OBJECTIVE
To determine if the seasonality of surgical site infections (SSIs)
may be explained by changes in temperature.
DESIGN
Retrospective cohort analysis.
SETTING
The National Inpatient Sample.
PATIENTS
All hospital discharges with a primary diagnosis of SSI from
1998–2011 served as cases. Discharges with a primary or secondary
diagnosis 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
temperature 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. Over 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 increase by roughly 2.1% per
5°F increase in the average monthly temperature. Specifically, the
highest temperature group, 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 less than 40°F.
CONCLUSIONS
At population level, SSI risk is highly seasonal and associated with
warmer weather.