirmed S. aureus isolates and trends in the incidence of S. aureus bloodstream infections and explore the relationship of patient demographics and infection onset type to the likelihood of S. aureus infections being methicillin resistant. MATERIALS AND METHODSSetting. Kaiser Permanente of Northern California (KPNC) is a nonprofit, integrated health care delivery system providing care to over 3 million members. The member population reflects the general population in the Northern California region, although as an insured population, it underrepresents persons with annual incomes less than $25,000 (9% of the KPNC member population 25 to 79 years of age versus 21% of the general Northern California population 25 to 79 years of age) and overrepresents persons with annual incomes greater than $100,000 (30% versus 26%). Although racially diverse, the KPNC population also somewhat underrepresents Latinos/Hispanics (19% versus 22%) and overrepresents African-Americans (8% versus 5.5%) (15). KPNC provides services in more than 15 counties and operates more than 40 outpatient clinics and 18 hospitals throughout Northern California. Laboratory results and diagnostic data from hospital discharges and ambulatory settings, including emergency departments, are archived in databases. KPNC databases contain individual patient records and are readily linked using the patient's unique medical record number, which remains with the patient for life.

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