Introduction: The aim of this study was to describe the rate of surgical site infections (SSI), classified according to the NNIS index and its components, as well as to evaluate this scale and assess the importance of several factors that influence the development of an SSI.
Patients and method: All episodes of SSI were prospectively registered over a 5-year period. All patients (infected or not) were given an NNIS index and an NNIS category. Postoperative hospital stay and bacteria cultured from the surgical site were also analyzed. Chi2 test, Student's t-test and multiple logistic regression were used.
Results: There were 6,218 patients and 513 SSI (8.25%). The infection rate was 2.27% for clean surgery, 9.17% for clean-contaminated surgery, 11.40% for contaminated surgery, and 19.14% for dirty surgery; 4% for ASA I, 8.23% for ASA II, 13.54% for ASA III, 19.55% for ASA IV, and 33.33% for ASA V; 6.97% for length of procedure = 75th percentile, and 23.01% for > 75th percentile; 3.95% for NNIS 0, 8.17% for NNIS 1, 22.08% for NNIS 2, and 37.23% for NNIS 3. Among the components of the NNIS index, the length of the surgical procedure had the greatest influence on the rate of SSI (OR = 3.43 versus OR = 2.60 for the grade of contamination and OR = 2.20 for ASA index). The infection rates according to the type of intervention were: 30.9% in hepatobiliopancreatic surgery, 24.3% in small bowel surgery, 16.1% in colorectal surgery, 15.4% in gastroduodenal surgery; 8.5% in other soft tissue interventions, 7.7% in exploratory laparotomies, 6.4% in appendicectomies for appendicitis, 5.0% in cholecystectomy, 5.0% in other interventions of the digestive tract, 3.3% in breast surgery, 1.5% in herniorrhaphies, and 0.7% in endocrine surgery.
Conclusions: The NNIS index is a valid instrument for classifying surgical patients according to the risk of developing an SSI. Of the three components, the length of the intervention has the greatest influence on increasing the risk of infection. The NNIS categories also distinguish different levels of risk of infection.