Parameters:
(1) number of infections
(2) number of days at risk
(3) NHSN rate per thousand days of risk (adjusted for hospital)
expected number of infections =
= (NHSN rate per 1000 days) * (total number of days at risk) / 1000
SIR =
= (number of infections observed) / (expected number of infections)
Restrictions:
(1) The SRI should not be calculated if the expected number of infections is less than 1.
Interpretation:
• A hospital performing at the expected rate will have an SIR = 1.
• An SIR > 1 indicates a number of infections greater than expected. An SIR of 1.25 indicates a rate 25% greater than expected.
• An SIR < 1 indicates a number of infections that is less than expected.
Limitations:
• Delgada-Rodriguez and Llorca indicate the SIR does not ensure reliability in the observed number of infections. They recommend that the standard distribution of the NNIS system index be given to adjust rates.
