Where does the predicted number of infections come from?

NHSN calculates it from multivariable regression models built on national baseline data, applied to your facility’s denominators and risk factors. You cannot derive it from a simple rate — take it from your NHSN SIR report and enter it here.

Why is the SIR not calculated when fewer than 1 infection is predicted?

NHSN does not calculate an SIR when the predicted number is below 1.0, to avoid statistically imprecise SIRs with extreme values. You can combine a longer time period, track rates, or use NHSN’s TAP reports instead.

What does an SIR of 0.62 mean?

The facility observed 62% of the infections predicted — 38% fewer than predicted from the national baseline. Whether that difference is statistically significant depends on the p-value and confidence interval.

Free tool · CIC® exam topic

SIR calculator

Standardized infection ratio = observed ÷ predicted infections, with the mid-P exact statistics NHSN uses to tell whether it differs from 1.

Result

Standardized infection ratio: 2.114
Compared with predicted: 111% more than predicted
p-value (mid-P exact): 0.125
95% confidence interval: 0.775 – 4.686

More infections were observed than predicted, but the difference is not statistically significant (p > 0.05).

SIR formula

SIR = observed (O) HAIs ÷ predicted (P) HAIs

The SIR compares the infections a facility reported with the number predicted for its patients from NHSN’s national baseline, adjusting for risk factors. NHSN calculates SIRs for CLABSI, CAUTI, SSI, C. difficile and MRSA bacteremia LabID events, VAE and MBI-LCBI.

How to interpret the SIR

SIR > 1.0: more HAIs were observed than predicted.
SIR < 1.0: fewer HAIs were observed than predicted.
SIR = 1.0: the same number observed as predicted.
p-value ≤ 0.05 or a 95% confidence interval that excludes 1: the difference is statistically significant. NHSN calculates p-values with a mid-P exact test; this calculator uses the same approach for the p-value and the interval.
When the SIR is 0, NHSN does not calculate the lower bound of the confidence interval.

Worked example

NHSN’s guide describes a facility with 5 CLABSIs where 2.365 were predicted: 5 ÷ 2.365 = 2.114, 111% more than predicted. Here the p-value is 0.125 (95% CI 0.775 – 4.686), so the excess is not statistically significant.

Your NHSN report is the official figure; small differences in the interval can come from rounding of the predicted number.

Test yourself: a CIC-style question

An infection preventionist calculates a CLABSI SIR of 0.62 for the past year. How should this be interpreted?

The facility had 62% more CLABSIs than predicted
The facility had 38% fewer CLABSIs than predicted from the national baseline
The surveillance method was inaccurate
An SIR below 1.0 means no statistical analysis is needed

Show answer

B. The facility had 38% fewer CLABSIs than predicted from the national baseline

An SIR of 0.62 means 62% of the predicted number of infections were observed — 38% fewer than predicted. The SIR is risk-adjusted, so it supports comparison with the national baseline; check the p-value or confidence interval for significance.

Questions like this come up in the Surveillance and Epidemiologic Investigation domain of the CIC® exam. The 654 questions in the app each come with a written explanation.

FAQ

Where does the predicted number of infections come from?: NHSN calculates it from multivariable regression models built on national baseline data, applied to your facility’s denominators and risk factors. You cannot derive it from a simple rate — take it from your NHSN SIR report and enter it here.
Why is the SIR not calculated when fewer than 1 infection is predicted?: NHSN does not calculate an SIR when the predicted number is below 1.0, to avoid statistically imprecise SIRs with extreme values. You can combine a longer time period, track rates, or use NHSN’s TAP reports instead.
What does an SIR of 0.62 mean?: The facility observed 62% of the infections predicted — 38% fewer than predicted from the national baseline. Whether that difference is statistically significant depends on the p-value and confidence interval.

Sources

CDC NHSN. The NHSN Standardized Infection Ratio (SIR) Guide, updated March 2024

Checked against these sources on October 21, 2026. For education and CIC® exam preparation only — not a substitute for your facility’s policies, NHSN’s official reports, or clinical judgment. Calculations run in your browser; nothing you enter is sent anywhere.