infection control & hospital epidemiology july 2017, vol. 38, no. 7 original article
The Seasonal Variability in Surgical Site Infections and the Association With Warmer Weather: A Population-Based Investigation
Chris A. Anthony, MD;1 Ryan A. Peterson, MS;2 Linnea A. Polgreen, PhD;3 Daniel K. Sewell, PhD;2 Philip M. Polgreen, MD, MPH4,5
objective. To determine whether the seasonality of surgical site infections (SSIs) can be explained by changes in temperature. design. Retrospective cohort analysis. setting. The National Inpatient Sample database.
patients. All hospital discharges with a primary diagnosis of SSI from 1998 to 2011 were considered cases. Discharges with a primary or secondary diagnoses of specific surgeries commonly associated with SSIs from the previous and current month served as our “at risk” cohort.
methods. We modeled the national monthly count of SSI cases both nationally and stratified by region, sex, age, and type of institution. We used data from the National Climatic Data Center to estimate the monthly average temperatures for all hospital locations. We modeled the odds of having a primary diagnosis of SSI as a function of demographics, payer, location, patient severity, admission month, year, and the average temperature in the month of admission.
results. SSI incidence is highly seasonal, with the highest SSI incidence in August and the lowest in January. During the study period, there were 26.5% more cases in August than in January (95% CI, 23.3–29.7). Controlling for demographic and hospital-level characteristics, the odds of a primary SSI admission increased by roughly 2.1% per 2.8°C (5°F) increase in the average monthly temperature. Specifically, the highest temperature group, >32.2°C (>90°F), was associated with an increase in the odds of an SSI admission of 28.9% (95% CI, 20.2–38.3) compared to temperatures <4.4°C (<40°F).
conclusions. At population level, SSI risk is highly seasonal and is associated with warmer weather. Infect Control Hosp Epidemiol 2017;38:809–816
Surgical site infections (SSIs) are among the most common healthcare-associated infections;1–3 they represent an important cause of morbidity following surgeries.4,5 SSIsresultinincreased use of antimicrobials,6 increased lengths of hospital stay,7,8 and increased rates of mortality.8,9 They are also a leading cause of hospital readmission,8,10,11 and they contribute to excess healthcare costs.4,5,7,11 Reports of SSI rates typically vary from 2%to 5%,2 but lower
and higher rates have been reported.4,5 SSI rates also vary across different procedures. Surgeries following trauma and some procedures (eg, colorectal surgeries) are much more likely to generate an SSI.1–3,12 At the patient level, risk factors for SSIs include smoking,13,14 diabetes,14,15 obesity,16 increasing age,14,17 and poor nutrition.18 In addition to individual and procedure- related risk factors for SSIs, environmental-level risk factors may
based on investigations in single centers, on specific proce- dures (eg, spinal surgeries), or in specific geographic regions. Furthermore, these specific investigations did not all use the proper time series methods for analyzing autocorrelated data, and they did not incorporate local weather patterns across large regions to determine how much of SSI seasonality can be explained by weather effects. The first objective of this study was to determine whether and to what extent the incidence of SSIs is seasonal, using a large, population-based, national
also exist. At the institutional level, the volume of procedures19,20 or institution size21 may increase the SSI risk, and other environmental-level risk factors may also exist. For example, some studies have demonstrated an increased incidence of SSIs for surgeries performed during summer months.12,22–24 To date, most reports regarding the seasonality of SSIs are
Affiliations: 1. Department of Orthopaedic Surgery and Rehabilitation, University of Iowa Hospitals and Clinics, Iowa City, Iowa; 2. Department of
Biostatistics, University of Iowa, Iowa City, Iowa; 3. Department of Pharmacy Practice and Science, University of Iowa, Iowa City, Iowa; 4. Departments of Internal Medicine and Epidemiology, University of Iowa Hospitals and Clinics, Iowa City, Iowa; 5. University of Iowa Health Ventures’ Signal Center for Health Innovation, Iowa City, Iowa.
© 2017 by The Society for Healthcare Epidemiology of America. All rights reserved. 0899-823X/2017/3807-0007. DOI: 10.1017/ice.2017.84 Received December 21, 2016; accepted March 29, 2017; electronically published May 16, 2017
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