NRD_DaysToEvent is one of two data elements created specifically for the Nationwide Readmissions Database (NRD) for tracking patients across hospitals in a year. The timing information in NRD_DaysToEvent must be used in tandem with the visit linkage variable (NRD_VisitLink). These variables enable users to study multiple hospital visits for the same patient across hospitals and time while adhering to strict privacy regulations.

The timing variable (NRD_DaysToEvent) was calculated consistently for each verified patient linkage (NRD_visitLink) based on a randomly assigned "start date." Each patient linkage is assigned a unique start date that is used to calculate NRD_DaysToEvent for all visits associated with that visitLink value. The variable NRD_DaysToEvent is the difference between the visit's admission date and the start date associated with the visitLink.

The calculation of days between visits is the difference of NRD_DaysToEvent between two selected visits for a patient linkage number (NRD_visitLink). Consider the following example:

• A patient with congestive heart failure that has a 3-day hospital admission on 1/10/13 and another inpatient stay on 1/25/13.
• Suppose the NRD_DayToEvent value is "1109" for the 1/10/13 admission and the NRD_DayToEvent value is "1124" for the 1/25/13 stay.
• The number of days between the start of each admission is 15 days (1124 - 1109 = 15)
• Readmission analyses often consider the time between the end of one admission and the start on the next admission. The number of days between the admissions (from discharge date of the first admission to the start of the second admission) is 12 days (1124 -1109 - 3 = 12) because the patient had a 3-day length of stay.

The lowest value of NRD_DaysToEvent will be on the first or earliest event for a patient. It is important to remember that if patient A has a value of 605 for NRD_DaysToEvent and patient B has a value of 300 for NRD_DaysToEvent, patient B's event did not necessarily take place prior to patient A's event. In fact, Patient B's NRD_DaysToEvent value has no relation to Patient A's NRD_DaysToEvent value. Because of the use of a random start date in the calculation of NRD_DaysToEvent, the value of NRD_DaysToEvent cannot be compared across patients.