Surgical Site Infections: Volume-Outcome Relationship and Year-to-Year Stability of Performance Rankings

Abstract

Background: Surgical site infection (SSI) rates are publicly reported as quality metrics and increasingly used to determine financial reimbursement.

Objective: To evaluate the volume-outcome relationship as well as the year-to-year stability of performance rankings following coronary artery bypass graft (CABG) surgery and hip arthroplasty.

Research design: We performed a retrospective cohort study of Medicare beneficiaries who underwent CABG surgery or hip arthroplasty at US hospitals from 2005 to 2011, with outcomes analyzed through March 2012. Nationally validated claims-based surveillance methods were used to assess for SSI within 90 days of surgery. The relationship between procedure volume and SSI rate was assessed using logistic regression and generalized additive modeling. Year-to-year stability of SSI rates was evaluated using logistic regression to assess hospitals' movement in and out of performance rankings linked to financial penalties.

Results: Case-mix adjusted SSI risk based on claims was highest in hospitals performing <50 CABG/year and <200 hip arthroplasty/year compared with hospitals performing ≥200 procedures/year. At that same time, hospitals in the worst quartile in a given year based on claims had a low probability of remaining in that quartile the following year. This probability increased with volume, and when using 2 years' experience, but the highest probabilities were only 0.59 for CABG (95% confidence interval, 0.52-0.66) and 0.48 for hip arthroplasty (95% confidence interval, 0.42-0.55).

Conclusions: Aggregate SSI risk is highest in hospitals with low annual procedure volumes, yet these hospitals are currently excluded from quality reporting. Even for higher volume hospitals, year-to-year random variation makes past experience an unreliable estimator of current performance.

FIGURE 1

These are plots of the coding for surgical site infection by volume of procedures performed on fee-for-service Medicare patients who underwent coronary artery bypass graft surgery (A) and primary hip arthroplasty (B) in US hospitals in 2011. The dots represent the coding percentages from individual hospitals, jittered slightly to prevent overplotting. The solid line represents 100 times the predicted probability of surgical site infection coding obtained from a generalized additive model, and the dashed lines represent the 95% confidence interval for the predicted probability line. SSI indicates surgical site infection.

FIGURE 2

For each volume category, 5 hospitals were selected at random to show the case mix-adjusted rank from 2005 to 2011 relative to other hospitals performing coronary artery bypass graft surgery and hip arthroplasty for fee-for-service Medicare patients. This shows the variability in performance ranking over time.