Standardized Infection Ratio (SIR)

The Standardized Infection Ratio (SIR) is not a count and not a rate — it is a risk-adjusted ratio that the CDC National Healthcare Safety Network (NHSN) uses to summarize healthcare-associated infection (HAI) performance, calculated as the number of infections actually observed at a facility or unit divided by the number statistically predicted for that same facility or unit given its patient mix and reporting period. It is the summary measure behind most public HAI comparisons — including CLABSI, CAUTI, and surgical site infection reporting — and it underlies CMS’s Hospital-Acquired Condition (HAC) Reduction Program scoring.

Operational Definition

SIR = Observed number of infections ÷ Predicted number of infections.

The predicted number is not a flat national average. NHSN calculates it using a risk-adjustment model (logistic regression, fit against a national baseline period of NHSN data) that accounts for facility- and unit-level factors known to be associated with differing infection risk — for example, ICU type, hospital bed size, teaching affiliation, or procedure type, depending on the specific infection measure. Two units with identical observed infection counts can have very different SIRs if their predicted counts differ because their patient populations carry different baseline risk.

• SIR = 1.0 means the observed number of infections exactly matches what was predicted for that population.
• SIR > 1.0 means more infections were observed than predicted (worse than the risk-adjusted expectation).
• SIR < 1.0 means fewer infections were observed than predicted (better than the risk-adjusted expectation).

NHSN’s statistics calculator also reports a 95% confidence interval and a p-value for each SIR; NHSN’s standard cut-point treats a p-value below 0.05 as indicating the SIR is statistically significantly different from 1.0.

Why Small Denominators Make the SIR Unstable

Because the SIR is built from a predicted count derived from a relatively small number of device-days or procedures at the unit level, facilities and units with low patient volume or low device utilization can see wide swings in SIR from period to period even with no real change in infection prevention practice — a single additional CLABSI on a low-device-day unit can move the SIR dramatically, and the accompanying confidence interval is correspondingly wide. This is a standard property of ratio measures built on small-count Poisson-like data, and it is the reason NHSN reports the confidence interval alongside the point estimate rather than the SIR alone: a wide interval that includes 1.0 means the difference from the predicted rate is not statistically distinguishable from chance, regardless of whether the SIR itself looks high or low. Comparing SIRs across small units, or using a single quarter’s SIR to judge a small unit’s performance, routinely overstates how much the number has actually moved.

Worked Examples

• A hospital’s ICU has 4 observed CLABSIs against a NHSN-predicted count of 4.0 for its central-line days and risk profile during the reporting period: SIR = 4 ÷ 4.0 = 1.0, meaning observed infections matched the risk-adjusted prediction exactly.
• A different unit has 6 observed CAUTIs against a predicted count of 3.0: SIR = 6 ÷ 3.0 = 2.0, meaning twice as many infections were observed as predicted for that unit’s risk profile — a candidate for closer review, though the confidence interval around that 2.0 should be checked before treating it as a confirmed, stable finding, especially if the unit’s catheter-days for the period were low.

Counter-Example: What the SIR Is Not

A raw infection count (e.g., “this unit had 5 CLABSIs this quarter”) is not a SIR — it has no risk adjustment and cannot be meaningfully compared across units with different patient populations. Similarly, a simple infection rate (infections per 1,000 device-days) is a different, non-risk-adjusted measure; NHSN and CMS use the SIR specifically because it adjusts for known risk factors before comparison, which a raw rate does not do.

Related Terms

See the CLABSI and CAUTI guides for how the observed infection counts that feed into an SIR are themselves determined through NHSN’s surveillance criteria, and the Patient Safety & Infection Prevention hub for the broader surveillance landscape.