Objectives Patients discharged from hospitals on a Friday (Friday discharges) are readmitted sooner (a shorter time-to-emergency-readmission) than those discharged on any other day of the week. To evaluate the cost-effectiveness of increasing weekend capacity, the effect estimate of Friday discharge on time-to-emergency-readmission needs to be precise. However, precise effect estimation is complicated by the confounding effect of differing healthcare-seeking behaviour and admission practices, and therefore different admission probability, by day of the week. The objective of this research was to examine how differing admission probability by day of the week influences the effect of discharge day on time-to-emergency-readmission. Methods We used a Markov model to determine how day of the week admission probability would theoretically affect the time-to-emergency-readmission for Friday and Wednesday discharges. We tested this in a cohort of patients who have had a history of respiratory illness, using a Cox proportional hazards model to fit the time-to-emergency-readmission to any Quebec hospital as a function of the day of the week of discharge and admission. We fitted another Cox model with an additional time-varying covariate for the current day of the week, to model differing admission probabilities by day of the week. Results Our Markov model showed that if admission probability is lower on the weekends, Friday discharges will be readmitted later (longer time-to-emergency-readmission) than Wednesday discharges. Using hospital admission data, we found that Friday discharges were readmitted slightly earlier than Wednesday discharges (HR: 1.03, 95% CI: (1.02, 1.05)). After adding a time-varying covariate for the current day of the week, the length of time-to-emergency-readmission for a Friday discharge increased, but it was still earlier than a Wednesday discharge (HR: 1.04, 95% CI: (1.01, 1.07)). Conclusions The lower admission probabilities on the weekend confound the effect of Friday discharge on time-to-emergency-readmission by increasing the time-to-emergency-readmission. This confounding effect causes an underestimate of the effect of Friday discharge on time-to-emergency-readmission.
Keywords: day of the week; hospital readmissions; survival analysis.