Possible Immortal Time Bias in Study of Antibiotic Treatment and Outcomes in Patients Hospitalized for Asthma

Newman discusses an article by Stefan et al on a 1-day higher median length of stay among patients admitted for acute asthma who were treated with antibiotics for at least 2 days starting in the first 2 days of their hospitalization, compared with propensity score-matched controls not so treated. He argues that the authors should clarify how the timing of antibiotic exposure and length of stay were determined because it seems like the study might be susceptible to immortal time bias, in which there is a period in the exposed group during which they are not at risk of the outcome (which in this case is discharge from the hospital).