Impact of length of stay after coronary bypass surgery on short-term readmission rate: An instrumental variable analysis

Objective: To determine the effect of postoperative length of stay (LOS) on 30-day readmission after coronary artery bypass surgery. Data Sources/Study Setting: We analyzed a final database consisting of Medicare claims of a cohort (N=157,070) of all fee-for-service beneficiaries undergoing bypass surgery during 2007-2008, the American Hospital Association annual survey file, and the rural urban commuting area file. Study Design: We regressed the probability of 30-day readmission on postoperative LOS using (1) a (naive) logit model that controlled for observed patient and hospital covariates only; and (2) a residual inclusion instrumental variable (IV) logit model that further controlled for unobserved confounding. The IV was defined using a measure of the hospital's risk-adjusted LOS for patients admitted for gastrointestinal hemorrhage. Principal Findings: The naive logit model predicted that a 1-day reduction in median postoperative LOS (ie, from a median of 6-5 d) lowered the 30-day readmission rate by 2 percentage points. The IV model predicted that a 1-day reduction in median postoperative LOS increased 30-day readmission rate by 3 percentage points. Conclusions: The findings indicate that a reduction in postoperative LOS is associated with an increased risk for 30-day readmission among Medicare patients undergoing bypass surgery, after both observed and unobserved confounding effects are corrected.