The data element DaysToEvent is one of two data elements that are supplemental information created for HCUP States for which there are encrypted person identifiers. The timing information in DaysToEvent must be used in tandem with the visit linkage variable (VisitLink). VisitLink is created from verified person numbers. 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 (DaysToEvent) was calculated consistently for each verified person number (visitLink) based on a randomly assigned "start date." Each verified person number is assigned a unique start date that is used to calculate DaysToEvent for all visits associated with that visitLink value. The variable DaysToEvent is the difference between the visit's admission date and the start date associated with the visitLink. Maps indicating the availability of the revisit variables provide detailed information regarding which States release this information in the Central Distributor SID, SASD, and/or SEDD by year.
The calculation of days between visits is the difference of DaysToEvent between two selected visits for a unique verified person number (visitLink). For example, consider a patient with congestive heart failure that has a hospital admission on 1/10/2008 and an ED visit on 1/25/2008. If the DaysToEvent value is "9" for the 1/10/2008 admission and the DaysToEvent value is "24" for the 1/25/2008 ED visit, then the number of days between the two visits is 15 days (24 - 9 = 15). It should be noted that often readmission analyses consider the time between the end of one admission and the start on the next admission. To adjust for the length of the admission, subtract the length of stay to the difference. In the example, above, it the first admission had a length of stay of 2 days then the number of days between the end of the first visit and the start of the second visit is 13 days (24 - 9 - 2 = 13).
The lowest value of 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 DaysToEvent and patient B has a value of 300 for DaysToEvent, patient B's event did not necessarily take place prior to patient A's event - in fact, Patient B's DaysToEvent value has no relation to Patient A's DaysToEvent value. Because of the use of a random start date in the calculation of DaysToEvent, the value of DaysToEvent cannot be compared across patients.
Beginning with the 2009 HCUP data, the revisit variables (VisitLink and DaysToEvent) are included in the Core file of the SID, SASD, and SEDD files, when possible. For 2003-2008 data, the revisit variables are in separate HCUP Supplemental Files for Revisit Analyses.
More information on the revisit variables VisitLink and DaysToEvent is available under the HCUP Supplemental Variables for Revisit Analyses.