AJIC and and NHSN Collaborative Case Study: How infection present at time of surgery (PATOS) data impacts your surgical site infection (SSI) standardized infection ratios (SIR), with focus on the Complex 30-day SSI SIR model

Introduction

This is the first analytic case study published in the American Journal of Infection Control (AJIC) since the Centers for Disease Control and Prevention/ National Healthcare Safety Network (NHSN) updated its healthcare-associated infection (HAI) risk adjustment models and rebaselined the standardized infection ratios (SIRs) in 2015. This case describes an analytic scenario that Infection Preventionists (IPs) have encountered during their analysis of surgical site infection (SSI) surveillance data. The case study is intended to illustrate how specific models can impact the SIR results by highlighting differences in the criteria for NHSN’s older and newer risk models: the original versions and the updated models introduced in 2015. Understanding these differences provides insight into how SSI SIR calculations differ between the older and newer NHSN baseline models. NHSN plans to produce another set of HAI risk adjustment models in the future, using newer HAI incidence and risk factor data. While the timetable for these changes remains to be determined, the statistical methods used to produce future models and SIR calculations will continue the precedents that NHSN has established. For a description of how to use logistic regression to calculate the risk adjusted number of predicted SSIs, readers should consult the NHSN SIR Guide¹. Objectives of AJIC case studies have been previously published for this case study series.²

All individual participant answers will remain confidential, although it is the authors’ intention to share an aggregate of the findings later. Cases, answers, and explanations have been reviewed and approved by NHSN staff.

We strongly recommend review/reference of the NHSN SIR Guide and the NHSN Patient Safety Component Manual, Procedure-associated Events Module for information that may be needed to answer the case study questions. The website links are:

https://www.cdc.gov/nhsn/pdfs/ps-analysis-resources/nhsn-sir-guide.pdf
https://www.cdc.gov/nhsn/pdfs/pscmanual/9pscssicurrent.pdf

The findings and conclusions in this case study are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

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