Standardized Infection Ratio (SIR) is calculated as which of the following?

• A. Observed/Predicted
• B. Predicted/Observed
• C. Observed/Expected
• D. Expected/Observed

Answer: (A)

Standardized Infection Ratio compares what actually happened to what would be expected under a standard benchmark. It is calculated by dividing the number of observed infections by the number of predicted (or expected) infections. The predicted figure comes from a model using national baseline rates adjusted for the facility’s patient mix and risk factors, giving a benchmark to judge performance. A SIR of 1 means observed equals predicted; a ratio above 1 indicates more infections than expected, while a ratio below 1 indicates fewer. Therefore, the best answer is the observed infections divided by the predicted infections. The other forms either invert the ratio or swap the concepts of what is being compared, which would misrepresent the comparison. For example, using predicted over observed would invert the interpretation.