The Standardized Infection Ratio: What, Why and When

Jonathan R. Edwards, Mstat

Maggie Dudeck, MPH, CPH

Division of Healthcare Quality Promotion

CDC

APIC Webinar

January 26, 2011

Objectives

● Describe the standardized infection ratio (SIR)

● Discuss business cases for the SIR by federal

and state agencies

● Explain the current approach for calculating the

SIR for two specific healthcare-associated

infection (HAI) events:

– Central line-associated BSI (CLABSI)

– Risk model methodology used for SSI SIRs

Standardized Infection Ratio

● Standardized Infection Ratio, SIR, is a

summary measure used to compare the

HAI experience among one or more

groups of patients to that of a standard

population’s (e.g. NHSN)

● Indirect standardization method

● Accounts for differences in incidence of

HAI within the groups

● To calculate O, sum the number of

HAIs among a reporting entity

● To calculate E, requires the use of the

appropriate aggregate data from a

standard population

SIR = Observed (O) HAIs

Expected (E) HAIs

Calculating an SIR

Business Cases for the SIR

● Development of a national action plan to prevent

healthcare-associated infections beginning in

2008 sponsored by the Department of Health and

Humans Services (a.k.a. HHS Action Plan)

● State health departments with legislation requiring

HAI reporting for all their eligible hospitals

● CMS Inpatient Prospective Payment System

– CLABSI reporting beginning January 2011

– SSI reporting beginning January 2012

Central Line-

associated

Bloodstream

Infections

Edwards JR et al. Am J Infect Control 2009;37:783-805.

Annual NHSN Report

Type of

Location

#

CLABSI

# Central

line-days

CLABSI

Rate

NHSN

Ratep-value Expected

# of

CLABSI

Medical cardiac 2 380 5.26 2.0 0.09 0.76

Medical 1 257 3.89 2.6 0.15 0.67

Med/Surgical 3 627 4.78 1.5 0.11 0.94

Neurosurgical 2 712 2.81 2.5 0.32 1.78

Total 8 1976 4.05 --- --- 4.15

Example CLABSI Rate Table

Expected Number = 380 * (2.0 / 1000) = 380 * 0.002 = 0.76

Type of

Location

#

CLABSI

# Central

line-days

CLABSI

Rate

NHSN

Ratep-value Expected

# of

CLABSI

Medical cardiac 2 380 5.26 2.0 0.09 0.76

Medical 1 257 3.89 2.6 0.15 0.67

Med/Surgical 3 627 4.78 1.5 0.11 0.94

Neurosurgical 2 712 2.81 2.5 0.32 1.78

Total 8 1976 4.05 --- --- 4.15

Example CLABSI Rate Table

Observed 8 Standardized Infection Ratio (SIR) = = = 1.93

Expected 4.15

External Comparison of an SIR

to its Nominal Value of 1.0

SIR = 1.0 Observed = Expected

Is the SIR of Hosp A (1.93) significantly greater

than 1.0?

Answer: p-value = 0.06 (Poisson test)

Conclusion: NO, the observed number of

CLABSI is not "significantly greater" than

expected

State HAI Summary Report

State CLABSI SIRs

95% CI for SIR Graphic Representation

State

No. of

Facilities

Reporting SIR Lower Upper 0 1.0 2.0

A 50 0.68 0.52 0.87

B 30 0.94 0.72 1.19

C 8 0.59 0.36 0.91

D 140 0.90 0.80 1.01

E 48 1.30 1.14 1.48

http://www.cdc.gov/hai/statesummary.html

♦

♦

State HAI Summary Report

Obtaining SIRs From NHSN:

CLABSI

Example: CLABSI Standardized Infection Ratio

You have been asked to provide a summary measure

for all 2009 CLABSIs that can be presented to the

administrators at your organization.

CLABSI SIR Reports in NHSN

CLABSI SIR Reports in NHSN

Output for both in-plan and all data SIR reports will include 4 tables

SIR reports can be modified similarly to rate tables:

Selection criteria (OrgID, CDC location types, etc)

Time period selection

Time unit variable selection (month, quarter, half year, year)

When # expected <1, can look at longer time period to get SIR by year, for example

CLABSI rates should continue to be an important tool in prevention efforts - SIR provides summary measure adjusting for locations within facility

CLABSI SIR Reports in NHSNTable 1: Overall SIR for Facility

Represents an overall CLABSI SIR for those locations for which CLABSIs were monitored in your facility during the specified time period.

CLABSI SIR Reports in NHSN Table 2: SIR by Location Type

Provides SIRs by location group and time period(s) specified.

All locations would be defined by one of these groups.

ICU-OTHER includes ICUs and inpatient wards (i.e., non-NICU/SCA locations)

CLABSI SIR Reports in NHSNTable 3: SIR by CDC Location

Provides SIRs by CDC Location and time period(s) specified.

Example, if your facility reported CLABSI data for 2 Medical ICUs, the SIR for this CDC location would include both of those locations.

CLABSI SIR Reports in NHSNTable 4: By Individual Location

Most granular level of SIR analysis reports

Provides SIR for each individual location and time period(s)

Months variable allows user to see how many months of summary data are contributing to SIR (similar to rate table)

Interpretation: Overall CLABSI SIR for Facility

74 CLABSIs in 10,065 CL days were reported by this facility for first half of 2009

Based on aggregate data, 26.606 CLABSIs were expected

SIR is 2.78 – almost 3 times more CLABSIs than expected

P value and 95% CI indicate this is statistically significant

Surgical Site

Infections

NHSN SSI Rates

* Based on 2009 NHSN Report– Am J Infect Control 2009;37:783-805

Procedure

Code DescriptionRisk

Category SSI Rate*

VHYS Vaginal hysterectomy 0 0.73

1,2,3 1.16

CSEC Cesarean section 0 1.46

1 2.43

2,3 3.82

CBGB Coronary bypass with chest and donor incisions

0 0.35

1 2.55

2 4.26

3 8.49

Improved Risk Adjustment

● Risk index relies on three risk factors only

Allow all available factors to be considered

● These same risk factors must differentiate risk for all types of procedures

Allow the set of risk factors to be procedure-specific

● The relative contribution of these factors are constrained to be equal

Allow each factor’s contribution to vary according to it’s significant association with risk

● What can be done to improve risk adjustment?

Build logistic regression models

Available NHSN Risk Factors

For All Procedures General anesthesia Age

Wound class Emergency Gender

ASA score Trauma Endoscope

Duration of procedure Bed size∆ Med School Affiliation∆

For C-section Duration of labor

Weight Height Estimated blood loss

For Spinal fusion Diabetes Mellitus

Spinal level Approach/Technique

For Hip/Knee prosthesis

Total/Partial Primary/Revision

∆Hospital-level factor

Logistic Regression AnalysisSSI after VHYS (N=19,012)*

Factor Parameter Estimate OR p-value

Intercept -5.448 - -

Age (≤44 vs >44) 0.520 1.659 <0.0001

ASA (3/4/5 vs 1/2) 0.425 1.529 0.0415

Duration (>100 vs ≤100) 0.501 1.650 0.0019

Med school affiliation (Y vs N) 1.069 2.912 <0.0001

* Unpublished NHSN data, 2006-2008

Logistic Model

for VHYS:

Factor Parameter Estimate

Intercept -5.448

Age (≤44 ) 0.500

ASA (3/4/5 ) 0.425

Duration (>100 min) 0.501

Med school affiliation (Y) 1.069

logit ( ) = + 1 X1+ 2 X2 + 3 X3 + 4 X4

= -5.448 + 0.500 (Age ≤ 44* ) +

0.425 (ASA 3/4/5* ) +

0.501 (Duration >100* ) +

1.069 (Med school affiliation* )

*For these risk factors, if present = 1; if not = 0

p̂

Example VHYS Patient #1Risk Factors

● Age = 40

● ASA score = 4

● Duration = 117 min

● Med school affiliation = Y

Logistic ModelCalculation for Example VHYS Patient #1

logit( ) = + 1 X1+ 2 X2 + 3 X3 + 4 X4

logit( ) = -5.448 + 0.500 (1) + 0.425 (1) + 0.501 (1) + 1.069 (1) = -2.953

Solve for : = e logit ( ) / (1 + e logit ( ))

= e -2.953 / (1 + e -2.953)= 0.050 or 5.0% riskp̂

p̂

p̂ p̂ p̂ p̂

p̂

List of Patient Risk FactorsSSI after VHYS

Total O = 3 E = 0.85 E = 2.91

Patient Age Duration

ASA

Score

Med School

Affiliation SSI Risk Index Model

1 40 117 4 Y 0 0.012 0.050

2 53 95 2 N 0 0.007 0.004

3 30 107 2 Y 1 0.007 0.033

. . . . . . . .

. . . . . . . .

. . . . . . . .

100 37 128 4 Y 1 0.012 0.050

p̂Prob of SSI ( )

Standardized Infection Ratio (SIR) = 3 / 0.85 = 3.5 3 / 2.91= 1.0

List of Patient Risk FactorsSSI after CSEC

Risk Index Model

1 2 60 CC Y Y 25 277 Y 31 2 0 0.024 0.064

2 4 95 CO Y Y 22 321 N 32 0 1 0.038 0.158

3 2 50 C N N 31 179 N 40 1 0 0.015 0.018

. . . . . . . . .

. . . . . . . . .

. . . . . . . . .

100 2 28 4 Y Y 34 219 Y 30 3 1 0.024 0.050

Total O = 4 E = 1.52 E = 4.52

p̂Prob of SSI ( )

SIR = 4 / 1.52 = 2.6 4 / 4.52= 0.9

Obtaining SIRs From NHSN:

SSI

Example: SSI Standardized Infection Ratio (SIR)

You have been asked to provide a summary measure

for all 2009 surgical site infections that can be

presented to the administrators at your organization.

SSI SIR

Can obtain SIRs at most granular level to overall,

“big picture” level

Can obtain Surgeon-specific SIRs

Can obtain “All SSI” SIRs or “Complex A/R” SIRs

Includes:

All SSI SIR Includes Superficial, Deep & Organ/Space

Includes Primary only

Includes Admit, Readmit & Post discharge

Complex A/R Includes only SSIs identified on Admit/Readmit

Includes only inpatient procedures

Includes only Deep & Organ/Space

Includes Primary Deep incisional

SSI SIR

SSI SIRTable 1: Overall SSI SIR for Facility

Overall SIR for those operative procedures for which SSI were monitored in your facility during the specified time period.

SSI SIRTable 2: SSI SIRs by NHSN Operative Procedure

SIR by operative procedure category and time period specified.

In this “All SSI SIR” table 2, inpatient and outpatient procedures are grouped together.

SSI SIRTable 3: SSI SIRs by NHSN Operative Procedure & Setting

SIR by NHSN operative procedure category and outpatient status, as well as time period.

This table will only be available in the “All SSI SIR” output options.

SSI SIRTable 4: Incomplete Procedures

Provides a count of the procedures and SSIs that were excluded from the SIRs.

Will only list those procedures that were excluded due to missing risk factors, or records that were recorded with outliers, as well as custom procedures.

Next step: Review the “Line Listing –

Incomplete Procedures for SSI SIR”

SSI SIRLine List – Incomplete Procedures

Recommend modifying

this line list to limit to the

procedure, time period,

and risk factors in

question.

Note that procedures will

be excluded from the

SIR if they meet one or

more of the defined

exclusion criteria

SSI SIRLine List – Incomplete Procedures

SSI SIR exclusion criteria*:

– Missing one or more of the model’s risk factors

– Procedure duration is <5 minutes or >IQR5• IQR5: 5 times the interquartile range above the 75th percentile. EXAMPLE:

the interquartile range (Q3-Q1) = 30 minutes and the 75th percentile is 100

minutes; the IQR5 would be 100+(5*30) = 250 minutes

– Procedure date is ≤ patient’s DOB

– Patient’s age at procedure is ≥ 109 years

– Wound class (swClass) = ‘U’

– Approach = ‘N’ (for FUSN and RFUSN only)

– Spinal Level = ‘N’ (for FUSN only)

*Full details can be found in the SIR Newsletter: http://www.cdc.gov/nhsn/newletters.html

SSI SIRLine List – Incomplete Procedures

After reviewing this line list, the following steps are

recommended:

1. Obtain missing information (in this example, obtain ASA).

2. Edit the procedure record in NHSN to include this

information.

3. Re-generate datasets.

4. Run the desired SSI SIR output option.

SSI SIRInterpretation

During 2009, there were 425 procedures performed and 6 SSIs

identified.

Based on the NHSN 2006-2008 baseline data, 4.653 SSIs were

expected.

This results in an SIR of 1.29 (6/4.653), signifying that during this

time period our facility identified 29% more SSIs than expected.

The p-value and 95% Confidence Interval indicate that the number

of observed SSIs is not significantly higher than the number of

expected SSIs.

Measuring Progress

Group Baseline SIR Follow-up SIR Comparison

A 1.2 1.3 Same

B 0.9 0.8 Same

C 2.7 1.9 Different Lower

D 0.7 1.1 Different Higher

E 1.3 1.1 Different Lower

● Risk-adjustment incorporated in the SIR is limited

primarily by data collection burden

● The SIR summary measures could mask

differences in HAI experience within the strata

being summarized

● Differences in patient composition could explain

some of the variation when comparing SIRs

Limitations

● Standardized Infection Ratio (SIR) is a useful

metric for summarizing HAI experience

● Procedure-specific multivariate models are an

improvement over simple stratification methods

– Better estimate and differentiate SSI risk

● Longitudinal comparisons using the SIR will help to

measure prevention of HAI

Summary

Where can I find more information about the SIR?

NHSN Newsletter

Special Edition: Your

Guide to the

Standardized Infection

Ratio (SIR)

Includes CLABSI and

SSI information and

interpretations

Examples of output

Predictive Risk Factors

for SSI SIR modelshttp://www.cdc.gov/nhsn/newletters.html

Thank You

nhsn@cdc.gov