surgical site infections and warm weather 813


table 4. Logistic Regression Model Resultsa Covariate


Month effects January February March April May June July


August


September October


November December


Average temperature <4.44°C°C (<40°F) 4.44–7.22°C (40–45°F) 7.22–10°C (45–50°F) 10–12.77°C (50–55°F) 12.77–15.55°C (55–60°F) 15.55–18.33°C (60–65°F) 18.33–21.11°C (65–70°F) 21.11–23.88°C (70–75°F) 23.88–26.66°C (75–80°F) 26.66–29.44°C (80–85°F) 29.44–32.22°C (85–90°F) >32.22°C (>90°F)


Comorbiditiesb None


Diabetes mellitus Obese


Odds Ratio (95% CI) Baseline


1.09 (1.07–1.11) 1.12 (1.09–1.14) 1.13 (1.11–1.16) 1.15 (1.12–1.19) 1.13 (1.10–1.17) 1.18 (1.13–1.22) 1.21 (1.16–1.25) 1.16 (1.12–1.20) 1.17 (1.14–1.20) 1.18 (1.15–1.20) 1.13 (1.11–1.15)


Baseline


1.02 (1.00–1.035) 1.04 (1.02–1.06) 1.03 (1.01–1.06) 1.08 (1.05–1.10) 1.07 (1.04–1.10) 1.11 (1.08–1.15) 1.09 (1.05–1.13) 1.12 (1.08–1.17) 1.13 (1.08–1.19) 1.23 (1.15–1.32) 1.29 (1.20–1.39)


Baseline


1.27 (1.26–1.28) 1.38 (1.36–1.40)


Covariate Payer


Medicare Medicaid


Private insurance Self-pay


No charge Other


Age Group, y 18–30 30–40 40–50 50–60 60–70 70–80 80+


Region


Northeast Midwest South West


Gender Male


Female


Continuous variables Time trend, y Length of stay


Latitude (scaled) Longitude (scaled)


Baseline


1.12 (1.10–1.15) 1.16 (1.14–1.17) 1.73 (1.64–1.81)


Baseline 0.91 (0.90–0.91)


1.02 (1.02–1.02) 1.02 (1.02–1.02) 1.10 (1.10–1.11) 1.18 (1.16–1.21)


aThe outcome variable is SSI admission. Estimated odds ratios and associated confidence intervals are presented. bAll 29 Elixhauser comorbidities are included in the model as indicator variables, but only those for diabetes mellitus and obese are presented here.


[>90°F], was associated with an increase in the odds of an SSI admission of 28.9% (95% CI, 20.2%–38.3%) when compared to temperatures <4.4°C [<40°F]. In the demographics-only model, the odds of an SSI


discharge increased by 32.1% from January to August (95% CI, 29.5%–34.8%). However, when we controlled for the effects of temperature and demographics, the odds of an SSI discharge are only 20.7% higher in August (95% CI, 16.4%– 25.3%) (Figure 3). By adding average monthly temperature to the model, we were able to explain approximately 35% of the change in the odds of an SSI discharge due to seasonality.


figure 2. The effect of monthly average regional temperature on the odds of surgical-site infection primary admissions.


a primary SSI admission increase by roughly 2.1% per 2.8°C [5°F] increase in the average monthly temperature; all else held constant. Specifically, the highest temperature group, >32.2°C


discussion


Our results show that SSIs are seasonal, with 26.5% more SSI-related hospital discharges in the peak month of August than in the nadir month of January. SSIs are seasonal for men, women, all age groups, and all geographic regions.


Odds Ratio (95% CI) Baseline


0.795 (0.78–0.81) 1.012 (1.00–1.03) 0.647 (0.63–0.66) 0.791 (0.72–0.87) 1.361 (1.33–1.39)


Baseline


1.776 (1.74–1.81) 2.987 (2.93–3.05) 2.758 (2.71–2.81) 2.270 (2.22–2.32) 1.664 (1.63–1.70) 0.896 (0.88–0.92)


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