Understanding Standardized Infection Ratio (SIR) in Healthcare-Associated Infections
The Standardized Infection Ratio (SIR) is a key measure used to monitor healthcare-associated infections (HAIs) at different levels. It compares observed HAIs with predicted values based on specific risk factors. An SIR > 1 indicates more infections than predicted, an SIR = 1 means observed equals predicted, and an SIR < 1 indicates fewer than predicted. Statistical significance is indicated by a p-value < 0.05 or a 95% CI excluding the value 1. Interpreting SIR values considers both observed and predicted infection rates to assess infection control effectiveness.
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Sir, could you explain the SIR? Dana Burshell, MPH, CPH, CIC Andrea Alvarez, MPH Healthcare-Associated Infections Program Virginia Department of Health June 2012
What is a standardized infection ratio? The standardized infection ratio (SIR) is a summary measure used to track healthcare-associated infections (HAIs) at a national, state, or local level over time. The SIR adjusts for patients of varying risk within each facility. - The National Healthcare Safety Network (NHSN)
The SIR calculation In HAI data analysis, the SIR compares the actual number of HAIs reported (observed) with the baseline U.S. experience (predicted) adjusting for several risk factors that have been found to be significantly associated with differences in infection incidence. SIR is a ratio that is a comparison of two values SIR = number of observed HAIs number of predicted HAIs
What is the baseline U.S. experience? 2006-2008 NHSN aggregate data are used as the standard population and considered to be the baseline U.S. experience for the SIR calculations. NHSN baseline data used in an SIR are used to calculate the predicted number of HAIs adjusting for the identified risk factors.
What does the SIR number mean? An SIR greater than 1.0 indicates that more HAIs were observed than predicted. An SIR of 1.0 indicates that the number of HAIs observed was equal to the number predicted. An SIR less than 1.0 indicates that fewer HAIs were observed than predicted. However, the SIR alone does not imply statistical significance. The SIR is only a point estimate and needs additional information to indicate if the finding is significant and not likely due to chance (that is, statistically significantly different from 1).
Statistical significance A p-value and 95% confidence interval (CI) are calculated by NHSN for each SIR. The p-value identifies if the information is statistically significant. The 95% CI can sometimes be used to approximate statistical significance. If the p-value is < 0.05, the SIR is statistically significant". If the SIR 95% CI does not contain the value 1, the SIR is considered "statistically significant".
How do I interpret the SIR? Facility name CLABSI (#) Central line days (#) Predicted CLABSIs (#) SIR SIR p-value SIR 95% CI Hospital X 8 1,976 4.15 1.93 0.06 0.83, 3.80 During 2011, there were 8 CLABSIs identified and 1,976 central line days observed in Hospital X s intensive care units. Based on the NHSN 2006-2008 baseline data and the composition of ICU locations in Hospital X, 4.15 CLABSIs were predicted. This results in an SIR of 1.93 (O/P= 8/4.15), signifying that during this time period, Hospital X identified 93% more CLABSIs than predicted. The p-value (0.06) and 95% confidence interval (CI) (0.83, 3.80) indicate that the number of observed CLABSIs is not statistically significantly higher than the number of predicted CLABSIs. (Reminder: If the p-value is not less than 0.05 and the 95% CI does crosses 1, the SIR is not statistically significantly different than 1.)
How is the SIR being used currently? NHSN SIR reports CLABSI, SSI, CAUTI CMS Hospital Compare website CLABSI Updated VDH HAI report CLABSI Other states Within hospitals
Benefits of using the SIR Single metric One number that can be used to make comparisons Scalable National, regional, facility-wide, location-specific, by surgeon for SSIs, etc. Can combine the SIR values at any level of aggregation Can perform more detailed comparisons within any individual risk group Risk-adjusted Adjusts for factors known to be associated with differences in HAI rates Risk-adjustment differs between types of HAIs and types of surgical procedures - HHS HAI Action Plan - http://www.hhs.gov/ash/initiatives/hai/appendices.html#appendix_g_comparison
Intra-facility data sharing benefits: Top benefits identified by SSI pilot study IPs (2011) Increased awareness of HAIs within the facility Presented data to those who can make a difference Provided benchmark data to support improvement initiatives Kept HAIs in the spotlight
Sharing the SIR with hospital leadership: One IP s experience Reasons IP decided to educate hospital staff: Hospital Compare website was using the SIR VDH had started using the SIR Corporate 2011 report used SIR Important leadership staff members to educate: Leadership in IP (Chairman, Chief Nursing personnel, Quality Director) Presentation included: Explanation of SIR and example calculation Hospital Compare screenshot Examples of CLABSI corporate SIRs compared with CLABSI rates Tables from VDH newsletter comparing CLABSI rates from 2009-11 next to SIRs. NHSN data summary reports to show where comparative data comes from
SIR data presentation: Components to consider Stratification type and time period Presented data Format Interpretation # infections # CL days SIR # predicted p-value 95% CI Comparison National State Bedsize Hospital Historical data Table Graph Order Considerations No infections <1 predicted Aggregate by Hospital Adult/PICU and NICU Unit Time period Annual Semi-annual Quarterly Cues Color Symbols Words SIR language Always customize for your audience whenever possible.
Customizing the data: What numbers? Number of infections Number of central line days SIR Number of infections predicted p-value 95% confidence interval Comparison data National State Other hospitals in same bedsize category Hospital Historical data
Customizing the data: What format? Table Graph Order Alphabetical by unit Highest to lowest SIR How will you account for the situation where: There are no infections? <1 infection is predicted?
Customizing the data: What setting and time period? Setting Hospital Adult/pediatric ICU and neonatal ICU Unit Time period Annual Semi-annual Quarterly Monthly
Customizing the data: How will you help your audience interpret? Cues Color Red/yellow/green ( stoplight ) Red/blue/green Symbols Circles, triangles, squares Arrows to show up or down trend Words SIR language Expected/predicted Fewer/less/better Greater/more/worse Similar/same
Questions? Dana.Burshell@ vdh.virginia.gov Andrea.Alvarez@ vdh.virginia.gov To speak to any member of the VDH HAI Team: 804-864-8141
SIR 101 and 201 available online SIR 101: Interpretation and public reporting Reviewed basic SIR calculation and interpretation Introduced publicly available SIR reports NHSN, Hospital Compare, Virginia Department of Health SIR 201: Calculating the measure, generating reports, and presenting the data Nuts and bolts, step-by-step presentation for IPs Archived and available at: http://www.vdh.virginia.gov/epidemiology/ surveillance/hai/communication.htm
SIR and surveillance resources NHSN e-News: SIRs Special Edition http://www.cdc.gov/nhsn/PDFs/Newsletters/ NHSN_NL_OCT_2010SE_final.pdf VDH HAI website Surveillance http://www.vdh.virginia.gov/Epidemiology/ Surveillance/HAI/SurveillanceReporting.htm
