Measuring readmissions: focus on the time factor

OBJECTIVE: To assess the effects of choosing different time-intervals of observation when using unplanned readmissions as an outcome indicator. DESIGN: A conceptual model was developed based on the risk curve. The model assigned readmissions above a background level as 'related' to the earlier episode of illness. The characteristics of the hazard curve were used to estimate how the rates of related and unrelated readmissions varied with time. SETTING: Patients living in a region of Middle Norway served by eight acute-care hospitals and discharged in the year 1996. MAIN OUTCOME MEASURE: The conditional risk (hazard rate) of having an unplanned readmission. The information gathered allowed inclusion of readmissions to all hospitals in the area, and to make risk corrections for deaths. RESULTS: The identified proportion of readmissions judged as related to the earlier episode of illness was found to be very sensitive to changes in the time interval. With the commonly used interval of 30 days, 0.5 of all related readmissions were identified, while 0.7 of the readmissions included at this time were estimated as related ones ('true positives'). The hazard curve was different for medical and surgical patients, but the corresponding proportions of related and unrelated readmissions were relatively similar. Adjusting for deaths in the observation period did not result in significantly different risk curves. CONCLUSION: When unplanned readmissions are used as an outcome indicator, the measure is susceptible to the choice of time interval. The operative characteristics must be interpreted in the context of where it is intended that the indicator should be used.