942 Using a Standardized Infection Ratio as a Risk-Adjusted Measure of Surgical Site Infections to Assess Surgeon Performance

Sunday, March 21, 2010
Grand Hall (Hyatt Regency Atlanta)
Benrong Chen, PhD , University of Michigan Health System, Ann Arbor, MI
Vinita Bahl, DMD, MPP , University of Michigan Health System, Ann Arbor, MI
Lisa Sturm, MPH, CIC , University of Michigan Health System, Ann Arbor, MI
Carol Chenoweth, MD , University of Michigan Health System, Ann Arbor, MI
Background: Surgical site infections (SSI) continue to be a significant source of morbidity and mortality.  Feedback to individual providers about their SSI rates has been shown to be an effective improvement strategy.  Provider-specific SSI rates are typically based on raw rates, risk-stratified rates or standardized infection ratio and have small sample sizes making it difficult to compare performance across providers. 

Objective: To combine risk stratification with standardization to produce a single standardized infection ratio with its confidence interval and to evaluate use of this single measure as an easy comparison among surgeon peers.

Methods: The study population included all inpatients in our hospital who underwent colorectal surgery between July 2007 and Dec. 2008.  Patients with evidence of prior infection, who received antibiotics within 24 hours prior to arrival, or had age less than 18-years were excluded from study.  Each patient was reviewed by infection control staff to identify SSI, using NHSN definitions.  Procedure duration, ASA score and wound classification were used to stratify operations into one of 4 NHSN risk categories.  SSI rates of each index category was calculated for the hospital and used as the expected rate.  Each individual surgeon’s SSI rate by risk category was calculated.  To produce a single measure for each surgeon that is adjusted for risk, a standardized infection ratio (SIR, Observed/Expected) with 95% confidence interval was calculated using the hospital expected SSI rates.

Results: Surgical site infection rates by risk category, and the raw SSI rates and SIR were generated for 28 providers and reported to the department.  A few selected providers were reported here (Table).  The raw rates varied from a low of 8.3% to high of 27.3%.  The SIR exhibited similar variation and was associated with large confidence intervals.  The SIR for each surgeon was not significantly different from the hospital average, which is partially explained by low numbers to date.


Category 0
Rate (N)
Category 1 Rate (N)
Category 2 Rate (N)
Category 3 Rate (N)
Raw Rate (N)

SIR (95% CI)
0% (3)
42.1% (19)
20.0% (25)
20.0% (5)
26.9% (52)

1.04 (0.57, 1.75)
12.5% (16)
14.3% (14)
0% (3)

12.1% (33)

0.65 (0.18, 1.67)
0% (7)
25.0% (4)
0% (1)

8.3% (12)

0.47 (0.01, 2.59)
40% (5)
0% (4)
50.0% (2)

27.3% (11)

1.38 (0.28, 4.04)


Conclusions: The SIR is a promising method for providing a single risk adjusted measure for performance feedback to surgeons, especially when there are low numbers of procedures.  Because the SIR can provide a means to report a surgeon’s overall SSI rates for a combination of different surgical procedures, it will be more powerful, than other procedure-specific SSI rates, at differentiating performance.