Immortal Time Bias in Assessing Evidence-Based Care Processes for Staphylococcus aureus Bacteremia—Reply

In Reply Tong et al call attention to the potential for immortal time bias in our analyses¹ of relationships between use of evidence-based care processes and mortality among patients with Staphylococcus aureus bacteremia (SAB). Immortal time bias may occur in cohort studies if there is a period of time when occurrence of the outcome of interest—mortality in our study¹—prevents patients from experiencing the exposure(s) of interest. In short, associations between use of evidence-based care processes and survival at 30 days may occur because patients who survived longer were more likely to receive the care processes and not because receipt of the care processes extended survival.