Utilization of the Standardized Infection Ratio for Summarizing Neonatal Central Line-Associated Bloodstream Infection Data

Background: Within the National Healthcare Safety Network (NHSN), central line-associated bloodstream infection (CLABSI) rates in neonatal intensive care units (NICUs) are stratified by NICU level (III vs. II/III), line type (central vs. umbilical), and five birth weight categories (≤750 g, 751-1000 g, 1001-1500 g, 1501-2500 g, >2500 g).  Thus, each NICU may have as many as ten CLABSI rates, which are cumbersome to compare against national rates and difficult to display for multiple NICUs simultaneously.

Objective: Our objective was to use a simplified standard measure to present NICU CLABSI data in Tennessee’s second public report on healthcare associated infections (HAIs), which was the first Tennessee state report to include CLABSI data from NICUs.

Methods: We utilized the standardized infection ratio (SIR) as a summary measure to compare CLABSI data in 26 Tennessee NICUs from July 2008–June 2009 to published national NICU data from 2006-8.  The SIR is an indirect standardization method for summarizing the HAI experience across any number of stratified groups of data.  Each NICU’s SIR was calculated by dividing the total number of observed CLABSI events in a given NICU by the “predicted” (“expected”) number of CLABSIs for that NICU.  The predicted number of infections was calculated by multiplying the observed number of line-days for each line type/birth weight stratum by the corresponding infection rate in the standard population (published national NHSN 2006-8 data).  The numbers of predicted and observed infections were then summed across strata in each NICU in order to calculate the SIR.  Fisher’s exact test was used to calculate a 95% confidence interval (CI) for each SIR.  A SIR that was significantly greater than 1 indicated that the NICU experienced more CLABSIs than predicted; a SIR that was significantly less than 1 indicated that the NICU experienced fewer infections than predicted.  A SIR that was not significantly different from 1 indicated that the observed number of infections was not statistically different from the number of predicted infections.

Results: The observed and predicted numbers of CLABSIs, SIR point estimates, and 95% CIs for all 26 Tennessee NICUs were calculated and published on a single, color-coded “caterpillar plot” (Figure)  in Tennessee’s second public report on HAIs.  The SIR provided a risk-adjusted summary measure of the CLABSI experience in each NICU, and the graphical display facilitated interpretation and presentation of NICU-level data.  In particular, the plot allowed for rapid identification of facilities with significantly low or high SIRs.

Conclusions: The SIR is a useful tool for analysis and presentation of NICU-level CLABSI data because it provides a single metric for performing comparisons across line type/birth weight strata.  For more granular comparisons, infection rates remain the metric of choice.