The Effect of Increasing Age on the Risk for Surgical Site Infection

Introduction: The National Nosocomial Infection Surveillance System (NNIS) risk adjustment system is used to predict risk of SSI. The objective of this study was to determine the relationship between age and SSI, and to determine whether the addition of age improves the predictive value of the current NNIS risk index. Methods: This was a retrospective cohort study of 144, 485 patients who underwent surgery between February 1, 1991 and July 31, 2002 at one of 11 hospitals: Duke University Medical Center and 10 community hospitals in the Duke Infection Control Outreach Network (DICON). Our cohort was randomly divided into two groups: a derivation cohort and a validation cohort. Logistic regression was used to control for the effect of NNIS variables (wound class, American Society of Anesthesiologists (ASA) score and duration of procedure). The function of age that best predicts SSI in the derivation cohort was obtained by restricted cubic splines and these results were confirmed in the validation cohort. Results: There were 1684 SSis in the cohort (SSI rate=1.2%). The mean age of the cohort was 52.4 years (range 17-108). The most common procedures were orthopedic (41.8%), obstetric/gynecologic (10.7%) and cardiothoracic (10.0%). In the derivation cohort, there were 72,139 procedures and 873 SSI (SSI rate=1.2%). After adjusting for NNIS variables, hospital and type of procedure, there was a significant linear relationship between age and SSI (p=0.006). SSI risk increased by 1% per year between ages 17-65 (p=0.002). Interestingly, after age 65, SSI risk decreased by 1% for each additional year (p=0.008, relative to the pre-65 effect). In the validation cohort, there were 72,334 procedures and 811 infections (SSI rate=1.1 %). The relationship between age and SSI was the similar to the relationship in the derivation cohort. Conclusions: Age independently predicted SSI and added significant predictive power to the NNIS risk index after adjustment for NNIS risk variables. SSI risk increased in a linear fashion until age 65. After age 65, SSI risk decreased in a similar fashion for each additional year of age. The decrease in SSI risk after age 65 should be further investigated.Master of Public Healt