KEY is a record identifier intended to be unique across all data years. However, there may be instances in which KEY is repeated across data years. For best practice, use YEAR + KEY to uniquely identify discharges if pooling data years. Beginning in the 1998 data, all HCUP databases are sorted by KEY.

KEY can be used to link within a HCUP database, such as linking records in the Core and Charges files in the SID.

KEY cannot be used to link across HCUP databases.

KEY is a unique record identifier and not a person identifier; therefore, it cannot be used to identify different hospital encounters for the same person.

KEY replaces the database-specific record identifiers used in the 1988-1997 HCUP databases (SEQ, SEQ_SID, and SEQ_ASD).

KEY is included as the unique record number for the 2000 KID. RECNUM is the unique record number for the 1997 KID, 2003 KID, and subsequent years.

The length of the value of KEY was increased from 14 characters to 15 characters starting in the 2011 HCUP databases. KEY is loaded into SAS, SPSS, and Stata as a numeric data element.