Surgical site infections (SSI)s are one of the key healthcare-associated infections (HAIs) tracked by the Centers for Disease Control and Prevention’s National Healthcare Safety Network (NHSN). The NHSN’s SSI surveillance definitions and metrics are used by hospitals and organizations to identify areas for improvement, to benchmark hospital and surgeon performance, and to track the progress of prevention efforts.

The current NHSN SSI rate calculation has some inherent flaws. The NHSN creates large categories for surgical procedures based on International Classification of Diseases, Tenth Revision (ICD-10) or current procedural terminology (CPT) procedure codes. When multiple surgical procedures are performed in the same operation, the NHSN definitions state that the operation is counted in each of the individual NHSN procedure categories; however, when an SSI is identified, it is only counted in 1 procedure category. When multiple procedures are performed through the same incision and the infection is not clearly associated with 1 surgical procedure, the infection is counted toward the “highest-ranking” procedure based on an NHSN-defined hierarchy.¹ For example, an abdominal surgery that involved small bowel (SB), colon (COLO), and rectum (REC) would count in each of the SB, COLO, and REC procedure denominators, but an SSI associated with this operation would only count as a COLO SSI. This method of calculating SSI rates and SIRs is problematic because some procedures counted in an SSI rate denominator could, by definition, never have an associated infection.

To compound this problem, potential inconsistent application of surveillance definitions may skew hospital-to-hospital comparison of SSI rates and standardized infection ratio (SIR) calculations. Hospitals may count denominators differently due to the electronic medical record used and the level of IT support at the individual hospital. For example, many community hospitals only count the highest-ranking procedure in the denominator for SSI rate calculations, potentially inflating their reported SSI rates in comparison to hospitals strictly following NHSN guidelines. On the other hand, hospitals with more advanced information technology support have been able to assign NHSN procedure codes automatically based on ICD-10 procedure codes and follow NHSN rules for counting operations in multiple procedure denominators.

Results

We reviewed surveillance data from 11 hospitals where 200 or more primary spinal fusions were performed and 5 hospitals where 200 or more primary colon surgeries were performed during the 3-year study period. Infection preventionists identified 87 SSIs associated with 17,247 (NHSN SSI rate: 0.50 infections per 100 procedures) laminectomies and 7 SSIs associated with 740 rectal procedures (NHSI SSI rate, 0.95 infections per 100 procedures). Laminectomy was the primary procedure in 12,015 cases (69.7%), whereas rectal surgery was the primary procedure performed in only 305 reported rectal procedures (41.2%). From these data, we calculated the change in SSI rates comparing the adjusted SSI rate to the NHSN SSI rate. The NHSN SSI rate and adjusted SSI rate for laminectomies were 0.50 and 0.72, respectively, which represents a 30.6% increase. The NHSN SSI rate and adjusted SSI rate for rectal surgeries was 0.95 and 2.30, which represents a 58.7% increase. The impact of the change in denominator on SSI rate varied by individual hospital (Fig. 1).

Fig. 1.

Hospital-specific surgical site infecton (SSI) rates comparing adjusted versus the National Health Safety Network (NHSN) denominators for (a) laminectomies and (b) rectal surgeries. Note. SSIs, surgical site infections; RR, rate ratio.