A population-based study of the epidemiology and clinical features of methicillin-resistant Staphylococcus aureus infection in Pennsylvania, 2001-2010

Abstract

No U.S. general population-based study has characterized the epidemiology and risk factors, including skin and soft tissue infection (SSTI), for healthcare-associated (HA) and community-associated (CA) methicillin-resistant Staphylococcus aureus (MRSA). We estimated the incidence of HA- and CA-MRSA and SSTI over a 9-year period using electronic health record data from the Geisinger Clinic in Pennsylvania. MRSA cases were frequency-matched to SSTI cases and controls in a nested case-control analysis. Logistic regression was used to assess risk factors, while accounting for antibiotic administration. We identified 1713 incident CA- and 1506 HA-MRSA cases and 78 216 SSTI cases. On average, from 2005 to 2009, the annual incidence of CA-MRSA increased by 34%, HA-MRSA by 7%, and SSTI by 4%. Age, season, community socioeconomic deprivation, obesity, smoking, previous SSTI, and antibiotic administration were identified as independent risk factors for CA-MRSA.

Fig. 1.

Flow chart depicting MRSA case selection and diagnosis location. GHS, Geisinger Health System; G1, Group 1; G2, Group 2; G3, Group 3; ICD-9, International Classification of Diseases, 9th Revision, Clinical Modification; EP, Geisinger Clinic generated electronic health record code; PL, problem list table; HA-MRSA, healthcare-associated methicillin-resistant Staphylococcus aureus; CA-MRSA, community associated MRSA; S. aureus, Staphylococcus aureus. ^a Geisinger Health System, outpatient data was available 2001–2010, and inpatient data 2004–2010. ^b G1, G2 and G3 indicate the method by which MRSA cases were identified. The numbered hierarchy was used when a case was identified by multiple methods on the same day, otherwise the case was linked to the method that identified the earliest MRSA diagnosis. ^c These numbers are not mutually exclusive. ^d Before 2007 there was no MRSA-specific ICD-9 code and MRSA was identified at GHS by the analogous MSSA code plus a V09.0 code, indicating infection with microorganisms resistant to penicillins. ^e Checked for PCR indicating MRSA colonization. ^f Cases originally selected with codes 041·10 (Staphylococcus infection, unspecified) and 038·10 (Staphylococcus septicaemia, unspecified) were excluded. ^g Due to the lack of inpatient data before 2004, cases could not be categorized as CA- or HA-MRSA before 2005. ^h If a patient met any of the six criteria they were classified as HA-MRSA. ⁱ V12.04, the ICD-9 code for history of MRSA infection in record. ^j Patients were disqualified from the CA-MRSA category if their procedures file contained a surgery or their inpatient, outpatient or emergency department file contained a post-operative visit in the 330 days before MRSA infection. ^k n₁, n₂, n₃ correspond to the numbered hierarchy and represent the total number of patients with each characteristic by group (i.e. these numbers are not mutually exclusive). ^l Patients were disqualified from the CA-MRSA category if their address matched an address of a nursing home facility in Pennsylvania listed on the Nursing Home Compare database website provided by the U.S. Department of Health and Human Services.

Fig. 2.

Incidence of MRSA infection and SSTI per 100 000 person-years among the Geisinger Clinic's primary-care patients, 2001–2009 (MRSA cases are combined before 2005 because lack of inpatient data did not allow for assignment to HA- and CA-MRSA groups). CA-MRSA, Community-associated MRSA; HA-MRSA, healthcare-associated MRSA; SSTI, skin and soft tissue infection.

Fig. 3.

Adjusted odds ratios for skin and soft tissue infection in the year preceding diagnosis, comparing CA- and HA-MRSA and SSTI cases to controls. SSTIs include: cellulitis and abscess, carbuncle and furuncle, erysipelas, impetigo, and unspecified local infection of skin and subcutaneous tissue. CA-MRSA, Community-associated MRSA; CI, confidence interval; HA-MRSA, healthcare-associated MRSA; SSTI, skin and soft tissue infection.