812 infection control & hospital epidemiology july 2017, vol. 38, no. 7
table 2. Subgroup Analysis of Surgical-Site Infection (SSI) Trends and Seasonality, Controlling for the Number of Surgeries in the Month of the SSI Admission and the PriorMonth Within Each Subgroup
Subgroup Region
Northeast Midwest South West
Age group, y 18–30 30–40 40–50 50–60 60–70 70–80 80+
Gender Male
Female
Hospitalb Teaching
Nonteaching Urban Rural
Annual Trend, % (95% CI)
2.15 (0.9–3.4) 1.37 (0.0–2.8)
0.08 (−0.9 to 1.1) 1.42 (0.5–2.4)
0.68 (0.1–1.3)
0.30 (−0.4 to 1.0) 0.18 (−0.4 to 0.7) −1.71 (−3.0 to −0.5) 1.23 (−1.2 to 3.7) −1.20 (−2.2 to −0.2) 0.21 (−0.9 to 1.3)
1.02 (−0.4 to 2.4) 0.60 (−0.2 to 1.5)
−0.13 (−1.7 to 1.5) 0.17 (−0.5 to 0.9) −1.59 (−2.0 to −1.1) 0.32 (−0.3 to 1.0)
Average Amplitude of Seasonality, % (95% CI)a
28.10 (22.7–33.7) 23.86 (19.0–29.0) 22.50 (17.7–27.5) 22.51 (16.7–28.6)
23.28 (16.1–30.9) 27.59 (21.4–34.1) 31.44 (25.4–37.7) 36.03 (30.5–41.8) 21.81 (16.2–27.7) 23.66 (17.3–30.4) 12.58 (4.7–21.0)
28.24 (24.0–32.6) 22.71 (19.4–26.1)
22.89 (19.0–26.9) 24.15 (20.5–27.9) 29.39 (21.8–37.4) 22.77 (19.9–25.7)
NOTE, CI, confidence interval. aAverage amplitude of seasonality is the percentage increase in SSI
between the peak and nadir month. bNot mutually exclusive categories in the Nationwide Inpatient Sample (NIS) dataset.
average amplitude of seasonality was 22.89% (95% CI, 19.0%–26.9%) for teaching hospitals and 24.15% (95% CI, 20.5%–27.9%) for nonteaching hospitals.
Weather Models
Descriptive statistics for our weather model are presented in Table 3. SSI cases were generally similar to the control group in terms of their mean age, sum of Elixhauser comorbidities, latitude, longitude, and region. However, cases had a higher mean length of stay (7.15 days vs 4.83 days), and they were admitted during a month with a slightly higher mean tem- perature than the control group (12.96°C [55.33°F] vs 12.43°C [54.39°F]). Additionally, although the mean age of cases was similar to that of the controls (56.88 years vs 56.46 years), the vast majority of cases were middle aged, while admissions for older ages were much more likely to be controls. SSI patients had higher rates of diabetes (19.0% vs 14.7%) and obesity (9.5% vs 5.6%) than the controls. Results from the weather logistic regression model are pre-
sented in Table 4. Patients aged in their 40s were 199% more likely to be an SSI admission (95% CI, 193%–205%) than the baseline group of 18- to 30-year-old patients. However,
table 3. Descriptive Statistics for Variables of Interest in the Surgical-Site Infection (SSI) and Control Groupsa
Variable No. of patients
Continuous variables Age, y
Length of stay, d Latitude
Longitude
Average temperature Elixhauser sum
Categoric variables Gender
Female Male
Payer
Medicare Medicaid
Private insurance Self-pay
No Charge Other
Region
Northeast Midwest South West
Comorbidityc Diabetes mellitus Obese
Age group, y 18–30 30–40 40–50 50–60 60–70 70–80 80+
17,310 (31.2) 11,880 (21.4) 10,068 (18.2) 16,171 (29.2)
8,149 (14.7) 3,106 (5.6)
7,544 (13.6) 6,977 (12.6) 6,588 (11.9) 7,422 (13.4) 7,954 (14.3) 9,358 (16.9) 9,587 (17.3)
73.21 (31.0) 47.51 (20.1) 43.86 (18.6) 71.38 (30.2)
44.94 (19.0) 22.46 (9.5)
15.0 (6.3)
25.83 (10.9) 42.0 (17.8) 48.4 (20.5) 45.5 (19.3) 38.8 (16.4) 20.4 (8.6)
aMean and standard deviation for continuous variables (top) and
number and percentage of patients for categorical variables (bottom). bAll variables were statistically significantly different between the SSI
and control groups (all P<.001). cThere are 29 Elixhauser comorbidities, but only diabetes mellitus and obese are presented in this table.
admissions for older patients (80+ ) were 10.4% less likely to be SSI related (95% CI, 8.1%–12.5%). The weather model also indicated a significant time trend over the course of the study: the odds of an SSI admission grew by 2% per year (95% CI, 1.9%–2.1%). Higher rates of SSI admission were associated with diabetes, which had 26.9% higher odds (95% CI, 25.5%– 28.3%), and obesity, which had 38.2% higher odds (95% CI, 36.3%–40.3%). Finally, the effect of temperature on the odds of SSI admission is presented in Figure 2. The odds of
24,172 (43.6) 7,924 (14.3) 18,999 (34.3) 2,443 (4.4) 109.5 (0.2) 1,782 (3.2)
93.69 (39.7) 25.81 (10.9) 97.3 (41.2) 6.82 (2.9)
0.403 (0.17) 11.92 (5.1)
33,625 (60.7) 21,805 (39.3)
131.74 (55.8) 104.23 (44.2)
Controlb 55,429,859
Mean (±SD) 56.88 (±21.1)
4.83 (±6.8) 39.61 (±3.4)
–91.27 (±18.0) 54.39 (±16.1) 1.77 (±1.6)
No. (thousands), % of sample
SSI 235,969
Mean (±SD) 56.46 (±16.8)
7.15 (±8.8) 39.71 (±3.5)
–91.47 (±18.2) 55.33 (±16.0) 1.78 (±1.6)
No. (thousands), % of sample
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