Two multicenter, prospective, cross-sectional studies were conducted from May 1 to June 30 of 2018 and the same time of 2019, and enrolled adult patients (> 18 years) who received colorectal surgery in the department of general surgery of 55 hospitals in China during these periods (Supplementary file 1). The follow-up period was defined as 30 days after surgery. The exclusion criteria were pregnancy, patients undergoing urological, transplantation or gynecological surgery. The ethics committees of all institutions involved in the study approved our study. All enrolled patients should provide written informed consent ahead of their participation in the study. For data collecting, we only included those patients who have agreed to use the information from their medical records for the purpose of scientific research.
Aside from baseline variables, we collected other data which may be related to the likelihood of SSI occurrence. Firstly, each hospital identified patients who met the inclusion criteria and could comply with follow-up, and collected their essential data. Secondly, we had specialized team members to evaluate the whole data collection process and the accuracy of all data.
Baseline variables of enrolled patients covered gender, age, body mass index (BMI), American Society of Anesthesiologists (ASA) physical status score, diagnosis, the prior diagnosis of diabetes mellitus, hypertension, chronic renal dysfunction (renal failure or dialysis), chronic hepatic dysfunction (abnormal concentrations of liver enzymes, hepatocellular carcinoma, cirrhosis or hepatitis), chronic cardiac dysfunction (heart failure, myocardial infarction, or previous cardiac surgery), tuberculosis, use of immunosuppressive medication, smoking status (nonsmoker, former smoker, or current smoker), preoperative blood biochemical parameters (albumin, hemoglobin and fasting blood glucose; collected at the preoperative day of surgery), and length of preoperative stay.
Patient perioperative characteristics covered type of surgery based on the urgency of surgery (emergency surgery or elective surgery), surgical site (colon or rectum), timing of hair removal (none, night before surgery, or day of surgery), surgical wound classes (dirty, clean-contaminated, or contaminated), ways of bowel preparation (oral antibiotics bowel preparation [OABP] without mechanical bowel preparation [MBP], MBP without OABP, or OABP combined with MBP), type of surgical hand preparation (scrubbing or disinfectant), surgery approach (open surgery, laparoscopy surgery or robotic surgery), incisional protection (adhesive incise drapes, wound edge protector, or gauze which is put around the incision to avoid the friction of surgical instruments on the incision skin and subcutaneous tissue), type of fluids for incisional wound irrigation (povidone-iodine solution, hydrogen peroxide solution, saline, or something else), fascial or muscle suture materials (silk suture, absorbable sutures, or antimicrobial-coated and absorbable sutures), skin closure materials (silk suture, absorbable sutures, skin staples, or something else), duration of surgery, grade of lead surgeon on the basis of their professional titles, colostomy/ileostomy, and the national nosocomial infections surveillance (NNIS) risk index.
The NNIS risk index is an internationally recognized method for stratifying surgical risk, while the NNIS risk index  varied from 0 to 3 by assessing three variables: duration of surgery, surgical wound class and ASA score. Each variable’s cutoff values were a contaminated or dirty surgical incision, the operative duration of 225 min and an ASA score of 3, with 1 point evaluated if any one variable was over the cutoff value.
In this study, the primary outcome was the occurrence of SSI within postoperative 30 days defined by Center for Disease Control criteria , including organ-space infections, deep or superficial incisional infections. The follow-up was carried out via standard telephone interview or review of the readmission records if patients were discharged from hospital after less than postoperative 30 d. If patients had more than one type of SSI within 30 days, only a single form of SSI with the deepest anatomy was included in subsequent analysis. In addition, once SSI happened, the bacterial culture of secretion, the pus, pelvic puncture fluid, or distal catheter would be conducted, which depended on the criteria of each hospital.
The secondary outcomes were duration of total hospital stay and postoperative stay, costs, and postoperative 30-day mortality.
Results were shown as the mean ± standard deviation (SD) or median with interquartile range (IQR), as appropriate. Comparisons of all continuous variables between the two groups were conducted by using the Mann-Whitney U-test or Student’s t-test depending on Gaussian distribution. Comparisons of categorical variables were conducted using Fisher’s exact test or chi-square test, as appropriate. The criterion of statistical significance was P < 0.05.
Variables with statistical significance in univariable logistic regressions would be included into multivariable logistic regression analysis to identify the independent risk factors of SSI within postoperative 30 days.
We used SPSS v24 software to analyze all data.