What does SIR stand for in infection surveillance?

• A. Standardized Infection Ratio
• B. Standardized Infection Rate
• C. Surveillance Infection Ratio
• D. Systematic Infection Ratio

Answer: (A)

SIR stands for Standardized Infection Ratio. It compares the number of infections actually observed in a facility to the number predicted for that facility using a risk-adjusted national model. Because the prediction accounts for factors like hospital size, patient mix, and unit type, the ratio lets you compare performance across different hospitals fairly. If the observed infections exceed the predicted number, the SIR is greater than 1, indicating more infections than expected; if fewer, the SIR is less than 1, indicating better-than-expected performance. For example, 8 observed infections with 4 predicted gives an SIR of 2.0, while 3 observed with 6 predicted gives 0.5. This is a ratio of counts (observed to predicted), not a rate per device-days, which is why the term “Rate” isn’t the standard phrasing in infection surveillance.