Setting

This retrospective study was performed at an urban, tertiary care, 136-bed pediatric children’s hospital from April 1, 2009, through March 31, 2014. Approximately 3,000 ambulatory surgeries are performed annually in this center. The operating suites, located on 3 separate campuses, are used for both inpatient and ambulatory surgeries. All patients from birth to 22 years old on the day of surgery were included in this analysis.

Definitions

Ambulatory surgeries were defined as any patient who received a surgery in the operating room but was not admitted overnight to the hospital. All surgery types were included. A surgery was defined as follows: “At least one incision (including laparoscopic approach) is made through the skin or mucous membrane, or reoperation via an incision that was left open during a prior operative procedure.” ¹⁰ This designation was applied to the primary procedure code for every patient. When a procedure code was vague regarding the use of an incision (eg, a cardiac catheterization with incision over the femoral vein), the procedure was excluded. The National Healthcare Safety Network (NHSN) 2014 definition for inpatient SSIsReference Pronovost, Needham and Berenholtz ¹¹ was adapted for the ambulatory arena. A superficial surgical or procedural site infection was defined as occurring within 30 days after the surgery with day 1 as the surgery date, and as defined by the NHSN “involves only skin and subcutaneous tissue of the incision and the patient has at least one of the following: a. purulent drainage from the superficial incision. b. organisms isolated from an aseptically-obtained culture of fluid or tissue from the superficial incision. c. superficial incision that is deliberately opened by a surgeon, attending physician or other designee and is culture positive or not cultured and patient has at least one of the following signs or symptoms: pain or tenderness; localized swelling; redness; or heat. A culture negative finding does not meet this criterion. d. diagnosis of a superficial incisional SSI by the surgeon or attending physician or other designee.” ¹⁰ A deep SSI was similarly defined regarding dates and as defined by the NHSN “involves deep soft tissues of the incision (eg, fascial and muscle layers) and patient has at least one of the following: a. purulent drainage from the deep incision. b. a deep incision that spontaneously dehisces or is deliberately opened by a surgeon, attending physician or other designee and is culture-positive or not cultured and patient has at least one of the following signs or symptoms: fever (>38°C); localized pain or tenderness. A culture-negative finding does not meet this criterion. c. an abscess or other evidence of infection involving the deep incision that is detected on direct examination, during invasive procedure, or by histopathologic examination or imaging test.” ¹⁰ A positive wound culture or any wound culture are not requirements for a patient to meet the NHSN SSI definition. Surgical wound class (clean, clean-contaminated, contaminated, or dirty) was defined by the NHSN criteria. ¹⁰ Medical comorbidities were defined as any listed condition in the patient’s problem list or preoperative anesthesia or surgical history and physical examination.

Identification of Infections

Demographic data on all ambulatory surgeries in the study time window were abstracted from an administrative database (age, gender, race, ethnicity, insurance status, and procedure type). All patients who had billing International Classification of Diseases, Ninth Revision (ICD-9), codes of 998.5 “postoperative infection,” 998.51 “infected postoperative seroma,” or 998.59 “other postoperative infection” were identified. Each patient’s chart with any of these ICD-9 codes was searched for evidence of an adapted NHSN SSI for 30 days following the surgery, using the definitions above. The medical chart review included all notes, laboratory values, subsequent primary care or surgical clinic visits, admissions, and emergency department visits. Those with a suspected adapted NHSN SSI based on the first medical chart review (J.N.) had their infection confirmed by a second author (M.L.R). An infection preventionist helped train both reviewers and was used for consultation and for final adjudication when there was conflict between the 2 reviewers (N=1).

In order to confirm the accuracy of the ICD-9 billing codes and obtain complete case capture, a 5% random sample of patient charts from each surgical group was searched to determine whether evidence of SSI exists without receipt of the billing code for postoperative infection (Figure 1). To further ensure no cases were missed, a second non-overlapping 5% random sample of “high-risk for SSI” patient charts without receipt of the billing code for postoperative infection was also searched. “High-risk for SSI” patients were defined as patients who were evaluated in the emergency department, evaluated in clinic more than once, or admitted in the 30 days following their ambulatory surgery.

FIGURE 1

Design of study of surgical site infections following pediatric ambulatory surgery. ED, emergency department; ICD-9, International Classification of Disease, Ninth Revision; NHSN, National Healthcare Safety Network.

Outcome data were collected from medical charts identified as meeting the adapted NHSN SSI case definition.

SSI Risk Factors

Adapted NHSN SSI cases were compared with the rest of the surgical cohort without found infections to identify administratively available risk factors, denoted as a “case–rest of cohort” design. We acknowledge that the rest of the surgical cohort may include a small number of SSI cases, although this analysis is still valid because those few cases are overwhelmed by the 15,000 other controls in the cohort. Risk factors abstracted included age, gender, race, ethnicity, and insurance status.

A nested case-control design was used for risk factor data available only through medical chart review. Risk factors abstracted for the nested case-control study included number of medical comorbidities, history of prematurity, neonatal age (younger than 30 days), body mass index at time of surgery, day of week of surgery, antibiotics in the 7 days prior to initial surgery, duration of surgery, antibiotics administered immediately before or during the surgery (patients received antibiotic in the 60 minutes prior to surgery or before completion of surgery), use of central venous catheters, use of urinary catheters, and immediate postoperative complications. Patients who received antibiotics before surgery were grouped with those who received antibiotics during surgery because patients who received antibiotics during surgery were more similar to patients who received antibiotics before surgery than to patients who received no antibiotics, and there were relatively few patients who received antibiotics during surgery (1 case and 5 controls). These risk factors were identified in prior SSI studies or by the research team as logical risks for infection. Adapted NHSN SSI cases were matched 1:3 with control patients on the first basis of primary surgical code. When more than 3 controls were available for matching, controls were chosen on the second basis of proximity in time to the case’s surgical date. If more than 3 controls were available with the same primary surgical code and with a surgical date within 1 month of the case’s surgical date, controls were chosen on the third basis of matching secondary procedures. All cases had at least 3 identifiable controls. All control charts were specifically checked to exclude SSIs. The matching scheme prevented wound class from being analyzed as a risk factor. We considered investigating postoperative antibiotics as a preventative risk factor for SSI but these data were inconsistently recorded in the chart.

Data Analysis

Descriptive analysis was performed for demographic characteristics of pediatric patients with ambulatory surgeries and clinical outcomes of SSI following pediatric ambulatory surgeries. All patients with ICD-9 codes for SSI and those in the 5% random sample from charts without ICD-9 codes were included with sampling weights of 1 and 20, respectively, to estimate SSI rates identified by the adapted NHSN definition for ambulatory SSI and investigate sensitivity and specificity of ICD-9 codes. Despite the recent transition to ICD-10 codes, significant data exists utilizing ICD-9 SSI codes, and it is important for researchers and policymakers to understand the sensitivity and specificity of these codes. SSI rates following ambulatory surgeries per 1,000 ambulatory surgeries with 95% confidence intervals (CIs) were obtained by binomial regression incorporating the sampling design. SSI rates were compared between surgical groups by log-binomial regression.

Adapted NHSN SSI cases versus rest of cohort were compared by the χ² or Fisher exact tests for categorical risk factors and Wilcoxon rank sum tests for continuous risk factors given concern for nonnormally distributed case data. Adapted NHSN SSI case versus control data were analyzed using unadjusted conditional logistic regression. For thorough investigation, we additionally performed 2 ad hoc analyses. One compared the cases versus rest of cohort without 19 randomly selected patients. These 19 patients represent the 1 infection found in a 5% random sample with a sampling weight of 20 to simulate the potential for additional unfound SSI cases in the cohort. The other compared the adapted NHSN SSI cases identified only by ICD-9 codes versus their matched controls (ie, conducting a case-control analysis without the adapted NHSN SSI case found via random sampling and its matched controls) to assess the possible impact of the adapted NHSN SSI case found via random sampling on risk factors. Statistical analyses were completed in Stata 11 (StataCorp). This study was approved by the Albert Einstein College of Medicine Institutional Review Board.