Risk adjustment for determining surgical site infection in colon surgery: Are all models created equal?

Background: Colon surgical site infections (SSIs) are being utilized increasingly as a quality measure for hospital reimbursement and public reporting. The Centers for Medicare and Medicaid Services (CMS) now require reporting of colon SSI, which is entered through the U.S. Centers for Disease Control and Prevention's National Healthcare Safety Network (NHSN). However, the CMS's model for determining expected SSIs uses different risk adjustment variables than does NHSN. We hypothesize that CMS's colon SSI model will predict lower expected infection rates than will NHSN. Methods: Colon SSI data were reported prospectively to NHSN from 2012-2014 for the six Fairview Hospitals (1,789 colon procedures). We compared expected quarterly SSIs and standardized infection ratios (SIRs) generated by CMS's risk-adjustment model (age and American Society of Anesthesiologist [ASA] classification) vs. NHSN's (age, ASA classification, procedure duration, endoscope [including laparoscope] use, medical school affiliation, hospital bed number, and incision class). Results: The patients with more complex colon SSIs were more likely to be male (60% vs. 44%; p = 0.011), to have contaminated/dirty incisions (21% vs. 10%; p = 0.005), and to have longer operations (235 min vs. 156 min; p < 0.001) and were more likely to be at a medical school-affiliated hospital (53% vs. 40%; p = 0.032). For Fairview Hospitals combined, CMS calculated a lower number of expected quarterly SSIs than did the NHSN (4.58 vs. 5.09 SSIs/quarter; p = 0.002). This difference persisted in a university hospital (727 procedures; 2.08 vs. 2.33; p = 0.002) and a smaller, community-based hospital (565 procedures; 1.31 vs. 1.42; p = 0.002). There were two quarters in which CMS identified Fairview's SIR as an outlier for complex colon SSIs (p = 0.05 and 0.04), whereas NHSN did not (p = 0.06 and 0.06). Conclusion: The CMS's current risk-adjustment model using age and ASA classification predicts lower rates of expected colon SSIs than does NHSN. This may lead to financial penalties because of the use of limited risk factors. Further efforts at elucidating appropriate risk adjustment measures without unnecessarily burdening hospitals with expensive data collection are necessary.